Expiratory phase tracheal gas insufflation and pressure control in sheep with permissive hypercapnia

Carbon dioxide clearance in rabbits during expiratory phase intratracheal pulmonary ventilation

The purpose of this study was to compare the efficacy of CO2 removal during conventional mechanical ventilation (CMV) with and without expiratory phase intratracheal pulmonary ventilation (expiratory ITPV or Exp-ITPV); and to compare CO2 clearance during Exp-ITPV, in pressure-controlled ventilation (PCV) and in volume-controlled ventilation (VCV) modes. Seven anesthetized rabbits were tracheotomized and intubated using a 4 mm endotracheal tube. Venous and arterial lines were established. The rabbits were paralyzed, mechanically ventilated, and ventilation parameters were adjusted to achieve baseline arterial hypercapnia. Animals were then ventilated during 30-minute trials of CMV and Exp-ITPV, in both PCV and VCV modes. A custom-built, microprocessor-controlled solenoid valve was used to limit ITPV gas flow to the expiratory phase. Proximal and carinal airway pressures and hemodynamic variables were continuously recorded, and arterial blood gases were analyzed at the end of each trial. Exp-ITPV, as compared with CMV, reduced arterial PCO2 by 12% and 21% in PCV and VCV modes, respectively (p < 0.02 and p < 0.001; one-sided paired t test), without significant changes in other cardiorespiratory variables. In conclusion, Exp-ITPV is more effective than CMV in clearing CO2 through a small endotracheal tube. Exp-ITPV is also more effective in VCV mode than PCV mode.

Optimal phasic tracheal gas insufflation timing: An experimental and mathematical analysis

OBJECTIVE

To investigate the modulation of CO2 clearance by changes in the duration of tracheal gas flow application during tracheal gas insufflation (TGI).

DESIGN

Combination of bench studies using a commercial test lung and a commercially available intensive care ventilator and mathematical analysis using a clearance model derived from first principles.

SETTING

University pulmonary research laboratory.

PATIENTS

None.

INTERVENTIONS

Experiments using TGI were performed on a test lung at two combinations of tidal volume and frequency. TGI was limited to part of the expiratory phase (the terminal 10-100% of expiration), and two different TGI catheter flow rates were studied. Permutations over a range of compliances, dead-space volumes, catheter flows, and TGI durations were collected. A mathematical model incorporating key ventilatory and TGI-related variables was developed to provide a first-principles theoretical foundation for interpreting the experimental results.

MEASUREMENTS AND MAIN RESULTS

In the physical model, alveolar Pco2 attained a minimum value with TGI flow applied during the terminal 40-60% of the expiratory phase, a finding that was consistent over an almost eight-fold range of expiratory time constants. The mathematical model shows the same qualitative pattern as the experimental model, indicating that the observed behaviors are not an experimental artifact.

CONCLUSION

The optimal duration of expiratory TGI flow application is stable over a wide range of impedance characteristics. Such stability suggests that near maximal effect of expiratory TGI could be obtained by applying TGI flow solely within the final 50% of the expiratory phase. Such uniform restriction of the application profile might both simplify technique implementation and decrease adverse consequences.

Continuous tracheal gas insufflation during partial liquid ventilation in juvenile rabbits with acute lung injury

To examine the hypothesis that combined treatment with tracheal gas insufflation (TGI) and partial liquid ventilation (PLV) may improve pulmonary outcome relative to either treatment alone in acute lung injury (ALI), saline lavage lung injury was induced in 24 anesthetized, ventilated juvenile rabbits that were then randomly assigned to receive ( n = 6/group) 1) conventional mechanical ventilation (CMV) alone, 2) continuous TGI at 0.5 l/min, 3) PLV with perfluorochemical liquid, and 4) combined TGI and PLV (TGI + PLV), and subsequently ventilated with minimized pressures and tidal volume (Vt) to keep arterial Po2 (PaO2) >100 Torr and arterial Pco2 (PaCO2) at 45-60 Torr for 4 h. Gas exchange, lung mechanics, myeloperoxidase, IL-8, and histomorphometry [including expansion index (EI)] were assessed. The CMV group showed no improvement in lung mechanics and gas exchange; all treated groups had significant increases in compliance, PaO2, ventilation efficacy index (VEI), and EI, and decreases in PaCO2, oxygenation index, physiological dead space-to-Vt ratio (Vd/Vt), myeloperoxidase, and IL-8, relative to the CMV group. TGI resulted in lower peak inspiratory pressure, Vt, Vd/Vt, and greater VEI vs. PLV group; PLV resulted in greater compliance, PaO2, and EI vs. TGI. TGI + PLV resulted in decreased peak inspiratory pressure, Vt, Vd/Vt, and increased VEI compared with TGI, improved compliance and EI compared with PLV, and a further increase in PaO2 and oxygenation index and a decrease in PaCO2 vs. either treatment alone. These results indicate that combined treatment of TGI and PLV results in improved pulmonary outcome than either treatment alone in this animal model of ALI.

Tracheal double-lumen ventilation attenuates hypercapnia and respiratory acidosis in lung injured pigs

OBJECTIVE

Evaluation of ventilatory and circulatory effects with coaxial double-lumen tube ventilation for dead-space reduction as compared with standard endotracheal tube ventilation.

DESIGN

Experimental study in a pig model of lung lavage induced acute lung injury.

SETTING

University research laboratory.

MEASUREMENTS AND RESULTS

Tidal volumes of 6, 8 and 10 ml/kg body weight with a set respiratory rate of 20 breaths per minute were used in a random order with both double-lumen ventilation and standard endotracheal tube ventilation. Measurements of ventilatory and circulatory parameters were obtained after steady state at each experimental stage. With a tidal volume of 6 ml/kg, PaCO(2) was reduced from 10.9 kPa (95% CI 9.0-12.9) with a standard endotracheal tube to 8.2 kPa (95% CI 7.0-9.4) with double-lumen ventilation. This corresponds to a reduction in carbon dioxide levels by 25%. At 6 ml/kg, pH increased from 7.17 (95% CI 7.09-7.24) with a standard endotracheal tube to 7.27 (95% CI 7.21-7.32) with double-lumen ventilation. Tracheal pressure was monitored continuously and no difference between single- or double-lumen ventilation was noted at corresponding levels of ventilation. There was no formation of auto-PEEP. Partial tube obstruction due to secretions was not observed during the experiments.

CONCLUSIONS

Coaxial double-lumen tube ventilation is an effective adjunct to reduce technical dead space. It attenuates hypercapnia and respiratory acidosis in a lung injury pig model.

Effect of catheter design on tracheal pressures during tracheal gas insufflation

BACKGROUND AND OBJECTIVE

This study investigated the distribution of pressures within a model trachea, produced by five different tracheal gas insufflation devices. The aim was to suggest a suitable design of a tracheal gas insufflation device for clinical use.

METHODS

Each device was tested using insufflation flow rates of 5 and 10 L min(-1). For each flow rate, the pressure within the tracheal model was measured at 33 fixed points.

RESULTS

The Boussignac tracheal tube produced the most even pressure distribution, while a reverse-flow catheter produced pressure changes of the smallest magnitude.

CONCLUSIONS

We suggest that catheters producing the lowest pressure changes are likely to be safer for clinical use.

Distal projection of insufflated gas during tracheal gas insufflation

Tracheal gas insufflation (TGI) flushes expired gas from the ventilator circuitry and central airways, augmenting CO2 clearance. Whereas a significant portion of this washout effect may occur distal to the injection orifice, the penetration and mixing behavior of TGI gas has not been studied experimentally. We examined the behavior of 100% oxygen TGI injected at set flow rates of 1-20 l/min into a simulated trachea consisting of a smooth-walled, 14-mm-diameter tube. Models incorporating a separate coaxial TGI injector, a rough-walled trachea, and a bifurcated trachea were also studied. One-hundred percent nitrogen, representing expiratory flow, passed in the direction opposite to TGI at set flow rates of 1-25 l/min. Oxygen concentration within the "trachea" was mapped as a function of axial and radial position. Three consistent findings were observed: 1) mixing of expiratory and TGI gases occurred close to the TGI orifice; 2) the oxygenated domain extended several centimeters beyond the endotracheal tube, even at high-expiratory flows, but had a defined distal limit; and 3) more distally from the site of gas injection, the TGI gas tended to propagate along the tracheal wall, rather than as a central projection. We conclude that forward-directed TGI penetrates a substantial distance into the central airways, extending the compartment susceptible to CO2 washout.

[Tracheal gas insufflation avoids hypercapnia in patients with severe head trauma and acute lung injury]

PURPOSE

The purpose of the ventilatory management of acute respiratory distress syndrome (ARDS) is to avoid any barotrauma to the lungs by decreasing the tidal volume at the expense of permissive hypercapnia. This hypercapnia is extremely dangerous for severe head trauma patients because it increases intracranial pressure. The solution could be the use of tracheal gas which insufflation (TGI) allows the reduction of arterial carbon dioxide tension (PaCO(2)) while controlling airway pressures.

CLINICAL FEATURES

We report the cases of two patients with ARDS and severe head trauma. The decrease of tidal volume ( by 60 and 25% respectively) in association with tracheal gas insufflation allowed to reduce plateau airway pressure (<35 cm d'H(2)O) and PaCO(2) (in the first case by 23% and in the second case, by 11% for the second hour then by 24%), while intracranial pressure remained constant or was lowered (in the second case by 39% for the second hour). TGI consisted in insufflating fresh gas via a small catheter placed in the trachea (0(2) at 6 L*min(-1) in the first patient and 4 L*min(-1) in the second case).

CONCLUSION

TGI appears to be an important component of ventilatory management when ARDS is associated with severe head trauma.

Treatment of ARDS

Improved understanding of the pathogenesis of acute lung injury (ALI)/ARDS has led to important advances in the treatment of ALI/ARDS, particularly in the area of ventilator-associated lung injury. Standard supportive care for ALI/ARDS should now include a protective ventilatory strategy with low tidal volume ventilation by the protocol developed by the National Institutes of Health ARDS Network. Further refinements of the protocol for mechanical ventilation will occur as current and future clinical trials are completed. In addition, novel modes of mechanical ventilation are being studied and may augment standard therapy in the future. Although results of anti-inflammatory strategies have been disappointing in clinical trials, further trials are underway to test the efficacy of late corticosteroids and other approaches to modulation of inflammation in ALI/ARDS.

Permissive hypercapnia

The term permissive hypercapnia defines a ventilatory strategy for acute respiratory failure in which the lungs are ventilated with a low inspiratory volume and pressure. The aim of permissive hypercapnia is to minimize lung damage during mechanical ventilation; its limitation is the resulting hypoventilation and carbon dioxide (CO2) retention. In this article we discuss the rationale, physiologic implications, and implementation of permissive hypercapnia. We then review recent clinical studies that tested the effect of various approaches to permissive hypercapnia on the outcome of patients with acute respiratory failure.

Auto-positive end-expiratory pressure during tracheal gas insufflation: testing a hypothetical model

OBJECTIVE

The major benefit of tracheal gas insufflation (TGI) is an increase in CO2 elimination efficiency by removal of CO2 from the anatomical deadspace. In conjunction with mechanical ventilation, TGI may also alter variables that affect CO2 elimination, such as minute ventilation and peak airway pressure (peak Paw) and cause the development of auto-positive end-expiratory pressure (auto-PEEP). We tested the hypothesis that TGI-induced auto-PEEP alters ventilatory variables. We predicted that TGI-induced auto-PEEP offsets the beneficial effects of TGI on CO2 elimination and that keeping total PEEP (ventilator PEEP + auto-PEEP) constant enhances the CO2 elimination efficiency afforded by TGI.

DESIGN

Prospective study of two series of patients with acute respiratory distress syndrome receiving mechanical ventilation.

SETTING

Intensive care units at a university medical center.

PATIENTS

Each series consisted of eight sequential hypercapnic patients.

INTERVENTIONS

In series 1, we examined the effect of continuous TGI at 0 and 10 L/min on PaCO2, without compensating for the development of auto-PEEP. In series 2, we examined this same effect of continuous TGI while reducing ventilator PEEP to keep total PEEP constant. TGI-induced auto-PEEP was calculated based on dynamic compliance measurements during zero TGI flow conditions (deltaV/deltaP) after averaging the two baseline values for peak Paw and tidal volume and assuming compliance did not change between the zero TGI and TGI flow conditions (deltaVTGI/deltaPTGI).

MEASUREMENTS AND MAIN RESULTS

In series 1, total PEEP increased from 13.2 +/- 3.2 cm H2O to 17.8 +/- 3.5 cm H2O without compensation for auto-PEEP (p = .01). PaCO2 decreased (p = .03) from 56.2 +/- 10.6 mm Hg (zero TGI) to 52.9 +/- 9.3 mm Hg (TGI at 10 L/min), a 6% decrement. In series 2, total PEEP was unchanged (p = NS). PaCO2 decreased (p = .03) from 59.5 +/- 10.4 mm Hg (zero TGI) to 52.2 +/- 8.3 mm Hg (TGI at 10 L/min), a 12% decrement. There was no significant change in PaO2; there were no untoward hemodynamic effects in either series.

CONCLUSIONS

These data are consistent with the hypothesis that mechanical ventilation + TGI causes an increase in auto-PEEP that can blunt CO2 elimination. In addition to the ventilator modifications necessary to keep ventilatory variables constant when TGI is used, it is also necessary to reduce ventilator PEEP to keep total PEEP constant and further enhance CO2 elimination efficiency.

Continuous tracheal gas insufflation in preterm infants with hyaline membrane disease. A prospective randomized trial

In mechanically ventilated neonates, the instrumental dead space is a major determinant of total minute ventilation. By flushing this dead space, continuous tracheal gas insufflation (CTGI) may allow reduction of the risk of overinflation. We conducted a randomized trial to evaluate the efficacy of CTGI in reducing airway pressure over the entire period of mechanical ventilation while maintaining oxygenation. A total of 34 preterm newborns, ventilated in conventional pressure-limited mode, were enrolled in two study arms, to receive or not receive CTGI. Transcutaneous Pa(CO(2)) (tcPa(CO(2))) was maintained at 40 to 46 mm Hg in both groups to ensure comparable alveolar ventilation. Respiratory data were collected several times during the first day and daily until Day 28. Both groups were similar at the time of inclusion. During the first 4 d of the study, the difference between peak pressure and positive end-expiratory pressure was significantly lower in the CTGI group by 18% to 35%, with the same tcPa(CO(2)) level and with no difference in the ratio of tcPa(O(2)) to fraction of inspired oxygen (245 +/- 29 versus 261 +/- 46 mm Hg [mean +/- SD] over the first 4 d). Extubation occurred sooner in the CTGI group (p < 0.05), and the duration of mechanical ventilation was shorter (median: 3.6 d; 25th to 75th quartiles: 1.5 to 12.0 d; versus median: 15.6 d; 25th to 75th quartiles: 7.9 to 22.2; p < 0.05) than in the non-CTGI group. CTGI allows the use of low-volume ventilation over a prolonged period and reduces the duration of mechanical ventilation.

Mechanical ventilation in acute lung injury and acute respiratory distress syndrome

Mechanical ventilation provides life-sustaining support for most patients with acute lung injury and acute respiratory distress syndrome; however, traditional approaches to mechanical ventilation may cause ventilator-associated lung injury, which could exacerbate or perpetuate respiratory failure caused initially by conditions such as pneumonia, sepsis, and trauma. This article reviews the theory, laboratory data, and results of recent clinical trials that suggest that modified ventilator strategies can reduce ventilator-associated lung injury and improve clinical outcomes.

Reverse-thrust ventilation in hypercapnic patients with acute respiratory distress syndrome. Acute physiological effects

Techniques of tracheal gas insufflation (TGI) have been shown to enhance CO(2) clearance efficiency in mechanically ventilated patients with acute respiratory distress syndrome (ARDS). Clinical studies have explored the effects of such techniques only at moderate intratracheal gas flow rates, with TGI superimposed to mechanical ventilation in a continuous fashion, or synchronized to the expiratory phase of the duty cycle. We examined the effects of intratracheal pulmonary ventilation (ITPV), delivering the entire tidal volume (VT) in the proximity of the tracheal carina, with all the gas flow supplied continuously through a reverse-thrust catheter (RTC). A potential limitation in the application of TGI is dynamic hyperinflation. Therefore, in a subgroup of patients, we also evaluated the effects of ITPV on end-expiratory lung volume (EELV) by respiratory inductive plethysmography (RIP). Eleven patients with ARDS under volume-cycled mechanical ventilation were subsequently switched to ITPV at the same baseline respiratory rate, I:E ratio, and VT. At the same minute volume, Pa(CO(2)) decreased from 70 +/- 12.3 to 59 +/- 9.5 mm Hg, with a percent reduction of 15 +/- 4% (range from 10 to 20%). The CO(2) decrease was greater in patients with higher baseline Pa(CO(2)) levels (DeltaPa(CO(2)) = 0.29 x Pa(CO(2)) - 9.48, r = 0.95). During transition from mechanical ventilation to ITPV, tracheal positive end-expiratory pressure (PEEP(tr)) decreased with a correspondent decrease in EELV. Both were restored by increasing the PEEP at the ventilator by 3.6 +/- 2.0 cm H(2)O. These data suggest that in patients with ARDS ITPV effectively reduces dead space ventilation and the employment of the RTC may limit or avoid dynamic hyperinflation.

[Tracheal gas insufflation associated with mechanical ventilation for CO2 removal]

OBJECTIVE

Tracheal gas insufflation (TGI) either continuously, or at inspiration, or at expiration, is a technique associated with mechanical ventilation aimed to enhance CO2 elimination in favouring washout of anatomical dead space. This article analyses the mechanism of action, the techniques and the effects of TGI in presence of hypercapnia, especially in the fame of ARDS in adults.

DATA SOURCES

In addition to some historical or major references, the articles on TGI published over the past five years have been searched in the Medline data base.

STUDY SELECTION

Articles with data on TGI associated with mechanical ventilation were selected.

DATA EXTRACTION

Data on mechanisms of action, technical and practical aspects of TGI were extracted.

DATA SYNTHESIS

CO2 elimination is increased when the TGI catheter tip is close to the carina, when the gas jet is directed towards the latter, by a continuous gas jet, by a high washing gas volume. The effect on oxygenation is minor. The work of breathing is decreased. An increased intracranial pressure is decreased. Circulatory effects are minor. The major risk is dynamic pulmonary over distension. Local complications include dessiccation and lesion of bronchial mucosa by the gas jet.

CONCLUSION

In mechanically ventilated patients, additional TGI is a valuable technique for decreasing anatomical dead space. TGI decreases hypercapnia during mechanical ventilation with limited tidal volumes in permissive hypercapnia. Further clinical studies with large series of patients are required to assess the benefits and the effect of TGI on outcome.

Pressure-release tracheal gas insufflation reduces airway pressures in lung-injured sheep maintaining eucapnia

Although tracheal gas insufflation (TGI) has proved to be a useful adjunct to mechanical ventilation, end-inspiratory as well as end-expiratory pressures may increase. We investigated the ability of continuous-flow TGI to maintain eucapnia while reducing airway pressure (Paw) and tidal volume (VT). Seven sheep (36 +/- 2 kg) were ventilated using the Dräger Evita 4 in the pressure control plus mode where flow is released via the expiratory valve to maintain constant inspiratory pressure. To avoid TGI-generated positive end-expiratory pressure (PEEP), a prototype reverse flow TGI tube was used. Two TGI flows (5 and 10 L/min) were investigated pre- and postsaline lavage-induced lung injury. Inspiratory pressures and VT were significantly reduced as TGI flow increased. At 10 L/min TGI flow the carinal pressures (Pcar) and VT were reduced pre- and postinjury by 15% and 20%, and by 28% and 34%, respectively. Tidal volume to dead space ratio (VD/VT) decreased preinjury from 0.49 +/- 0.1 to 0.18 +/- 0.2 and postinjury from 0.62 +/- 0.1 to 0.33 +/- 0.1 at a TGI flow of 10 L/min. The combination of the reverse flow TGI tube and a ventilator with an inspiratory pressure relief mechanism kept set end-inspiratory and end-expiratory pressures constant. This TGI system effectively reduced set Paw and VT while maintaining eucapnia.