Ventilatory management of ARDS: can it affect the outcome?

Pediatric acute respiratory distress syndrome - current views

Acute respiratory distress syndrome (ARDS) mainly involves acute respiratory failure. In addition to this affected patients feel progressive arterial hypoxemia, dyspnea, and a marked increase in the work of breathing. The only clinical solution for the above pathological state is ventilation. Mechanical ventilation is necessary to support life in ARDs but it itself worsen lung injury and the term is known clinically as ‘ventilation induced lung injury’ (VILI). At the cellular level, respiratory epithelial cells are subjected to cyclic stretch, i.e. repeated cycles of positive and negative strain, during normal tidal ventilation. In aerated areas of diseased lungs, or even normal lungs subjected to injurious positive pressure mechanical ventilation, the cells are at risk of being over distended, and worsening injury by disrupting the alveolar epithelial barrier. Further, hypercapnic acidosis (HCA) in itself confers protection from stretch injury, potentially via a mechanisms involving inhibition of nuclear factor κB (NF-κB), a transcription factor central to inflammation, injury and repair. Mesenchymal stem cells are the latest in the field and are being investigated as a possible therapy for ARDS.

On the complexity of scoring acute respiratory distress syndrome: do not forget hemodynamics!

Acute respiratory distress syndrome (ARDS) remains associated with a poor outcome despite recent major therapeutic advances. Forecasting the outcome of patients suffering from such a syndrome is of a crucial interest and many scores have been proposed, all suffering from limits responsible for important discrepancies. Authors try to elaborate simple, routine and reliable scores but most of them do not consider hemodynamics yet acknowledged as a major determinant of outcome. This article aims at reminding the approach of scoring in ARDS and at deeply describing the most recently published one in order to highlight their main pitfall, which is to forget the hemodynamics.

The effect of diet-induced serum hypercholesterolemia on the surfactant system and the development of lung injury

Acute respiratory distress syndrome (ARDS) is a pulmonary disorder associated with alterations to the pulmonary surfactant system. Recent studies showed that supra-physiological levels of cholesterol in surfactant contribute to impaired function. Since cholesterol is incorporated into surfactant within the alveolar type II cells which derives its cholesterol from serum, it was hypothesized that serum hypercholesterolemia would predispose the host to the development of lung injury due to alterations of cholesterol content in the surfactant system.

Wistar rats were randomized to a standard lab diet or a high cholesterol diet for 17–20 days. Animals were then exposed to one of three models of lung injury: i) acid aspiration ii) ventilation induced lung injury, and iii) surfactant depletion. Following physiological monitoring, lungs were lavaged to obtain and analyze the surfactant system.

The physiological results showed there was no effect of the high cholesterol diet on the severity of lung injury in any of the three models of injury. There was also no effect of the diet on surfactant cholesterol composition. Rats fed a high cholesterol diet had a significant impairment in surface tension reducing capabilities of isolated surfactant compared to those fed a standard diet exposed to the surfactant depletion injury. In addition, only rats that were exposed to ventilation induced lung injury had elevated levels of surfactant associated cholesterol compared to non-injured rats.

It is concluded that serum hypercholesterolemia does not predispose rats to altered surfactant cholesterol composition or to lung injury. Elevated cholesterol within surfactant may be a marker for ventilation induced lung damage.

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Hypercholesterolemia in rats did not alter the susceptibility to lung injury.

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Elevated cholesterol within surfactant is observed in ventilation induced lung injury.

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Increases in surfactant-associated cholesterol depend on the type of lung injury.

Prognosis of critically ill patients with multiple organ failure

The purpose of this study was to determine the mortality rate in 527 critically ill patients with multiple organ failure (MOF), treated in our ICU between August, 1986 and January, 1992, and to compare it with the results obtained in a group of patients studied who had been treated between October, 1978 and July, 1986. The relationship between the mortality rate and each type of organ failure and the extent of organ system involvement was also investigated. The overall mortality rate was 25%, and the rate increased with the number of failed organs. Sepsis and disseminated intravascular coagulation were closely associated with the development of MOF. The mortality rate of patients with the failure of two organs in the present study was significantly lower than that found in those in the previous study. Although artificial organ mechanical life support technology other than that for patients with renal failure is still unsatisfactory, these results suggest that the prognosis of patients with MOF is improving.

Respiratory management of inhalation injury

Advances in the care of patients with major burns have led to a reduction in mortality and a change in the cause of their death. Burn shock, which accounted for almost 20 percent of burn deaths in the 1930s and 1940s, is now treated with early, vigorous fluid resuscitation and is only rarely a cause of death. Burn wound sepsis, which emerged as the primary cause of mortality once burn shock decreased in importance, has been brought under control with the use of topical antibiotics and aggressive surgical debridement. Inhalation injury has now become the most frequent cause of death in burn patients. Although mortality from smoke inhalation alone is low (0-11 percent), smoke inhalation in combination with cutaneous burns is fatal in 30 to 90 percent of patients. It has been recently reported that the presence of inhalation injury increases burn mortality by 20 percent and that inhalation injury predisposes to pneumonia. Pneumonia has been shown to independently increase burn mortality by 40 percent, and the combination of inhalation injury and pneumonia leads to a 60 percent increase in deaths. Children and the elderly are especially prone to pneumonia due to a limited physiologic reserve. It is imperative that a well organized, protocol driven approach to respiratory care of inhalation injury be utilized so that improvements can be made and the morbidity and mortality associated with inhalation injury be reduced.

Measuring end-expiratory lung volume and pulmonary mechanics to detect early lung function impairment in rabbits

We investigated whether end-expiratory lung volume (EELV) or lung mechanical parameters are more sensitive for the detection of a compromised gas exchange during bronchoconstriction and after surfactant depletion. EELV was determined via SF(6) multiple breath wash-outs in mechanically ventilated rabbits while a positive end-expiratory pressure (PEEP) of 1, 3 or 7 cm H(2)O was maintained. Airway resistance (R(aw)) and parenchymal elastance (H) were estimated from the pulmonary input impedance measured at each PEEP level by means of forced oscillations. Measurements were repeated during i.v. methacholine (MCh) infusions and following lung injury induced by saline lavage. MCh induced marked elevations in R(aw), with no significant change in EELV or H at any PEEP. After lavage, the severity of hypoxia was reflected systematically in significant decreases in EELV at all PEEP levels (-42+/-13%, -26+/-4%, and -18+/-5% at 1, 3 and 7 cm H(2)O, respectively), whereas compromised gas exchange was not associated with consistent changes in the mechanical parameters at a PEEP of 7 cm H(2)O (20+/-9% and 14+/-9% in R(aw) and H, respectively; p=0.2). We conclude that R(aw) is the only sensitive indicator for the detection of a compromised lung function during MCh infusions, whereas the estimation of EELV is necessary to follow the progression of a lung injury when a high PEEP level is applied.

High frequency percussive ventilation in burn patients: hemodynamics and gas exchange

High frequency percussive ventilation (HFPV) is a recent ventilatory mode, which combines conventional cycles with high frequency percussions. HFPV was initially instituted as salvage therapy after acute respiratory failure following smoke inhalation injury achieving in each case a dramatic improvement of blood oxygenation, PaCO(2) and ventilatory pressures. This study investigates the influence of HFPV on hesmodynamics, blood oxygenation and ventilatory parameters in eight stable ICU burn patients requiring artificial ventilatory support during a postoperative period following traumatic injury. Periods of 2h were analysed receiving conventional ventilation and HFPV with a high frequency of 400 and 800 cycles/min. Hemodynamic data were not significantly modified; peak inspiratory pressure was significantly lower under HFPV but mean airway pressure was unchanged. Blood oxygenation and CO(2) elimination were significantly improved under HFPV. No side effects were noted. These observations suggest that HFPV could improve pulmonary gas exchanges under lower peak pressures and without hemodynamic compromise. HFPV could represent an interesting alternative open lung strategy method to improve alveolar recruitment.

Innovative practices of ventilatory support with pediatric patients

OBJECTIVES

The recognition that alveolar overdistension rather than peak inspiratory airway pressure is the primary determinant of lung injury has shifted our understanding of the pathogenesis of ventilator-induced side effects. In this review, contemporary ventilatory methods, supportive treatments, and future developments relevant to pediatric critical care are reviewed.

DATA SYNTHESIS

A strategy combining recruitment maneuvers, low-tidal volume, and higher positive end-expiratory pressure (PEEP) decreases barotrauma and volutrauma. Given that appropriate tidal volumes are critical in determining adequate alveolar ventilation and avoiding lung injury, volume-control ventilation with high PEEP levels has been proposed as the preferable protective ventilatory mode. Pressure-related volume control ventilation and high-frequency oscillatory ventilation (HFOV) have taken on an important role as protective lung strategies. Further data are required in the treatment of children, confirming the preliminary results in specific lung pathologies. Spontaneous breathing supported artificially during inspiration (pressure support ventilation) is widely used to maintain or reactivate spontaneous breathing and to avoid hemodynamic variation. Volume support ventilation reduces the need for manual adaptation to maintain stable tidal and minute volume and can be useful in weaning. Prone positioning and permissive hypercapnia have taken on an important role in the treatment of patients undergoing artificial ventilation. Surfactant and nitric oxide have been proposed in specific lung pathologies to facilitate ventilation and gas exchange and to reduce inspired oxygen concentration. Investigation of lung ventilation using a liquid instead of gas has opened new vistas on several lung pathologies with high mortality rates.

RESULTS

The conviction emerges that the best ventilatory treatment may be obtained by applying a combination of types of ventilation and supportive treatments as outlined above. Early treatment is important for the overall positive final result. Lung recruitment maneuvers followed by maintaining an open lung favor rapid resolution of pathology and reduce side effects.

CONCLUSIONS

The methods proposed require confirmation through large controlled clinical trials that can assess the efficacy reported in pilot studies and case reports and define the optimal method(s) to treat individual pathologies in the various pediatric age groups.

Mechanical ventilatory support

This chapter reviews the most updated knowledge regarding mechanical ventilation, its' indications and its' features both diagnostic and therapeutic. Further, the various modes of mechanical ventilation are described. The reader will also gain insight into the pathophysiology of various disease processes and the mode of ventilation that may be the most helpful in their treatment. Weaning is also discussed as well as a relatively new type of ventilation, non-invasive.

Critical care of the severely injured child

It is the responsibility of the surgeon committed to care of these critically injured patients to understand the nature of injuries being treated, and to orchestrate this treatment in a manner that maximizes recovery, avoids unnecessary morbidity, and assures the injured child the best quality of life humanly possible.

The outcome of early pressure-controlled inverse ratio ventilation on patients with severe acute respiratory distress syndrome in surgical intensive care unit

BACKGROUND

Pressure-controlled inverse ratio ventilation (PC-IRV) was used in patients with acute respiratory distress syndrome (ARDS) after failed volume-cycled conventional ratio ventilation (VC-CRV). The aim of this study was to evaluate the outcome of early PC-IRV in severe ARDS.

METHODS

Twenty patients with severe ARDS were switched from VC-CRV to PC-IRV if they failed to maintain SaO(2) >90% by the following criteria: peak inspiratory pressure (PIP) >35 cm H(2)O, FIO(2) = 60%, and positive end-expiratory pressure (PEEP) 10 cm H(2)O.

RESULTS

The values of PIP, mean airway pressure, minute volumes, and lung injury score in VC-CRV were 43.9 +/- 8.0 cm H(2)O, 19.5 +/- 6.4 cm H(2)O, 11.0 +/- 2.1 L/min, and 2.8 +/- 0.2 respectively. In PC-IRV, the corresponding data were 31.8 +/- 5.1 cm H(2)O, 25.4 +/- 4.6 cm H(2)O, 8.3 +/- 0.9 L/min, and 2.5 +/- 0.4. All of these parameters were significantly different. Fifteen patients (75%) survived their intensive care unit stay.

CONCLUSIONS

Early PC-IRV in severe ARDS improves oxygenation, facilitates tapering of high fraction of inspiratory oxygen, and decreases high PEEP or PIP, and then results in the improvement of the patient's outcome.

Tracheal gas insufflation

Tracheal gas insufflation is a technique in which gas is injected intratracheally during positive pressure ventilation. The fresh gas rinses expired gas from the tracheal tube and anatomical dead space, aiding carbon dioxide elimination. This reduces ventilatory volume and pressure, helping to reduce ventilator-induced lung damage. Complications of tracheal gas insufflation include interference with ventilator function, tracheal damage and barotrauma. Expiratory washout is a variation of tracheal gas insufflation. We designed and constructed an original expiratory washout system and evaluated its safety and performance in lung and animal models. We found that expiratory limb and tracheal tube occlusion tests caused the device to disable itself at acceptable intratracheal pressures. We also demonstrated up to 31% reduction in tidal volume compared with conventional ventilation, supporting the possibility of using this device clinically to lessen volutrauma. We concluded that aspects of this design might alleviate many of the safety concerns of using tracheal gas insufflation.

Small dose of exogenous surfactant combined with partial liquid ventilation in experimental acute lung injury: effects on gas exchange, haemodynamics, lung mechanics, and lung pathology

A combination of exogenous surfactant and partial liquid ventilation (PLV) with perfluorocarbons should enhance gas exchange, improve respiratory mechanics and reduce tissue damage of the lung in acute lung injury (ALI). We used a small dose of exogenous surfactant with and without PLV in an experimental model of ALI and studied the effects on gas exchange, haemodynamics, lung mechanics, and lung pathology. ALI was induced by repeated lavages (PaO2/FIO2 less than 13 kPa) in 24 anaesthesized, tracheotomized and mechanically ventilated (FIO2 1.0) juvenile pigs. They were treated randomly with either a single intratracheal dose of surfactant (50 mg kg(-1), Curosurf, Serono AG, München, Germany) (SURF-group, n=8), a single intratracheal dose of surfactant (50 mg kg(-1), Curosurf) followed by PLV with 30 ml kg(-1) of perfluorocarbon (PF 5080, 3M, Germany) (SURF-PLV-group, n=8) or no further intervention (controls, n=8). Pulmonary gas exchange, respiratory mechanics, and haemodynamics were measured hourly for a 6 h period. In the SURF-group, the intrapulmonary right-to-left shunt (QS/QT) decreased significantly from mean 51 (SEM 5)% after lavage to 12 (2)%, and PaO2 increased significantly from 8.1 (0.7) to 61.2 (4.7) kPa compared with controls and compared with the SURF-PLV-group (P<0.05). In the SURF-PLV-group, QS/QT decreased significantly from 54 (3)% after induction of ALI to 26 (3)% and PaO2 increased significantly from 7.2 (0.5) to 30.8 (5.0) kPa compared with controls (P<0.05). Static compliance of the respiratory system (C(RS)), significantly improved in the SURF-PLV-group compared with controls (P<0.05). Upon histological examination, the SURF-group revealed the lowest total injury score compared with controls and the SURF-PLV-group (P<0.05). We conclude that in this experimental model of ALI, treatment with a small dose of exogenous surfactant improves pulmonary gas exchange and reduces the lung injury more effectively than the combined treatment of a small dose of exogenous surfactant and PLV.

Computerized decision support for mechanical ventilation of trauma induced ARDS: results of a randomized clinical trial

BACKGROUND

Variability and logistic complexity of mechanical ventilatory support of acute respiratory distress syndrome, and need to standardize care among all clinicians and patients, led University of Utah/LDS Hospital physicians, nurses, and engineers to develop a comprehensive computerized protocol. This bedside decision support system was the basis of a multicenter clinical trial (1993-1998) that showed ability to export a computerized protocol to other sites and improved efficacy with computer- versus physician-directed ventilatory support. The Memorial Hermann Hospital Shock Trauma intensive care unit (ICU) (Houston, TX; a Level I trauma center and teaching affiliate of The University of Texas Houston Medical School) served as one of the 10 trial sites and recruited two thirds of the trauma patients. Results from the trauma patient subgroup at this site are reported to answer three questions: Can a computerized protocol be successfully exported to a trauma ICU? Was ventilator management different between study groups? Was patient outcome affected?

METHODS

Sixty-seven trauma patients were randomized at the Memorial Hermann Shock Trauma ICU site. "Protocol" assigned patients had ventilatory support directed by the bedside respiratory therapist using the computerized protocol. "Nonprotocol" patients were managed by physician orders.

RESULTS

Of the 67 trauma patients randomized, 33 were protocol (age 40 +/- 3; Injury Severity Score [ISS] 26 +/- 3; 73% blunt) and 34 were nonprotocol (age 38 +/- 2; ISS 25 +/- 2; 76% blunt). For the protocol group, the computerized protocol was used 96% of the time of ventilatory support and 95% of computer-generated instructions were followed by the bedside respiratory therapist. Outcome measures (i.e., survival, ICU length of stay, morbidity, and barotrauma) were not significantly different between groups. Fio2 > or = 0.6 and Pplateau > or = 35 cm H2O exposures were less for the protocol group.

CONCLUSION

A computerized protocol for bedside decision support was successfully exported to a trauma center, and effectively standardized mechanical ventilatory support of trauma-induced acute respiratory distress syndrome without adverse effect on patient outcome.

New clinically relevant sheep model of severe respiratory failure secondary to combined smoke inhalation/cutaneous flame burn injury

OBJECTIVES

To develop a predictable, dose-dependent, clinically relevant model of severe respiratory failure associated with a 40% total body surface area, full-thickness (third-degree) cutaneous flame burn and smoke inhalation injury in adult sheep.

DESIGN

Model development.

SETTING

Research laboratory.

SUBJECTS

Adult female sheep (n = 22).

INTERVENTIONS

Animals were divided into three groups, determined by the number of smoke breaths administered (24, 36, 48) for a graded inhalation injury. The smoke was insufflated into a tracheostomy with a modified bee smoker at airway temperatures <40 degrees C. All animals concurrently received a 40% total body surface area (third-degree) cutaneous flame burn to the body (flanks). After injury, the animals were placed on volume-controlled ventilation to achieve PaO2 >60 mm Hg and PaCO2 <40 mm Hg. Arterial blood gases and ventilator settings were monitored every 6 hrs postinjury for up to 7 days.

MEASUREMENTS AND MAIN RESULTS

All animals survived the induction of injury. In the 24 smoke breath/40% total body surface area burn (24/40) group, PaO2/F(IO2) never decreased below 300, and peak inspiratory pressure was consistently <14 cm H2O with normal arterial blood gases throughout the observation period. With 36 smoke breaths/40% total body surface area burn (36/40) (n = 7), all animals had PaO2/F(IO2) of <200 and peak inspiratory pressure of 26 cm H2O within 40-48 hrs, as 30% died during the study period. With 48 smoke breaths/40% total body surface area burn (48/40) (n = 12), all animals developed respiratory distress syndrome (RDS) in 24-30 hrs, but none survived the experimental period.

CONCLUSIONS

Development of RDS by smoke and cutaneous flame bum injury depends on smoke inhalation dose. A combination of 36 breaths of smoke and a 40% total body surface area (third-degree) cutaneous flame burn injury can induce severe RDS (PaO2/F(IO2) <200) within 40-48 hrs to allow evaluation of various treatment modalities of RDS.

Comparison of intratracheal pulmonary ventilation and hybrid intratracheal pulmonary ventilation with conventional mechanical ventilation in a rabbit model of acute respiratory distress syndrome by saline lavage

OBJECTIVES

To study changes in PaCO2 and PaO2 during intratracheal pulmonary ventilation (ITPV) and hybrid intratracheal pulmonary ventilation (h-ITPV) compared with conventional mechanical ventilation (CMV) in a rabbit model of respiratory failure, and to define the technique of h-ITPV that combines conventional mechanical ventilation and ITPV.

DESIGN

Prospective, interventional study.

SUBJECTS

Twelve adult New Zealand White rabbits.

INTERVENTIONS

Surfactant deficiency was induced by saline lavage, and rabbits were randomized to either ITPV or h-ITPV. The study consisted of four phases: phase 0, CMV after saline lavage, ventilator rate 30 breaths/min; phase I, ITPV or h-ITPV initiated at the same pressure and rate as in phase 0; phase II, ITPV or 1.0 L/min h-ITPV bias flow, with peak inspiratory pressure (PIP) decreased and ventilator rate increased to achieve the lowest tidal volume while maintaining adequate gas exchange; and phase III, animals returned to CMV.

MEASUREMENTS AND MAIN RESULTS

In phase I, no difference in PaCO2 was observed between ITPV, h-ITPV, or CMV. There was a decrease in PaO2 when switching from CMV to ITPV but not to h-ITPV. In phase II, it was possible to decrease PIP (average of 37% for ITPV and 36% for h-ITPV) and tidal volume (average of 64% for ITPV and 53% for h-ITPV) without compromising gas exchange (p < .05). Oxygenation tended to improve from phase 0 to the end of phase II. In phase III, PaCO2 increased (average of 71% for ITPV and 79% for h-ITPV) and pH decreased (p < .05). Normocapnia was achieved using significantly higher PIP and tidal volume, compared with phase 0 (p < .05).

CONCLUSIONS

ITPV and h-ITPV can effectively ventilate and oxygenate rabbits with surfactant-deficient lungs at tidal volumes and therefore pressures lower than required with CMV. Maximum benefit appears to occur at high ventilator rates. These findings suggest that both modes of ventilation may represent powerful new tools in the management of patients with acute respiratory failure. (Crit Care Med 2000; 28:774-781)

Exacerbation of acute pulmonary edema during assisted mechanical ventilation using a low-tidal volume, lung-protective ventilator strategy

STUDY OBJECTIVES

To assess the magnitude of negative intrathoracic pressure development in a patient whose pulmonary edema acutely worsened immediately following the institution of a low-tidal volume (VT) strategy.

DESIGN

Mechanical lung modeling of patient-ventilator interactions based on data from a case report.

SETTING

Medical ICU and laboratory.

PATIENT

A patient with suspected ARDS and frank pulmonary edema.

INTERVENTIONS

The patient's pulmonary mechanics and spontaneous breathing pattern were measured. Samples of arterial blood and pulmonary edema fluid were obtained.

MEASUREMENTS

A standard work-of-breathing lung model was used to mimic the ventilator settings, pulmonary mechanics, and spontaneous breathing pattern observed when pulmonary edema worsened. Comparison of the pulmonary edema fluid-to-plasma total protein concentration ratio was made.

RESULTS

The patient's spontaneous VT demand was greater than preset. The lung model revealed simulated intrathoracic pressure changes consistent with levels believed necessary to produce pulmonary edema during obstructed breathing. A high degree of imposed circuit-resistive work was found. The pulmonary edema fluid-to-plasma total protein concentration ratio was 0.47, which suggested a hydrostatic mechanism.

CONCLUSION

Ventilator adjustments that greatly increase negative intrathoracic pressure during the acute phase of ARDS may worsen pulmonary edema by increasing the transvascular pressure gradient. Therefore, whenever sedation cannot adequately suppress spontaneous breathing (and muscle relaxants are contraindicated), a low-VT strategy should be modified by using a pressure-regulated mode of ventilation, so that imposed circuit-resistive work does not contribute to the deterioration of the patient's hemodynamic and respiratory status.

Reduced ventilator pressure and improved P/F ratio during percutaneous arteriovenous carbon dioxide removal for severe respiratory failure

OBJECTIVE

To evaluate the effect of percutaneous arteriovenous carbon dioxide removal (AVCO2R) on ventilator pressures and P/F ratio in a clinically relevant large-animal model of severe respiratory failure.

SUMMARY BACKGROUND DATA

AVCO2R was developed as a simple arteriovenous shunt with a commercially available low-resistance gas exchange device of sufficient surface area for near-total CO2 removal. With an AV shunt 10% to 15% of cardiac output, AVCO2R allows a reduction in ventilator airway pressures without hypercapnia or the complex circuitry and monitoring required for conventional ECMO.

METHODS

AVCO2R was applied to a new, clinically relevant large-animal model of severe respiratory failure created by smoke inhalation and cutaneous flame bum injury. Adult sheep (n = 9, 38+/-6 kg) received a 40% total body surface area, third-deinsufflation. After injury, all animals were placed on volume-controlled mechanical ventilation to achieve PaO2 > 60 mmHg and PacO2 < 40 mmHg. Animals were placed on AVCO2R within 40 to 48 hours of injury when the PaO2/FiO2 was <200. Animals underwent cannulation of the carotid artery and jugular vein with percutaneous 10F arterial and 14F venous cannulas. Shunt flow was continuously monitored using an ultrasonic flow probe and calculated as a percentage of cardiac output.

RESULTS

AVCO2R flows of 800 to 900 ml/min (11% to 13% cardiac output) achieved 77 to 104 ml/min of CO2 removal (95% to 97% total CO2 production) while maintaining normocapnia. Significant reductions in ventilator settings were tidal volume, 421.3+/-39.8 to 270.0+/-6.3 ml; peak inspiratory pressure, 24.8+/-2.4 to 13.7+/-0.7 cm H2O; minute ventilation, 12.7+/-1.4 to 6.2+/-0.8 L/min; respiratory rate, 25.4+/-1.3 to 18.4+/-1.8 breaths/min; and FiO2, 0.88+/-0.1 to 0.39+/-0.1. The P/F ratio increased from 151.5+/-40.0 at baseline to 320.0+/-17.8 after 72 hours.

CONCLUSIONS

Percutaneous AVCO2R allows near-total CO2 removal and significant reductions in ventilator pressures with improvement in the P/F ratio.

Extreme hypoventilation reduces ventilator-induced lung injury during ventilation with low positive end-expiratory pressure in saline-lavaged rabbits

OBJECTIVE

To compare the degrees of ventilator-induced lung injury caused by two ventilation protocols.

DESIGN

Randomized trial.

SETTING

University animal laboratory.

SUBJECTS

Sixteen New Zealand white rabbits.

INTERVENTIONS

After five sequential saline lung lavages, eight pairs of anesthetized rabbits were allocated randomly to receive either of two ventilation protocols for 4 hrs during neuromuscular blockade. Both groups received 3 cm H2O of positive end-expiratory pressure and 100% oxygen. Control group animals received an estimated tidal volume of 12 mL/kg, an inspiratory time of 0.7 sec, and a ventilatory rate adjusted for a PaCO2 of 35 to 45 torr (4.7 to 6 kPa). Study group animals were ventilated through an intratracheal catheter, with inspiratory time of 1.5 secs, ventilatory rate of 20 breaths/min, and peak inspiratory pressure of 4 to 8 cm H2O, adjusted to maintain PaCO2 at 150 to 250 torr (20 to 33 kPa).

MEASUREMENTS AND MAIN RESULTS

Arterial blood gases were measured every 30 mins. After 4 hrs, a final lung lavage was performed. Physiologic parameters, cell counts and protein concentration in the final lavage, and lung histology were compared between groups. The alveolar-arterial oxygen tension gradient was higher in the study group over the first 1.5 hrs, but the time profile showed significantly (p = .001) greater improvement in the study group. After 4 hrs, the mean alveolar-arterial oxygen tension gradient was lower in the study group (94 torr [12.5 kPa] vs. 201 torr [26.8 kPa]). The increase in neutrophil count from the initial to the final lung lavage was lower in the study group (0.27 x 10(7) cells/L vs. 2.01 x 10(7) cells/L, p = .037), as was the absolute value of the neutrophil count in the final lavage (1.33 x 10(7) cells/L vs. 3.02 x 10(7) cells/L, p = .04). The median hyaline membrane score was lower in the study group (0.5 vs. 3.0) but the difference was not statistically significant.

CONCLUSION

These findings suggest that a very low tidal volume reduces ventilator-induced lung injury in saline-lavaged rabbits during ventilation at low lung volume.

Intratracheal pulmonary ventilation at low airway pressures in a ventilator-induced model of acute respiratory failure improves lung function and survival

STUDY OBJECTIVE

The pulmonary parenchyma in patients with acute respiratory failure (ARF) is commonly not involved in a homogenous disease process. Conventional mechanical ventilation (MV) at elevated positive end-expiratory pressure (PEEP) and peak inspiratory pressure (PIP) aims at recruiting collapsed or nonventilated lung units. Invariably, those pressures are also transmitted to the healthiest regions, with possible extension of the disease process (barotrauma). During intratracheal pulmonary ventilation (ITPV), a continuous flow of fresh gas is delivered directly at the carina, bypassing the dead space proximal to the catheter tip. In healthy sheep, it allows lowering tidal volume (VT) to as low as 1.0 mL/kg, at respiratory rates (RR) up to 120 breaths/min, while maintaining normocapnia. In a model of ventilator-induced lung injury, we wished to explore whether ITPV, applied at low VT and low PEEP and tailored to ventilate the healthiest regions of the lungs, could provide adequate oxygenation and alveolar ventilation, without any attempt to recruit lungs.

DESIGN

Randomized study in sheep.

SETTING

Animal research laboratory.

PARTICIPANTS

We induced ARF in 12 sheep following 1 to 2 days of MV at a PIP of 50 cm H2O, except that 5 to 8% of lungs were kept on apneic oxygenation of 5 cm H2O, sparing those regions from the injury process.

INTERVENTIONS

Sheep were randomized to volume-controlled MV (control group) (n = 6) with VT of 8 to 12 mL/kg, PEEP of 5 to 10 cm H2O, or to ITPV (n = 6) at PEEP of 3 to 5 cm H2O, VT of 2.5 to 4 mL/kg, PIP of <20 cm H2O, at RRs sufficient to sustain normocapnia.

MEASUREMENTS AND RESULTS

Hemodynamic status in the ITPV group progressively improved, and all six sheep were weaned to room air within 83+/-54 h. Sheep in the control group had progressively deteriorating conditions and all animals died after a mean of 50+/-39 h. Barotrauma and postmortem histopathologic changes were more pronounced in the control group.

CONCLUSION

In this model of ventilator-induced lung injury, low PEEP-low VT ventilation with ITPV sustained normocapnia and prevented further lung injury, allowing weaning to room air ventilation.

The pressure-volume curve is greatly modified by recruitment. A mathematical model of ARDS lungs

A mathematical model of the ARDS lung, with simulated gravitational superimposed pressure, evaluated the effect of varying alveolar threshold opening pressures (TOP), PEEP and peak inspiratory pressure (PIP) on the static pressure-volume (PV) curve. The lower inflection point (Pflex) was affected by SP and TOP, and did not accurately indicate PEEP required to prevent end-expiratory collapse. Reinflation of collapsed lung units (recruitment) continued on the linear portion of the PV curve, which had a slope at any volume greater than the total compliance of aerated alveoli. As recruitment diminished, the reduced PV slope could produce an upper Pflex at 20 to 30 cm H2O pressure. An upper Pflex caused by alveolar overdistension could be modified or eliminated by recruitment with high TOP. With constant PIP as PEEP increased, and TOP range of 5 to 60 cm H2O, PEEP to prevent end-expiratory collapse was indicated by minimum PV slope above 20 cm H2O, minimum hysteresis, and maximum volume at a pressure of 20 cm H2O. With constant inflation volume as PEEP increased, the effect on PV slope was unpredictable. Although increased PV slope indicated recruitment, maximum PV slope usually underestimated PEEP required to prevent end-expiratory collapse. Therefore, with this model the PV curve did not reliably predict optimal ventilator settings.

High-frequency oscillation versus conventional ventilation following surfactant administration and partial liquid ventilation

Surfactant followed by partial liquid ventilation (PLV) with perfluorocarbon (PFC; LiquiVent) improves oxygenation, lung compliance, and lung pathology in lung-injured animals receiving conventional ventilation (CV). In this study, we hypothesize that high-frequency oscillation (HFO) and CV will provide equivalent oxygenation in lung-injured animals following surfactant repletion and PLV, once lung volume is optimized. After saline-lavage lung injury during CV, newborn piglets were randomized to either HFO (n = 10) or CV (n = 9). HFO animals were stabilized over 15 min without optimization of lung volume; CV animals continued treatment with time-cycled, pressure-limited, volume-targeted ventilation. All animals then received 100 mg/kg of surfactant (Survanta). Thirty minutes later, all received intratracheal PFC to approximate functional residual capacity. Thirty minutes after PLV began, mean airway pressure (MAP) in both groups was increased to improve oxygenation. MAP was directly adjusted during HFO; PEEP and PIP were adjusted during IMV, maintaining a pressure sufficient to deliver 15 mL/kg tidal volume. Animals were treated for 4 h. The CV group showed improved oxygenation following surfactant administration (OI: 26.79 +/- 1.98 vs. 8.59 +/- 6.29, P < 0.0004), with little further improvement following PFC administration or adjustments in MAP. Oxygenation in HFO-treated animals did not improve following surfactant, but did improve following PFC (0I: 27.78 +/- 6.84 vs. 15.86 +/- 5.53, P < 0.005) and adjustments in MAP (OI: 15.86 +/- 5.53 vs. 8.96 +/- 2.18, P < 0.03). After MAP adjustments, there were no significant intergroup differences in oxygenation. Animals in the CV group required lower MAP than animals in the HFO group to maintain similar oxygenation. We conclude that surfactant repletion followed by PLV improves oxygenation during both CV and HFO. The initial response to administration of surfactant and PFC was different for the conventional and high-frequency oscillation groups, likely reflecting the ventilation strategy used; animals in the CV group responded most to surfactant, whereas animals in the HFO group responded most after PFC instillation. The ultimately similar oxygenation of the two groups once lung volume had been optimized suggests that HFO may be used effectively during administration of, and treatment with, surfactant and perfluorocarbon.

Increasing tidal volumes and pulmonary overdistention adversely affect pulmonary vascular mechanics and cardiac output in a pediatric swine model

OBJECTIVES

In a pediatric swine model, the effects of increasing tidal volumes and the subsequent development of pulmonary overdistention on cardiopulmonary interactions were studied. The objective was to test the hypothesis that increasing tidal volumes adversely affect pulmonary vascular mechanics and cardiac output. An additional goal was to determine whether the effects of pulmonary overdistention are dependent on delivered tidal volume and/or positive end-expiratory pressure (PEEP, end-expiratory lung volume).

DESIGN

Prospective, randomized, controlled laboratory trial.

SETTING

University research laboratory.

SUBJECTS

Eleven 4- to 6-wk-old swine, weighing 8 to 12 kg.

INTERVENTIONS

Piglets with normal lungs were anesthetized, intubated, and paralyzed. After median sternotomy, pressure transducers were placed in the right ventricle, pulmonary artery, and left atrium. An ultrasonic flow probe was placed around the pulmonary artery.

MEASUREMENTS AND MAIN RESULTS

The swine were ventilated and data were collected with delivered tidal volumes of 10, 15, 20, and 25 mL/kg and PEEP settings of 5 and 10 cm H2O in a random order. Pulmonary overdistention was defined as a decrease in dynamic compliance of > or =20% when compared with a compliance measured at a baseline tidal volume of 10 mL/kg. At this baseline tidal volume, airway pressure-volume curves did not demonstrate pulmonary overdistention. Tidal volumes and airway pressures were measured by a pneumotachometer and the Pediatric Pulmonary Function Workstation. Inspiratory time (0.75 sec), FIO2 (0.3), and minute ventilation were held constant. We evaluated the pulmonary vascular and cardiac effects of the various tidal volume and PEEP settings by measuring pulmonary vascular resistance, pulmonary characteristic impedance, and cardiac output. When compared with a tidal volume of 10 mL/kg, a tidal volume of 20 mL/kg resulted in a significant decrease in dynamic compliance from 10.5 +/- 0.9 to 8.4 +/- 0.6 mL/cm H2O (p = .02) at a constant PEEP of 5 cm H2O. The decrease in dynamic compliance of 20% indicated the presence of pulmonary overdistention by definition. As the tidal volume was increased from 10 to 20 mL/kg, pulmonary vascular resistance (1351 +/- 94 vs. 2266 +/- 233 dyne x sec/cm5; p = .004) and characteristic impedance (167 +/- 12 vs. 219 +/- 22 dyne x sec/cm5; p = .02) significantly increased, while cardiac output significantly decreased (951 +/- 61 vs. 708 +/- 48 mL/min; p = .001). Each of these effects of pulmonary overdistention were further magnified when the tidal volume was increased to 25 mL/kg. The tidal volume-induced alterations in pulmonary vascular mechanics, characteristic impedance, and cardiac output occurred to a greater degree when the PEEP was increased to 10 cm H2O. Pulmonary vascular resistance and characteristic impedance were significantly increased and cardiac output significantly decreased for all tidal volumes studied at a PEEP of 10 cm H2O as compared with 5 cm H2O.

CONCLUSIONS

Increasing tidal volumes, increasing PEEP levels, and the development of pulmonary overdistention had detrimental effects on the cardiovascular system by increasing pulmonary vascular resistance and characteristic impedance while significantly decreasing cardiac output. Delivered tidal volumes of >15 mL/kg should be utilized cautiously. Careful monitoring of respiratory mechanics and cardiac function, especially in neonatal and pediatric patients, is warranted.

An approach to the treatment of severe adult respiratory failure

OBJECTIVES

The purpose of this article is to evaluate outcome in adult patients with severe respiratory failure managed with an approach using (1) limitation of end inspiratory pressure, (2) inverse ratio ventilation, (3) titration of PEEP by SvO2, (4) intermittent prone positioning, (5) limitation of FiO2, (6) diuresis, (7) transfusion, and (8) extracorporeal life support (ECLS) if patients failed to respond.

PATIENTS AND METHODS

This study was designed as a retrospective review in the intensive care unit of a tertiary referral hospital. One-hundred forty-one consecutive patients with hypoxic (n = 135) or hypercarbic (n = 6) respiratory failure referred for consideration of ECLS between 1990 and 1996. Overall, initial PaO2/FiO2 (P/F) ratio was 75+/-5 (median = 66).

RESULTS

Lung recovery occurred in 67% of patients and 62% survived. Forty-one patients improved without ECLS (83% survived); 100 did not and were supported with ECLS (54% survived). Survival was greater in patients cannulated within 12 hours of arrival (59%) compared with those cannulated after 12 hours (40%, P < .05). Multiple logistic regression identified age, duration of mechanical ventilation before transfer, four or more dysfunctional organs, and the requirement for ECLS as independent predictors of mortality.

CONCLUSIONS

An approach that emphasizes lung protection and early implementation of extracorporeal life support is associated with high rates of survival in patients with severe respiratory failure.

Evaluation of a ventilation strategy to prevent barotrauma in patients at high risk for acute respiratory distress syndrome. Pressure- and Volume-Limited Ventilation Strategy Group

BACKGROUND

A strategy of mechanical ventilation that limits airway pressure and tidal volume while permitting hypercapnia has been recommended for patients with the acute respiratory distress syndrome. The goal is to reduce lung injury due to overdistention. However, the efficacy of this approach has not been established.

METHODS

Within 24 hours of intubation, patients at high risk for the acute respiratory distress syndrome were randomly assigned to either pressure- and volume-limited ventilation (limited-ventilation group), with the peak inspiratory pressure maintained at 30 cm of water or less and the tidal volume at 8 ml per kilogram of body weight or less, or to conventional ventilation (control group), with the peak inspiratory pressure allowed to rise as high as 50 cm of water and the tidal volume at 10 to 15 ml per kilogram. All other ventilatory variables were similar in the two groups.

RESULTS

A total of 120 patients with similar clinical features underwent randomization (60 in each group). The patients in the limited-ventilation and control groups were exposed to different mean (+/-SD) tidal volumes (7.2+/-0.8 vs. 10.8+/-1.0 ml per kilogram, respectively; P<0.001) and peak inspiratory pressures (23.6+/-5.8 vs. 34.0+/-11.0 cm of water, P<0.001). Mortality was 50 percent in the limited-ventilation group and 47 percent in the control group (relative risk, 1.07; 95 percent confidence interval, 0.72 to 1.57; P=0.72). In the limited-ventilation group, permissive hypercapnia (arterial carbon dioxide tension, >50 mm Hg) was more common (52 percent vs. 28 percent, P=0.009), more marked (54.4+/-18.8 vs. 45.7+/-9.8 mm Hg, P=0.002), and more prolonged (146+/-265 vs. 25+/-22 hours, P=0.017) than in the control group. The incidence of barotrauma, the highest multiple-organ-dysfunction score, and the number of episodes of organ failure were similar in the two groups; however, the numbers of patients who required paralytic agents (23 vs. 13, P=0.05) and dialysis for renal failure (13 vs. 5, P= 0.04) were greater in the limited-ventilation group than in the control group.

CONCLUSIONS

In patients at high risk for the acute respiratory distress syndrome, a strategy of mechanical ventilation that limits peak inspiratory pressure and tidal volume does not appear to reduce mortality and may increase morbidity.

Pulmonary epithelial permeability and gas exchange: a comparison of inverse ratio ventilation and conventional mechanical ventilation in oleic acid-induced lung injury in rabbits

STUDY OBJECTIVE

(1) To explore the interaction between mechanical ventilation and oleic acid (OA)-induced lung injury on indexes of pulmonary gas exchange and epithelial permeability, and (2) to compare this interaction using two different modes of ventilation: pressure-controlled inverse ratio ventilation (PCIRV) and volume-controlled ventilation with positive end-expiratory pressure (VCV PEEP).

DESIGN

Randomized animal study.

SETTING

Experimental laboratory investigation at Södersjukhuset, Stockholm, Sweden.

ANIMALS

Twenty-four New Zealand white rabbits.

INTERVENTIONS

(1) Ventilation with PCIRV (n=6) or VCV PEEP (n=6) for 6 h at equal end-expiratory alveolar pressure levels of 5 cm H2O followed by induction of lung injury (IV injection of OA 0.15 mL/kg). (2) Induction of lung injury followed by 6 h of ventilation with either PCIRV (n=6) or VCV PEEP (n=6) as described above.

MEASUREMENTS AND RESULTS

Lung mechanics, heart rate, BP, and gas exchange results were equal at baseline. In group A, after 1 h of ventilation, mean airway pressure was 11.9+/-4.4 with PCIRV and 8.3+/-1.0 cm H2O with VCV PEEP (p<0.05). Forty minutes after OA injection, PaO2/fraction of inspired oxygen (FIO2) was 24+/-10 kPa with PCIRV and 44+/-15 kPa with VCV PEEP (p<0.05). Mean airway pressure was higher and peak airway pressure was lower with PCIRV. In group B, after 6 h of ventilation, PaO2/FIO2 was 17+/-5 kPa with PCIRV and 43+/-8 kPa with VCV PEEP (p<0.01). Systemic BP was lower with PCIRV and mean airway pressure was higher. Technetium-99m diethylene triamine penta-acetic acid lung clearance: In group A, curves were monoexponential with both PCIRV (half-life time [T 1/2], 21+/-8 min and VCV PEEP (T 1/2, 126+/-59 min, p<0.005) until injection of OA. In the VCV PEEP-treated animals, a marked increase in clearance rate was observed within 60 s of OA injection (T 1/2, 13+/-9 min, p<0.001). Fifteen minutes after OA injections, T 1/2 had decreased to 38+/-17 min with VCV PEEP. In the animals treated with PCIRV, OA injection did not lead to a significant change in clearance rate, although the elimination pattern was observed to change from single-compartment to multicompartment type. In group B, clearance curves were monoexponential with both ventilatory modes. There was no significant difference in clearance rate between PCIRV (T 1/2, 25+/-9 min) and VCV PEEP (T 1/2, 36+/-16 min, not significant).

CONCLUSIONS

The observation that PaO2 was lower in the PCIRV-treated groups must be interpreted with caution in this animal study with relatively few observations. The finding may reflect differences in the effect of OA injection in the two ventilatory modes. It is also possible that externally applied PEEP is more effective than PCIRV in increasing oxygen tension, either because of a less inhomogenous distribution of ventilation and perfusion or for other reasons. The clearance results imply that PCIRV causes an alteration in lung epithelial or membrane function in comparison to VCV PEEP. This functional difference is most likely caused by the large time-weighted lung volume produced by pressure control in combination with a prolonged inspiration. Induction of high permeability lung injury with OA eliminates the difference between PCIRV and VCV PEEP. It remains to be established whether these findings are relevant with regard to ventilator-associated structural lung injury in man.

Pulmonary function and radiographic abnormalities related to neurological outcome after aneurysmal subarachnoid hemorrhage

OBJECT

This observational study is based on a consecutive series of 207 patients with aneurysmal subarachnoid hemorrhage who were treated within 7 days of their most recent bleed. The purpose of the study was to evaluate the effect of respiratory failure on neurological outcome.

METHODS

Pulmonary function was assessed by determination of parameters describing pulmonary oxygen transport and exchange, by using composite scores for quantification of lung injury (lung injury score [LIS]) and mechanical ventilator settings (PIF score). Pulmonary function was related to the Hunt and Hess (H & H) grade assigned to the patient at hospital admission (p < 0.001). The pattern and time course of lung injury differed significantly between patients with H & H Grade I or II, Grade III, and Grade IV or V. Hunt and Hess grade, Fisher computerized tomography grade, intracranial pressure, cerebral perfusion pressure, LIS, ratio of PaO2 to the fraction of inspired oxygen (FiO2), and the ratio of the alveolar-minus-arterial oxygen tension difference (AaDO2) to FiO2 were related to neurological outcome (p < 0.001). The LIS on the day of maximum lung injury remained an independent predictor of outcome (p = 0.01) in a stepwise logistic regression analysis. The probability of poor neurological outcome significantly increased with both decreasing cerebral perfusion pressure and increasing severity of lung injury.

CONCLUSIONS

The overall mortality rate was 22.2% (46 of 207 patients). Subarachnoid hemorrhage and its neurological sequelae accounted for the principal mortality in this series. Medical (nonneurological and nontreatment-related) complications accounted for 37% of all deaths. Systemic inflammatory response syndrome with associated multiple organ dysfunction syndrome was the leading cause of death from medical complications. The authors conclude that respiratory failure is related to neurological outcome, although it is not commonly the primary cause of death from medical complications.

PEEP only partly restores disturbed distribution of regional pulmonary blood flow in lung injury

The effects of lung injury, positive end-expiratory pressure (PEEP), and norepinephrine on heterogeneity of regional pulmonary blood flow (rPBF, radioactive microspheres) were investigated. We hypothesized that lung injury increases heterogeneity of rPBF and that PEEP ventilation reduces these effects. Heterogeneity of rPBF is scale dependent and was therefore assessed in detail. Local correlation (p), relative dispersion (RD), fractal dimension (D), perfusion gradients, and histograms of rPBF each measures a different aspect of heterogeneity. In eight anesthetized dogs, lung injury was induced with oleic acid and glass bead injection. Afterward, PEEP of 10-20 cmH2O was instituted. Norepinephrine was infused at 20 cmH2O PEEP. Heterogeneity increased upon lung injury (p, 0.44 +/- 0.09 vs. 0.24 +/- 0.09; RD, 0.36 +/- 0.06 vs. 0.64 +/- 0.12; both P < or = 0.05), but fractal dimension remained constant. PEEP did not change p, RD, or D. Perfusion gradients were reversed after lung injury (right, -27 +/- 18 vs. 196 +/- 115%; -24 +/- 18 vs. 282 +/- 184%; P < or = 0.05). PEEP (10 cmH2O) reduced gradients (116 +/- 73 and 143 +/- 62%, respectively; P < or = 0.05). Norepinephrine, in part, further reduced gradients (right, 50 +/- 58%; P < or = 0.05; left, 102 +/- 94%; P = NS). We conclude that oleic acid- and glass bead-induced lung injury produces abnormal distribution of rPBF. Of these changes, application of PEEP only reverses perfusion gradients.

Use of intratracheal pulmonary ventilation versus conventional ventilation in meconium aspiration syndrome in a newborn pig model

OBJECTIVE

To determine whether intratracheal pulmonary ventilation (ITPV) allows for effective oxygenation and ventilation at lower mean airway pressures and peak inspiratory pressures than conventional ventilation in a piglet model of meconium aspiration syndrome.

DESIGN

Prospective, interventional study.

SETTING

The animal research laboratory at Children's National Medical Center, Washington, DC.

SUBJECTS

Twenty newborn piglets, 2 to 7 days of age, weighing 1.8 to 2.8 kg.

INTERVENTION

Animals were anesthetized, paralyzed, intubated, and ventilated. Femoral arterial and venous catheters were inserted; 5 mL/kg of 20% meconium in normal saline was instilled into the endotracheal tube. Animals were randomized to either ITPV or conventional ventilation, and settings were adjusted to maintain ideal blood gases, i.e., pH 7.35 to 7.45, PCO2 40 to 45 torr (5.3 to 6 kPa), PO2 80 to 100 torr (10.7 to 13.3 kPa), and SaO2 > or = 90%. Ventilatory settings were adjusted as needed to a maximum of: FIO2 1.0, peak inspiratory pressure 40 cm H2O, positive end-expiratory pressure 5 cm H2O, and respiratory rate 80 breaths/min.

MEASUREMENTS AND MAIN RESULTS

Arterial blood gases were taken every 30 mins for 4 hrs and ventilatory settings were adjusted to maintain optimal blood gases. Heart rate, mean arterial blood pressure, and arterial saturation were monitored continuously. The animals in the ITPV group had significantly lower peak inspiratory pressure at 1, 2, 3, and 4 hrs after meconium instillation (p < .018) and significantly lower mean airway pressure at 2, 3, and 4 hrs after meconium instillation (p < .03). The mean peak inspiratory pressure in the ITPV animals ranged from 17 +/- 2.7 cm H2O at baseline to 16.6 +/- 5.7 cm H2O at 4 hrs compared with 16.5 +/- 2.7 cm H2O at baseline to 31.8 +/- 9.1 cm H2O at 4 hrs in the conventionally ventilated animals (p < .04). The mean airway pressure ranged from 6.3 +/- 1.1 mm Hg at baseline to 6.8 +/- 2.5 mm Hg at 4 hrs in the ITPV group compared with 5.5 +/- 1.2 mm Hg at baseline to 10.7 +/- 3.4 mm Hg at 4 hrs in the conventional ventilation group (p < .03). The lungs of the ITPV animals were less hemorrhagic and had less pathologic evidence of injury than the lungs of the conventionally ventilated animals.

CONCLUSIONS

These results indicate that ITPV can be used to effectively ventilate and oxygenate piglets with meconium aspiration syndrome at lower mean airway pressures and peak inspiratory pressures than conventional ventilation. This lower pressure causes less injury to the lungs of the animals.

Total arteriovenous CO2 removal: simplifying extracorporeal support for respiratory failure

BACKGROUND

To reduce the complexity, complications, and cost of conventional extracorporeal membrane oxygenation, we have developed a technique of simplified arteriovenous extracorporeal CO2 removal (AVCO2R) with a low-resistance membrane gas exchanger for total CO2 removal to provide lung rest in the setting of severe respiratory failure.

METHODS

We initially used AVCO2R in healthy animals to quantify the gas exchange capabilities of the system and establish ventilator management protocols for the subsequent studies of AVCO2R in a large animal model of respiratory failure secondary to a severe smoke inhalation injury.

RESULTS

In healthy sheep the maximum spontaneous arteriovenous flow ranged from 1,350 to 1,500 mL/min, whereas CO2 removal plateaued at a blood flow of approximately 1,000 mL/min in which 112 +/- 3 mL/min CO2 was removed, allowing an 84% reduction in the minute ventilation of from 6.9 +/- 0.8 L/min to 1.1 +/- 0.4 L/min (p < 0.01) without triggering hypercapnia. A subsequent reduction in extracorporeal flow at a reduced minute volume led to the development of hypercapnia only if it decreased to less than 500 mL/min. We also applied AVCO2R in mechanically ventilated sheep with a severe smoke inhalation injury and removed 95% (111 +/- 4 mL/min) of the total CO2 production. This allowed the minute ventilation to be reduced by 95% and the peak inspiratory pressures by 52% (both p < 0.05) over 6 hours and produced no adverse hemodynamic effects. The partial pressure of arterial oxygen was maintained above 100 mm Hg at a maximally reduced minute volume. The mean AVCO2R flow was 1,213 +/- 29 mL/min, averaging 27% +/- 1% of the cardiac output.

CONCLUSIONS

We conclude that AVCO2R in a simple arteriovenous shunt is a less complicated technique than extracorporeal membrane oxygenation and is capable of total CO2 removal that allows a significant reduction in the minute ventilation and peak airway pressure during severe respiratory failure.

Prolonged hemodynamic stability during arteriovenous carbon dioxide removal for severe respiratory failure

OBJECTIVE

The effects of prolonged arteriovenous carbon dioxide removal on hemodynamics during severe respiratory failure were evaluated in adult sheep with severe smoke inhalation injury.

METHODS

Adult female sheep (n = 6,33.8 +/- 5.2 kg) were subjected to intratracheal cotton severe smoke insufflation to a mean carboxyhemoglobin level of 83% +/- 3%. Twenty-four hours after injury, a low-resistance 2.5 m2 membrane oxygenator was placed in a carotid-to-jugular pumpless arteriovenous shunt at unrestricted flow to allow complete carbon dioxide removal and reductions in ventilator support. Animals remained conscious, and heart rate, cardiac output, mean arterial pressure, and pulmonary arterial pressure were measured at baseline, after injury, and daily during support with the arteriovenous carbon dioxide removal circuit for 7 days.

RESULTS

All animals survived the study period. Carbon dioxide removal ranged from 99.7 +/- 13.7 to 152.2 +/- 16.2 ml/min, and five (83%) of the six animals were successfully weaned from the ventilator before day 7. During full support with the arteriovenous carbon dioxide removal circuit, shunt flow ranged from 1.24 +/- 0.06 to 1.43 +/- 0.08 L/min and accounted for 20.1% +/- 1.4% to 25.9% +/- 2.4% of cardiac output. No statistically significant changes in heart rate, cardiac output, mean arterial pressure, or pulmonary artery pressure were demonstrated over the study course despite the extracorporeal shunt flow.

CONCLUSIONS

Arteriovenous carbon dioxide removal as a simplified means of extracorporeal gas exchange support is relatively safe without adverse hemodynamic effects or complications.

Pressure-controlled ventilation in ARDS: a practical approach

Patients with ARDS typically have functionally small lungs. A growing body of clinical and experimental evidence has demonstrated that mechanical ventilation that results in high transpulmonary pressure gradients and overdistention of lung units will potentiate the acute lung injury in patients with ARDS. A relative form of "lung rest" using low tidal volume mechanical ventilation that prevents alveolar overdistention has therefore been advocated. This may be achieved with low-volume, volume-cycled ventilation with a decelerating inspiratory flow or pressure-controlled ventilation (PCV). The goal of this article is to provide a simple and practical approach to the management of PCV in patients with ARDS. Implicit in our approach is the use of a ventilator with PCV software and waveform capabilities.

Interest of a therapeutic optimization strategy in severe ARDS

STUDY OBJECTIVE

Evaluate the interest of the response to a therapeutic optimization as a predictor of prognosis in ARDS.

DESIGN

Prospective study.

SETTING

ICU of a University Hospital.

PATIENTS

Thirty-six consecutive patients with severe ARDS addressed for extracorporeal carbon dioxide removal (ECCO2R).

INTERVENTIONS

We studied the response during the first 2 days after arrival to the therapeutic optimization strategy consisting in a combination of the following: (1) decrease in extravascular lung water (diuretics or hemofiltration); (2) selection of the best ventilatory mode; (3) permissive hypercarbia; and (4) correction of hypoxemia by alveolar recruitment, additional continuous oxygen insufflation, body position changes (prone position), inhaled nitric oxide, enhancement of hypoxic pulmonary vasoconstriction with almitrine, and drainage of pleural or mediastinal effusions. In patients remaining severely hypoxemic despite these modalities, ECCO2R was then proposed.

MEASUREMENTS AND RESULTS

Thirty-six patients were addressed after 8.3+/-5.5 days of mechanical ventilation. On arrival, mean simplified acute physiologic score was 46.8+/-14.2, multiple system organ failure score was 1.8+/-1.6, Murray score was 3.4+/-0.4, PaO2 was 75.3+/-31.3 (fraction of inspired oxygen [FIO2]=1) for a positive end-expiratory pressure level of 12.3+/-3.4 cm H2O. Nineteen of 36 patients improved their gas exchange within 2 days and their mortality was 21%. The seventeen remaining patients did not improve PaO2/FIO2; PaCO2 and airway pressures remained high and their mortality was 88%. This different response to therapeutic optimization appeared using stepwise logistic regression as the most predictive factor for mortality (p<0.05).

CONCLUSIONS

In patients with severe ARDS, the response to an early performed therapeutic optimization used to improve hypoxemia appeared to be a highly discriminant factor distinguishing deceased from surviving patients.

Significant reduction in minute ventilation and peak inspiratory pressures with arteriovenous CO2 removal during severe respiratory failure

OBJECTIVES

To quantify CO2 removal using an extracorporeal low-resistance membrane gas exchanger placed in an arteriovenous shunt and evaluate its effects on the reduction of ventilatory volumes and airway pressures during severe respiratory failure induced by smoke inhalation injury.

DESIGN

Prospective study.

SETTING

Research laboratory.

SUBJECTS

Adult female sheep (n = 5).

INTERVENTIONS

Animals were instrumented with femoral and pulmonary arterial catheters and underwent an LD50 cotton smoke inhalation injury via a tracheostomy under halothane anesthesia. Twenty-four hours after smoke inhalation injury, the animals were reanesthetized and systemically heparinized for cannulation of the left carotid and common jugular vein to construct a simple arteriovenous shunt. A membrane gas exchanger was interposed within the arteriovenous shunt, and blood flow produced by the arteriovenous pressure gradient was unrestricted at the time of complete recovery from anesthesia. CO2 removal by the gas exchanger was measured as the product of the sweep gas flow (FIO2 of 1.0 at 2.5 to 3.0 L/min) and the exhaust CO2 content measured with an inline capnometer. CO2 removed by the animal's lungs was determined by the expired gas CO2 content in a Douglas bag. We made stepwise, 20% reductions in ventilator support hourly. We first reduced the tidal volume to achieve a peak inspiratory pressure of < 30 cm H2O, and then we reduced the respiratory rate while maintaining normocapnia. PaO2 was maintained by adjusting the FIO2 and the level of positive end-expiratory pressure.

MEASUREMENTS AND MAIN RESULTS

Mean blood flow through the arteriovenous shunt ranged from 1154 +/- 82 mL/min (25% cardiac output) to 1277 +/- 38 mL/min (29% cardiac output) over the 6-hr study period. The pressure gradient across the gas exchanger was always < 10 mm Hg. Maximum arteriovenous CO2 removal was 102.0 +/- 9.5 mL/min (96% of total CO2 production), allowing minute ventilation to be reduced from 10.3 +/- 1.4 L/min (baseline) to 0.5 +/- 0.0 L/min at 6 hrs of arteriovenous CO2 removal while maintaining normocapnia. Similarly, peak inspiratory pressure decreased from 40.8 +/- 2.1 to 19.7 +/- 7.5 cm H2O. PaO2 was maintained at > 100 torr (> 13.3 kPa) at maximally reduced ventilator support. Mean arterial pressure and cardiac output did not change significantly as a result of arteriovenous shunting.

CONCLUSIONS

Extracorporeal CO2 removal using a low-resistance gas exchanger in a simple arteriovenous shunt allows significant reduction in minute ventilation and peak inspiratory pressure without hypercapnia or the complex circuitry and monitoring required for conventional extracorporeal membrane oxygenation. Arteriovenous CO2 removal can be applied as an easy and cost-effective treatment to minimize ventilator-induced barotrauma and volutrauma during severe respiratory failure.

Mitigation of injury in canine lung grafts by exogenous surfactant therapy

BACKGROUND

Exogenous surfactant therapy of lung donors improves the preservation of normal canine grafts. The current study was designed to determine whether exogenous surfactant can mitigate the damage in lung grafts induced by mechanical ventilation before procurement.

METHODS AND RESULTS

Five donor dogs were subjected to 8 hours of mechanical ventilation (tidal volume 45 ml/kg). This produced a significant decrease in oxygen tension (p = 0.007) and significant increases in bronchoscopic lavage fluid neutrophil count (p = 0.05), protein concentration (p = 0.002), and the ratio of poorly functioning small surfactant aggregates to superiorly functioning large aggregates (p = 0.02). Five other animals given instilled bovine lipid extract surfactant and undergoing mechanical ventilation in the same manner demonstrated no significant change in oxygen tension values, lavage fluid protein concentration, or the ratio of small to large aggregates. All 10 lung grafts were then stored for 17 hours at 4 degrees C. Left lungs were transplanted and reperfused for 6 hours. After 6 hours of reperfusion the ratio of oxygen tension to inspired oxygen fraction was 307 +/- 63 mm Hg in lung grafts administered surfactant versus 73 +/- 14 mm Hg in untreated grafts (p = 0.007). Furthermore, peak inspired pressure was significantly (p < 0.05) lower in treated animals from 90 to 360 minutes of reperfusion. Analysis of lavage fluid of transplanted grafts after reperfusion revealed small to large aggregate ratios of 0.17 +/- 0.04 and 0.77 +/- 0.17 in treated versus untreated grafts, respectively (p = 0.009).

CONCLUSIONS

Instillation of surfactant before mechanical ventilation reduced protein leak, maintained a low surfactant small to large aggregate ratio, and prevented a decrease of oxygen tension in donor animals. After transplantation, surfactant-treated grafts had superior oxygen tension values and a higher proportion of superiorly functioning surfactant aggregate forms in the air space than untreated grafts. Exogenous surfactant therapy can protect lung grafts from ventilation-induced injury and may offer a promising means to expand the donor pool.

Tracheal gas insufflation during pressure-control ventilation: effect of using a pressure relief valve

OBJECTIVES

Pressure-control ventilation minimizes alveolar overdistention by limiting peak airway pressure, but a consequence of this pressure limitation may be a reduction in tidal volume with subsequent hypercarbia. Tracheal gas insufflation (TGI) can be used in combination with pressure-control ventilation to augment CO2 elimination. During pressure-control ventilation with continuous TGI, we observed that peak airway pressure increased above the set inspiratory pressure. Based on this observation, we investigated the ability of the pressure-control ventilator circuit to compensate for continuous TGI and the effect of insertion of a pressure relief valve to eliminate over-pressurization.

SETTING

University research laboratory.

DESIGN

Using an artificial lung model, we studied the effects of continuous TGI with varying catheter flows (0, 2, 6, and 10 L/ min); ventilator frequencies (10 and 20 breaths/min); inspiratory duty cycles (0.33, 0.50, and 0.67); lung compliance (0.01, 0.02, and 0.04 L/cm H2O); and airway resistance (5, 20, and 50 cm H2O/L/sec) on: a) peak airway pressure; b) total inspiratory tidal volume; c) ventilator-derived tidal volume; and d) intrapulmonary pressure at end-exhalation (auto-PEEP). Tests were performed with and without a pressure relief valve whose threshold "pop-off" pressure was adjusted to match the set inspiratory pressure (35 cm H2O) for a total of 432 experimental conditions.

MEASUREMENTS AND MAIN RESULTS

Our data demonstrate that pressure-control ventilation augmented with continuous TGI can increase peak airway pressure above set inspiratory pressure due to delivery of a higher than intended tidal volume. Predisposing conditions include catheter flow rates of 6 and 10 L/min, long inspiratory time, low compliance, and low resistance. With the pressure relief valve, peak airway pressure was maintained at the set inspiratory pressure and total inspiratory tidal volume remained constant.

CONCLUSION

A pressure relief valve is a necessary adjunct to maintain peak airway pressure at set inspiratory pressure and keep total inspiratory tidal volume constant when continuous TGI is administered in conjunction with pressure-control ventilation.

A comparison of pressure- and volume-controlled ventilation at different inspiratory to expiratory ratios

BACKGROUND

Inverse ratio ventilation (IRV) is frequently used in severe acute respiratory failure. IRV may lead to intrinsic positive end-expiratory pressure (PEEP) and is thought to improve oxygenation and to have advantageous effects on lung mechanics. Published data to support the use of IRV are scarce. This animal study compares external PEEP with intrinsic PEEP in pressure- and volume-controlled ventilation.

METHODS

Fifteen pigs were randomly treated with 1. volume-controlled PEEP ventilation (I:E ratio 1:2) (VCV PEEP), 2. volume-controlled ventilation (I:E ratio 4:1) (VCIRV) and 3. pressure-controlled ventilation (I:E ratio 4:1) (PCIRV). Baseline measurements were performed using volume-controlled ventilation (I:E ratio 1:2) (VCV ZEEP). Lung mechanics, haemodynamics and gas exchange were measured by standard methods and functional residual capacity (FRC) by the sulphur hexafluoride technique.

RESULTS

In comparison to VCV PEEP, PCIRV resulted in reduced peak airway pressure (32 +/- 3 vs. 27 +/- 6 cm H2O, P < 0.001) and increased mean airway pressure (14 +/- 2 vs. 22 +/- 5 cm H2O, P < 0.001). FRC was 942 +/- 264 ml in VCV PEEP and 1024 +/- 390 ml in PCIRV (n.s.). Oxygen delivery was lower in PCIRV (458 +/- 193 vs. 346 +/- 150 ml/min, P < 0.05). Physiologic dead space was 14 +/- 4% in PCIRV and 20 +/- 6% in VCV PEEP and VCIRV (P < 0.005).

CONCLUSIONS

Inverse ratio ventilation did not result in improved FRC in comparison to conventional volume-controlled PEEP ventilation. PCIRV allows for a reduction in minute ventilation but the increase in mean airway pressure compromises circulation.

Pressure-limited ventilation with permissive hypercapnia and minimum PEEP in saline-lavaged rabbits allows progressive improvement in oxygenation, but does not avoid ventilator-induced lung injury

OBJECTIVE

To determine whether pressure-limited intermittent mandatory ventilation with permissive hypercapnia and positive end-expiratory pressure (PEEP) titrated to arterial oxygen tension (PaO2) prevents or reduces acute lung injury, compared to conventional ventilation, in saline-lavaged rabbits.

DESIGN

Prospective randomised trial.

SETTING

University animal laboratory.

SUBJECTS

18 New Zealand White rabbits.

INTERVENTIONS

Following five sequential saline lung lavages, anaesthetised rabbits were randomly allocated in pairs to receive either of two ventilation protocols using intermittent mandatory ventilation. The study group had peak inspiratory pressure limited to 15 cm H2O and arterial partial pressure of carbon dioxide (PaCO2) was allowed to rise. The control group received 12 ml/kg tidal volume with rate adjusted for normocarbia. PEEP and fractional inspired oxygen (FIO2) were adjusted to maintain, PaO2 between 8 and 13.3 kPa (60 and 100 mm Hg) using a predetermined protocol. At 10 h or following death, lung lavage was repeated and lung histology evaluated.

MEASUREMENTS AND MAIN RESULTS

The mean increase in lavage cell counts and protein concentration and hyaline membrane scores were not significantly different between the groups. Oxygenation progressively improved more in the study group (p = 0.01 vs control for PaO2/FIO2 ratio and alveolar-arterial oxygen tension gradient (AaDO2)). PEEP was similar and the mean airway pressure higher in the control group, suggesting that this probably resulted from less ventilator-induced injury in the study group. Four deaths occurred in the control group (three due to pneumothorax and one to hypoxaemia) and none in the study group (p = 0.08).

CONCLUSIONS

This ventilatory protocol may have failed to prevent lung overdistension or it may have provided insufficient PEEP to prevent injury in this model; PEEP greater than the lower inflection point of the pressure-volume curve has been shown to prevent injury almost entirely.