Ventilatory pattern, intrapleural pressure, and cardiac output

Prehospital and emergency department airway management of severe penetrating trauma in Sweden during the past decade

BACKGROUND

Prehospital tracheal intubation (TI) is associated with increased mortality in patients with penetrating trauma, and the utility of prehospital advanced airway management is debated. The increased incidence of deadly violence in Sweden warrants a comprehensive evaluation of current airway management for patients with penetrating trauma in the Swedish prehospital environment and on arrival in the emergency department (ED).

METHODS

This was an observational, multicenter study of all patients with penetrating trauma and injury severity scores (ISSs) ≥ 15 included in the Swedish national trauma register (SweTrau) between 2011 and 2019. We investigated the frequency and characteristics of prehospital and ED TI, including 30-day mortality and patient characteristics associated with TI.

RESULT

Of 816 included patients, 118 (14.5%) were intubated prehospitally, and 248 (30.4%) were intubated in the ED. Patients who were intubated prehospitally had a higher ISS, 33 (interquartile range [IQR] 25, 75), than those intubated in the ED, 25 (IQR 18, 34). Prehospital TI was associated with a higher associated mortality, OR 4.26 (CI 2.57, 7.27, p < 0.001) than TI in the ED, even when adjusted for ISS (OR 2.88 [CI 1.64, 5.14, p < 0.001]). Hemodynamic collapse (≤ 40 mmHg) and low GCS score (≤ 8) were the characteristics most associated with prehospital TI. Traumatic cardiac arrests (TCAs) occurred in 154 (18.9%) patients, of whom 77 (50%) were intubated prehospitally and 56 (36.4%) were intubated in the ED. A subgroup analysis excluding TCA showed that patients with prehospital TI did not have a higher mortality rate than those with ED TI, OR 2.07 (CI 0.93, 4.51, p = 0.068), with OR 1.39 (0.56, 3.26, p = 0.5) when adjusted for ISS.

CONCLUSION

Prehospital TI was associated with a higher mortality rate than those with ED TI, which was specifically related to TCA; intubation did not affect mortality in patients without cardiac arrest. Mortality was high when airway management was needed, regardless of cardiac arrest, thereby emphasizing the challenges posed when anesthesia is needed. Several interventions, including whole blood transfusions, the implementation of second-tier EMS units and measures to shorten scene times, have been initiated in Sweden to counteract these challenges.

Comparison of Simultaneous Transthoracic Versus Transesophageal Echocardiography for Assessment of Aortic Stenosis

Transthoracic echocardiography (TTE) is the gold standard for aortic stenosis (AS) assessment. Transesophageal echocardiography (TEE) provides better resolution, but its effect on AS assessment is unclear. To answer this question, we studied 56 patients with ≥moderate AS. Initial TTE (TTE1) was followed by conscious sedation with simultaneous TEE and TTE2. Based on conservative versus actionable implication, AS types were dichotomized into group A, comprising moderate and normal-flow low-gradient, and group B, comprising high gradient, low ejection fraction low-flow low-gradient, and paradoxical low-flow low-gradient AS. Paired analysis of echocardiographic variables and AS types measured by TEE versus TTE2 and by TEE versus TTE1 was performed. TEE versus simultaneous TTE2 comparison demonstrated higher mean gradients (31.7 ± 10.5 vs 27.4 ± 10.5 mm Hg) and velocities (359 ± 60.6 vs 332 ± 63.1 cm/s) with TEE, but lower left ventricular outflow velocity-time-integral (VTI₁) (18.6 ± 5.1 vs 20.2 ± 6.1 cm), all p <0.001. This resulted in a lower aortic valve area (0.8 ± 0.21 vs 0.87 ± 0.28 cm²), p <0.001, and a net relative risk of 1.86 of group A to B upgrade. TEE versus (awake state) TTE1 comparison revealed a larger decrease in VTI₁ because of a higher initial awake state VTI₁ (22 ± 5.6 cm), resulting in similar Doppler-velocity-index and aortic valve area decrease with TEE, despite a slight increase in mean gradients of 0.8 mm Hg (confidence interval -1.44 to 3.04) and velocities of 10 cm/s (confidence interval -1.5 to 23.4). This translated into a net relative risk of 1.92 of group A to B upgrade versus TTE1. In conclusion, TEE under conscious sedation overestimates AS severity compared with both awake state TTE and simultaneous sedation state TTE, accounted for by different Doppler insonation angles obtained in transapical versus transgastric position.

Effect of Static Respiratory Volume on the Waveform of Cardiac-induced Sternal Vibrations

Cardio-respiratory activity originating in the chest creates vibrations that diffuse through the organs to the thoracic wall. The vibrational waves were detected in all six degrees of freedom by an inertial motion sensor at the xiphoid process of the sternum. Vibrational cardiography (VCG) combines the detection of vibrations via acceleration, termed as seismocardiography, and gyration, termed as gyrocardiography. The objective of this study was to determine the effect of static respiration volume on the morphology of cardiac-induced waveforms in the VCG signal. In this study, 24 subjects were tested while holding breath at peak inhalation, and at peak exhalation. Ensemble averages of the waveforms showed larger variations in the signal when the lungs were inhaled for both the primary and secondary heart sounds. Inter-subject variability was accounted for by averaging all waveforms and calculating the root mean squared value over a sliding window of 60 milliseconds. The peak amplitudes of both heart sounds were consistently larger for high lung volumes. However, the ratio of primary to the secondary heart sound was found to be inversely proportional to lung volume. These opposing effects offer a strong analysis tool for the determination of relative inhalation volume using VCG morphology alone.

Circulation first - the time has come to question the sequencing of care in the ABCs of trauma; an American Association for the Surgery of Trauma multicenter trial

Background

The traditional sequence of trauma care: Airway, Breathing, Circulation (ABC) has been practiced for many years. It became the standard of care despite the lack of scientific evidence. We hypothesized that patients in hypovolemic shock would have comparable outcomes with initiation of bleeding treatment (transfusion) prior to intubation (CAB), compared to those patients treated with the traditional ABC sequence.

Methods

This study was sponsored by the American Association for the Surgery of Trauma multicenter trials committee. We performed a retrospective analysis of all patients that presented to trauma centers with presumptive hypovolemic shock indicated by pre-hospital or emergency department hypotension and need for intubation from January 1, 2014 to July 1, 2016. Data collected included demographics, timing of intubation, vital signs before and after intubation, timing of the blood transfusion initiation related to intubation, and outcomes.

Results

From 440 patients that met inclusion criteria, 245 (55.7%) received intravenous blood product resuscitation first (CAB), and 195 (44.3%) were intubated before any resuscitation was started (ABC). There was no difference in ISS, mechanism, or comorbidities. Those intubated prior to receiving transfusion had a lower GCS than those with transfusion initiation prior to intubation (ABC: 4, CAB:9, p = 0.005). Although mortality was high in both groups, there was no statistically significant difference (CAB 47% and ABC 50%). In multivariate analysis, initial SBP and initial GCS were the only independent predictors of death.

Conclusion

The current study highlights that many trauma centers are already initiating circulation first prior to intubation when treating hypovolemic shock (CAB), even in patients with a low GCS. This practice was not associated with an increased mortality. Further prospective investigation is warranted.

Trial registration

IRB approval number: HM20006627. Retrospective trial not registered.

Pumpless extracorporeal CO(2) removal restores normocapnia and is associated with less regional perfusion in experimental acute lung injury

BACKGROUND

Lung protective ventilation may lead to hypoventilation with subsequent hypercapnic acidosis (HA). If HA cannot be tolerated or occurs despite increasing respiratory rate or buffering, extracorporeal CO2-removal using a percutaneous extracorporeal lung assist (pECLA) is an option. We hypothesised that compensation of HA using pECLA impairs regional perfusion. To test this hypothesis we determined organ blood flows in a lung-injury model with combined hypercapnic and metabolic acidosis.

METHODS

After induction of lung injury using hydrochloric acid (HCl) aspiration and metabolic acidosis by intravenous HCl infusion in nine pigs, an arterial-venous pECLA device was inserted. In randomised order, four treatments were tested: pECLA shunt (1) with and (2) without HA, and clamped pECLA shunt (3) with and (4) without HA. Regional blood flows were measured with the coloured microsphere technique.

RESULTS

HA resulted in higher perfusion in adrenal glands, spleen and parts of splanchnic area (P < 0.05) compared with normocapnia. During CO2-removal with pECLA, regional perfusion decreased to levels comparable with those without pECLA and normocapnia. Cardiac output (CO) increased during HA without a pECLA shunt and was highest during HA with a pECLA shunt compared with normocapnia. During CO2-removal with pECLA, this variable decreased but stayed higher than during normocapnia with clamped pECLA shunt (P < 0.05).

CONCLUSION

In our lung-injury model, HA was associated with increased systemic and regional blood flow in several organs. pECLA provides effective CO2 removal, requiring a higher CO for perfusion of the pECLA device without improvement of regional organ perfusion.

A prospective crossover comparison of neurally adjusted ventilatory assist and pressure-support ventilation in a pediatric and neonatal intensive care unit population

OBJECTIVE

To compare neurally adjusted ventilatory assist ventilation with pressure-support ventilation.

DESIGN

Prospective, crossover comparison study.

SETTING

Tertiary care pediatric and neonatal intensive care unit.

PATIENTS

Sixteen ventilated infants and children: mean age = 9.7 months (range = 2 days-4 yrs) and mean weight = 6.2 kg (range = 2.4-13.7kg).

INTERVENTIONS

A modified nasogastric tube was inserted and correct positioning was confirmed. Patients were ventilated in pressure-support mode with a pneumatic trigger for a 30-min period and then in neurally adjusted ventilatory assist mode for up to 4 hrs.

MEASUREMENTS AND MAIN RESULTS

Data collected for comparison included activating trigger (neural vs. pneumatic), peak and mean airway pressures, expired minute and tidal volumes, heart rate, respiratory rate, pulse oximetry, end-tidal CO2 and arterial blood gases. Synchrony was improved in neurally adjusted ventilatory assist mode with 65% (+/-21%) of breaths triggered neurally vs. 35% pneumatically (p < .001) and 85% (+/-8%) of breaths cycled-off neurally vs. 15% pneumatically (p = .0001). The peak airway pressure in neurally adjusted ventilatory assist mode was significantly lower than in pressure-support mode with a 28% decrease in pressure after 30 mins (p = .003) and 32% decrease after 3 hrs (p < .001). Mean airway pressure was reduced by 11% at 30 mins (p = .13) and 9% at 3 hrs (p = .31) in neurally adjusted ventilatory assist mode although this did not reach statistical significance. Patient hemodynamics and gas exchange remained stable for the study period. No adverse patient events or device effects were noted.

CONCLUSIONS

In a neonatal and pediatric intensive care unit population, ventilation in neurally adjusted ventilatory assist mode was associated with improved patient-ventilator synchrony and lower peak airway pressure when compared with pressure-support ventilation with a pneumatic trigger. Ventilating patients in this new mode seem to be safe and well tolerated.

High-frequency oscillatory ventilation (HFOV) and airway pressure release ventilation (APRV): a practical guide

Despite advances in ventilator management, 31% to 38% of patients with adult respiratory distress syndrome (ARDS) will die, some from progressive respiratory failure. Inability to adequately oxygenate patients with severe ARDS has prompted extensive efforts to identify what are now known as alternative modes of ventilation including high-frequency oscillatory ventilation and airway pressure release ventilation. Both modalities are based on the principles of the open-lung concept and aim to improve oxygenation by keeping the lung uniformly inflated for an extended period of time. Although a mortality benefit has not been proven, some patients may benefit from these alternative modes of ventilation as rescue measures while the underlying process resolves. The purpose of this article is to review the evidence and mechanisms underlying each modality and to describe the fundamental steps in initiating, adjusting, and terminating these modes of ventilation.

Assisted spontaneous breathing during early acute lung injury

In the early phase of their disease process, patients with acute lung injury are often ventilated with strategies that control the tidal volume or airway pressure, while modes employing spontaneous breathing are applied later to wean the patient from the ventilator. Spontaneous breathing modes may integrate intrinsic feedback mechanisms that should help prevent ventilator-induced lung injury, and should improve synchrony between the ventilator and the patient's demand. Airway pressure release ventilation with spontaneous breathing was shown to decrease cyclic collapse/recruitment of dependent, juxtadiaphragmatic lung areas compared with airway pressure release ventilation without spontaneous breathing. Combined with previous data demonstrating improved cardiorespiratory variables, airway pressure release ventilation with spontaneous breathing may turn out to be a less injurious ventilatory strategy.

The impact of spontaneous breathing during mechanical ventilation

PURPOSE OF REVIEW

In patients with acute respiratory distress syndrome, controlled mechanical ventilation is generally used in the initial phase to ensure adequate alveolar ventilation, arterial oxygenation, and to reduce work of breathing without causing further damage to the lungs. Although introduced as weaning techniques, partial ventilator support modes have become standard techniques for primary mechanical ventilator support. This review evaluates the physiological and clinical effects of persisting spontaneous breathing during ventilator support in patients with acute respiratory distress syndrome.

RECENT FINDINGS

The improvements in pulmonary gas exchange, systemic blood flow and oxygen supply to the tissue which have been observed when spontaneous breathing has been maintained during mechanical ventilation are reflected in the clinical improvement in the patient's condition. Computer tomography observations demonstrated that spontaneous breathing improves gas exchange by redistribution of ventilation and end-expiratory gas to dependent, juxtadiaphragmatic lung regions and thereby promotes alveolar recruitment. Thus, spontaneous breathing during ventilator support counters the undesirable cyclic alveolar collapse in dependent lung regions. In addition, spontaneous breathing during ventilator support may prevent increase in sedation beyond a level of comfort to adapt the patient to mechanical ventilation which decreases duration of mechanical ventilator support, length of stay in the intensive care unit, and overall costs of care giving.

SUMMARY

In view of the recently available data, it can be concluded that maintained spontaneous breathing during mechanical ventilation should not be suppressed even in patients with severe pulmonary functional disorders.

The effects of mechanical ventilation on the gut and abdomen

PURPOSE OF REVIEW

Mechanical ventilation generates an increase in airway pressure and, therefore, in intrathoracic pressure, which may decrease systemic and intraabdominal organ perfusion. Critically ill patients rarely die of hypoxia and/or hypercarbia but commonly develop a systemic inflammatory response that culminates in multiple-organ dysfunction syndrome and death. In the pathogeneses of this syndrome the gastrointestinal tract and liver have received considerable attention.

RECENT FINDINGS

Mechanical ventilation with high positive end-expiratory pressure has been found to decrease splanchnic perfusion. Hepatic arterial buffer response is preserved and an increased hepatic arterial blood flow will compensate the decrease in portal blood flow. Despite an increased cardiac output with an acute moderate increase in arterial PCO2 during protective ventilation it cannot be expected that splanchnic and gut perfusion is improved. In the absence of a significant rise in intraabdominal pressure without impairment in cardiovascular function, splanchnic and gastrointestinal function remained unchanged during short periods of prone positioning. Spontaneous breathing during ventilator support improves systemic blood flow and gastrointestinal and splanchnic perfusion.

SUMMARY

In critically ill patients mechanical ventilation should be adjusted to avoid conditions known to be associated with decreased gastrointestinal and splanchnic perfusion.

Pre-hospital endotracheal intubation and positive pressure ventilation is associated with hypotension and decreased survival in hypovolemic trauma patients: an analysis of the National Trauma Data Bank

BACKGROUND

Studies of pre-hospital endotracheal intubation (ETI) from single EMS systems have shown contradictory results, which may represent local differences in paramedic training and experience. An alternative hypothesis is that positive pressure ventilation increases mortality because positive pressure ventilation causes hypotension in severely injured hypovolemic patients.

METHODS

A national sample (National Trauma Data Bank, 1994-2002) was used to minimize effects of local paramedic training and experience. All patients with pre-hospital GCS < 8 (most likely to warrant early ETI) and ISS > 16 (most likely to be hypovolemic) were included. Patients intubated in the field (pre-hospital group, n = 871) and in the emergency department (ED group, n = 6581) were compared. To determine whether pre-hospital ETI was an independent predictor of hypotension and mortality, logistic regression was used to control for potential confounders, including age, ISS, body region injured, AIS scores, pre-hospital IV fluids, and other variables. Physiologic variables were not used, as they may be influenced by ETI and positive pressure ventilation, and were therefore considered outcomes, rather than predictors.

RESULTS

Groups were comparable in age, gender, anatomic distribution of injuries, likelihood of at least one severe injury (AIS >3) and other variables, except for head injury (ED 83%, pre-hospital 71%, p < 0.001) and ISS (ED 33 +/- 0.2, pre-hospital 36 +/- 0.6, p < 0.001). Patients intubated in the field were more likely to be hypotensive upon arrival in the ED (SBP < or = 90 mm Hg; ED 33%, pre-hospital 54%, p < 0.001), and had worse survival (ED 45% versus pre-hospital 24%, p < 0.001). Even after controlling for potential confounders, pre-hospital ETI was still an independent predictor of hypotension upon arrival in ED (OR 1.7, 95% CI 1.46 -2.09, p < 0.001) and decreased survival (OR 0.51, 95% C.I. 0.43-0.62, p < 0.001).

CONCLUSIONS

Pre-hospital endotracheal intubation in trauma patients is associated with hypotension and decreased survival. This may be mediated by the effect of positive pressure ventilation during hypovolemic states.

Assisted breathing is better in acute respiratory failure

PURPOSE OF REVIEW

Mechanical ventilation is usually provided in acute lung injury to ensure alveolar ventilation and reduce the patients' work of breathing without further damaging the lungs by the treatment itself. Although partial ventilatory support modalities were initially developed for weaning from mechanical ventilation, they are increasingly used as primary modes of ventilation, even in patients in the acute phase of pulmonary dysfunction. The aim of this paper is to review the role of spontaneous breathing ventilatory modalities with respect to their physiologic or clinical evidence.

RECENT FINDINGS

By allowing patients with acute lung injury to breathe spontaneously, one can expect improvement in gas exchange and in systemic blood flow, on the basis of both experimental and clinical trials. In addition, by increasing end-expiratory lung volume, as will occur when airway pressure release ventilation is used, recruitment of collapsed or consolidated lung is likely to occur, especially in juxtadiaphragmatic lung regions. Until recently, traditional approaches to mechanical ventilatory support of patients with acute lung injury have called for adaptation of the patient to the mechanical ventilator using heavy sedation and administration of neuromuscular blocking agents. Recent investigations have questioned the utility of sedation, muscle paralysis, and mechanical control of ventilation. Further, evidence exists that lowering sedation levels will decrease the duration of mechanical ventilatory support, the length of stay in the intensive care unit, and the overall costs of hospitalization.

SUMMARY

On the basis of currently available data, the authors suggest the use of techniques of mechanical ventilatory support that maintain, rather than suppress, spontaneous ventilatory effort, especially in patients with severe pulmonary dysfunction.

Detrimental hemodynamic effects of assisted ventilation in hemorrhagic states

OBJECTIVE

Our goal was to demonstrate explicitly that lower-frequency positive-pressure ventilation not only preserves adequate oxygenation and acid-base status in hemorrhagic states, but also that "normal" or higher respiratory rates significantly compromise hemodynamics, even with moderate degrees of hemorrhage.

DESIGN AND SUBJECTS

Eight intubated pigs (ventilated with 12 mL/kg tidal volume, 28% FIO2, respiratory rate = 12 breaths/min) were hemorrhaged to <65 mm Hg of systolic blood pressure. Respiratory rates were then sequentially changed every 10 mins to 6, 20, 30, and 6 breaths/min.

MEASUREMENTS AND MAIN RESULTS

With respiratory rates at 6 breaths/min, all subjects maintained pH of >7.25 and SaO2 of >99% while increasing systolic blood pressure (mean, 65-84 mm Hg; p < .05), time-averaged coronary perfusion pressure (50 +/- 2 to 60 +/- 4 mm Hg; p < .05), and cardiac output (2.4 to 2.8 L/min; p < .05). With respiratory rates of 20 and 30 breaths/min, systolic blood pressure (73 +/- 4 and 66 +/- 5 mm Hg, respectively), coronary perfusion pressure (47 +/- 3 and 42 +/- 4 mm Hg), and cardiac output (2.5 and 2.4 L/min) diminished. When returned to 6 breaths/min, systolic blood pressure (95 mm Hg), coronary perfusion pressure (71 + 6 mm Hg), and cardiac output (3.0 L/min) improved significantly (p < .05 for all comparisons).

CONCLUSIONS

After moderate hemorrhage, animals maintain adequate oxygenation and acid-base status with lower-frequency respiratory rates, whereas increasingly higher respiratory rates progressively and significantly impair hemodynamics. Current ventilatory protocols for trauma resuscitation should be re-examined and considered a possible cause of worsened clinical outcomes and unrecognized confounded study results.

Airway pressure release ventilation as a primary ventilatory mode in acute respiratory distress syndrome

BACKGROUND

Airway pressure release ventilation (APRV) is a ventilatory mode, which allows unsupported spontaneous breathing at any phase of the ventilatory cycle. Airway pressure release ventilation as compared with pressure support (PS), another partial ventilatory mode, has been shown to improve gas exchange and cardiac output. We hypothesized whether the use of APRV with maintained unsupported spontaneous breathing as an initial mode of ventilatory support promotes faster recovery from respiratory failure in patients with acute respiratory distress syndrome (ARDS) than PS combined with synchronized intermittent ventilation (SIMV-group).

METHODS

In a randomized trial 58 patients were randomized to receive either APRV or SIMV after a predefined stabilization period. Both groups shared common physiological targets, and uniform principles of general care were followed.

RESULTS

Inspiratory pressure was significantly lower in the APRV-group (25.9 +/- 0.6 vs. 28.6 +/- 0.7 cmH2O) within the first week of the study (P = 0.007). PEEP-levels and physiological variables (PaO2/FiO2-ratio, PaCO2, pH, minute ventilation, mean arterial pressure, cardiac output) were comparable between the groups. At day 28, the number of ventilator-free days was similar (13.4 +/- 1.7 in the APRV-group and 12.2 +/- 1.5 in the SIMV-group), as was the mortality (17% and 18%, respectively).

CONCLUSION

We conclude that when used as a primary ventilatory mode in patients with ARDS, APRV did not differ from SIMV with PS in clinically relevant outcome.

Emergency ventilatory management in hemorrhagic states: elemental or detrimental?

BACKGROUND

A study was performed to demonstrate that slower respiratory rates (RRs) of positive-pressure ventilation can preserve adequate oxygenation and acid-base status in hemorrhagic states, whereas "normal" or higher RRs worsen hemodynamics.

METHODS

Eight swine (ventilated with 12 mL/kg tidal volume, 0.28 Fio(2); RR of 12 breaths/min) were hemorrhaged to < 65 mm Hg systolic arterial blood pressure (SABP). RRs were then sequentially changed every 10 minutes to 6, 20, 30, and 6 breaths/min.

RESULTS

With RRs at 6 breaths/min, the animals maintained pH > 7.25/Sao(2) > 99%, but increased mean SABP (from 65 to 84 mm Hg; p < 0.05), time-averaged coronary perfusion pressure (CPP) (from 50 +/- 2 to 60 +/- 4 mm Hg; p < 0.05), and cardiac output (Qt) (from 2.4 to 2.8 L/min; p < 0.05). With RRs of 20 and 30 breaths/min, SABP (73 and 66 mm Hg), CPP (47 +/- 3 and 42 +/- 4 mm Hg), and Qt (2.5 and 2.4 L/min) decreased, as did Pao(2) and Paco(2) (< 30 mm Hg), with p < 0.05 for each comparison, respectively. When RR returned to 6 breaths/min, SABP (95 mm Hg), CPP (71 +/- 6 mm Hg), and Qt (3.0 L/min) improved significantly (p < 0.05).

CONCLUSION

After even moderate levels of hemorrhage in animals, positive-pressure ventilation with "normal" or higher RRs can impair hemodynamics. Hemodynamics can be improved with lower RRs while still maintaining adequate oxygenation and ventilation.

Controlled versus assisted mechanical ventilation

On the basis of currently available data, it can be suggested that maintained spontaneous breathing during mechanical ventilation should not be suppressed even in patients with severe pulmonary dysfunction if no contraindications, such as increased intracranial pressure, are present. Improvements in pulmonary gas exchange, systemic blood flow, and oxygen supply to tissues, which have been observed when spontaneous breathing was allowed during ventilatory support, are reflected in the clinical improvement in the patient's condition, as indicated by significantly fewer days with ventilation, earlier extubation, and shorter stays in the intensive care unit. The positive effects of spontaneous breathing have been documented only for some of the available partial ventilatory support modalities. If ventilatory modalities are limited to those whose positive effects have been documented, then partial ventilatory support can be used as a primary modality even in patients with severe pulmonary dysfunction. Whereas controlled mechanical ventilation followed by weaning with partial ventilatory support modalities has been the earlier standard in ventilation therapy, this approach should be reconsidered in view of the available data.

Preserved spontaneous breathing improves cardiac output during partial liquid ventilation

The aim of this study was to examine whether preserved spontaneous breathing (SB) supported by proportional-assist ventilation (PAV) would improve cardiac output (CO) during partial liquid ventilation (PLV) in rabbits with and without lung disease if compared with time-cycled, volume-controlled ventilation (CV) combined with muscle paralysis (MP). PLV was initiated in 17 healthy rabbits and 17 surfactant-depleted rabbits using 12 to 15 ml/kg of perfluorodecaline. Both ventilatory modes, SB+PAV and CV+MP, were applied in random sequence using a crossover design. CO was measured by thermodilution. CO was significantly higher during SB+PAV than during CV+MP: 136 +/- 21 ml/kg x min (mean +/- SD) versus 120 +/- 30 ml/kg x min (p = 0.004) in healthy rabbits, and 147 +/- 19 ml/kg x min versus 111 +/- 13 ml/kg x min (p < 0.0001) in surfactant-depleted rabbits, resulting in an improved oxygen delivery. This difference was mainly caused by a larger stroke volume during SB+PAV, whereas there was little change in heart rate. In surfactant-depleted rabbits, SB+PAV resulted in improved arterial blood pressure and arterial and mixed venous pH and in a higher PaO2 at the same level of PEEP and mean airway pressure. We conclude that during PLV, CO is higher during SB+PAV than during CV+MP, resulting in an improved oxygen delivery. In surfactant-depleted rabbits, improved CO, oxygen delivery, and arterial blood pressure resulted in higher pH, possibly reflecting improved tissue perfusion and oxygenation.

Spontaneous breathing during ventilatory support improves ventilation-perfusion distributions in patients with acute respiratory distress syndrome

Ventilation-perfusion (V A/Q) distributions were evaluated in 24 patients with acute respiratory distress syndrome (ARDS), during airway pressure release ventilation (APRV) with and without spontaneous breathing, or during pressure support ventilation (PSV). Whereas PSV provides mechanical assistance of each inspiration, APRV allows unrestricted spontaneous breathing throughout the mechanical ventilation. Patients were randomly assigned to receive APRV and PSV with equal airway pressure limits (Paw) (n = 12) or minute ventilation (V E) (n = 12). In both groups spontaneous breathing during APRV was associated with increases (p < 0.05) in right ventricular end-diastolic volume, stroke volume, cardiac index (CI), PaO2, oxygen delivery, and mixed venous oxygen tension (PvO2) and with reductions (p < 0.05) in pulmonary vascular resistance and oxygen extraction. PSV did not consistently improve CI and PaO2 when compared with APRV without spontaneous breathing. Improved V A/Q matching during spontaneous breathing with APRV was evidenced by decreases in intrapulmonary shunt (equal Paw: 33 +/- 4 to 24 +/- 4%; equal V E: 32 +/- 4 to 25 +/- 2%) (p < 0.05), dead space (equal Paw: 44 +/- 9 to 38 +/- 6%; equal V E: 44 +/- 9 to 38 +/- 6%) (p < 0.05), and the dispersions of ventilation (equal Paw: 0.96 +/- 0.23 to 0.78 +/- 0.22; equal V E: 0.92 +/- 0.23 to 0.79 +/- 0.22) (p < 0.05), and pulmonary blood flow distribution (equal Paw: 0.89 +/- 0.12 to 0.72 +/- 0.10; equal V E: 0.94 +/- 0.19 to 0.78 +/- 0.22) (p < 0.05). PSV did not improve V A/Q distributions when compared with APRV without spontaneous breathing. These findings indicate that uncoupling of spontaneous and mechanical ventilation during APRV improves V A/Q matching in ARDS presumably by recruiting nonventilated lung units. Apparently, mechanical assistance of each inspiration during PSV is not sufficient to counteract the V A/Q maldistribution caused by alveolar collapse in patients with ARDS.

Pressure support ventilation in adult respiratory distress syndrome: short-term effects of a servocontrolled mode

PURPOSE

To assess the short-term effects of pressure support ventilation in adult respiratory distress syndrome (ARDS), we studied 17 patients with moderate to severe ARDS using mandatory rate ventilation (MRV), a servocontrolled mode of PSV having respiratory rate as the targeted parameter.

MATERIALS AND METHODS

Based on the duration of ARDS, the patients were divided into two groups: Group 1, early ARDS (duration up to 1 week), 10 patients; Group 2, intermediate ARDS (duration between 1 and 2 weeks). The patients were initially ventilated with assisted mechanical ventilation then with MRV, and finally with controlled mechanical ventilation. After a 20-minute period allowed for stabilization in each mode, ventilatory variables, gas exchange, hemodynamics, and patient's inspiratory effort were evaluated.

RESULTS

During MRV blood gases, airway pressures and hemodynamic variables remained within acceptable limits in all patients. Compared with assisted mechanical ventilation, during MRV, patients of group 1 decreased their VT and V (from 0.64 +/- 0.04 to 0.42 +/- 0.03 L/sec) and increased their TI/TT (from 0.39 +/- 0.03 to 0.52 +/- 0.03). f did not change. PAO2 - PaO2 and QS/QT decreased (from 306 +/- 16 to 269 +/- 15 mm Hg, and from 20.2 +/- 1.4 to 17.5 +/- 1.1, respectively), while PaCO2 increased (from 44 +/- 3 to 50 +/- 3 mm Hg). On the contrary, patients of group 2 increased their VT (from 0.69 +/- 0.02 to 0.92 +/- 0.09 L), decreased their f (from 22.3 +/- 0.5 to 19.3 +/- 0.3 b/min), although they did not change their V and TI/TT. PAO2 - PaO2 and QS/QT remained stable. PaCO2 diminished (from 39 +/- 3 to 34 +/- 3 mm Hg). Pressure support level was higher in group 2 than in group 1 (29.4 +/- 3.0 v 19.8 +/- 2.9 cm H2O).

CONCLUSIONS

We conclude that (1) PSV delivered by MRV may adequately ventilate patients with moderate to severe ARDS, preserving gas exchange and hemodynamics, at least for the short period tested; (2) early and intermediate ARDS respond in a different manner to MRV in terms of breathing pattern, gas exchange, and level of pressure assistance; and (3) patients with early ARDS are those who have an improvement in intrapulmonary oxygenation probably due, at least in part, to alveolar recruitment augmented by active diaphragmatic contraction.

Pressure support ventilation attenuates the cardiopulmonary response to an acute increase in oxygen demand

Critically ill patients undergo interventions, such as chest physical therapy, that acutely increase metabolic rate. Previous observations revealed that chest physical therapy is accompanied by increases of 40 to 50% in oxygen consumption (Vo2) and 40% in minute ventilation contributes to the rise in Vo2 and its associated hemodynamic responses. This was done by increasing mandatory ventilatory support during the chest physical therapy session: In phase 1 the mandatory ventilation rate was increased by 35% and in phase 2 pressure support ventilation 15 cm h2O was added. In phase 1 (n = 12), the increase in mandatory rate did not attenuate the chest physical therapy induced rises in heart rate, arterial blood pressure and Vo2. The increase in minute ventilation when the mandatory rate was increased prevented a rise in PaCO2. In phase 2 (n = 15), no change in the increase in Vo2 with chest physical therapy was observed with the addition of pressure support. Yet the rises in heart rate and systemic and pulmonary artery pressures were attenuated, as was the increase in PaCO2. Respiratory rate did not increase as much with pressure support. There appears to be a role for pressure support ventilation in attenuating the pulmonary and hemodynamic responses to interventions that increase oxygen demand.

Physiologic implications of mechanical ventilation on pharmacokinetics

Numerous factors present in the critically ill patient decrease drug clearance. The contribution of one factor, mechanical ventilation, to this decrease is largely unknown and unquantified. This article attempts to review the physiologic effects of mechanical ventilation and to propose theoretical changes in the pharmacokinetics of concomitantly administered drugs. Mechanical ventilation with or without positive end-expiratory pressure is a well-documented cause of decreases in cardiac output, hepatic and renal blood flow, glomerular filtration rate, and urine flow. The mean airway pressure delivered, the pathophysiologic state of the patient, and coexisting therapeutic interventions affect the degree of hemodynamic alteration. Theoretically, these hemodynamic changes can decrease the clearance of several drugs frequently administered to critically ill patients. Decreased hepatic blood flow decreases the clearance of nonrestrictively cleared drugs. The pharmacokinetics of drugs predominantly renally cleared, by either glomerular filtration or tubular secretion, are affected by a decrease in renal blood flow or glomerular filtration rate. Also, the clearance of agents for which tubular reabsorption is important may decrease because the reduction in urine flow resulting from mechanical ventilation allows increased time for drug reabsorption. Interventions that minimize the decrease in cardiac output and organ blood flow and, theoretically, the risk of the adverse drug reactions from decreased drug clearance include expansion of intravascular volume, administering positive inotropic agents, and decreasing mean airway pressure. Monitoring serum concentration of critical and toxic agents suspected to have altered clearance in patients receiving mechanical ventilation is recommended. We hope that our article will stimulate future research in this area to give clinicians guidelines for drug dosing in patients receiving mechanical ventilation.

Assisted ventilation in patients with preexisting cardiopulmonary disease. The effect on systemic oxygen consumption, oxygen transport, and tissue perfusion variables

We have evaluated systemic oxygen consumption (VO2), systemic oxygen transport, and tissue perfusion variables in 30 patients with preexisting cardiac and underlying pulmonary disease during continuous positive-pressure ventilation and positive end-expiratory pressure [PEEP], during intermittent mandatory ventilation (IMV and PEEP), and during spontaneous ventilation (continuous positive airway pressure [CPAP]), with end-expiratory pressure held constant during all ventilatory modes. Using radionuclide angiography together with invasive determinations of pressure and flow, we also measured left and right ventricular ejection fractions and calculated the end-systolic (ESVI) and end-diastolic (EDVI) volume indices of both ventricles. We found that oxygen transport was significantly greater during CPAP (583 +/- 172 ml/min/M2)(mean +/- SD) than during either IMV and PEEP (543 +/- 151 ml/min/sq; p less than 0.01) or CPPV and PEEP (526 +/- 159 ml/min/M2; p less than 0.01); however, we found no significant change in systemic VO2 with conversion from CPPV and PEEP to CPAP. The increase in oxygen transport was related to a greater cardiac index and, more specifically, to a higher heart rate during CPAP (CPAP, 106 +/- 16 beats per minute; CPPV and PEEP, 97 +/- 14 beats per minute) (p less than 0.01). Enhanced oxygen transport during CPAP was also associated with an increase in mixed venous oxygenation and a decrease in arterial lactate. Although neither the mean left ventricular EDVI nor ESVI changed from CPPV and PEEP to CPAP, the mean pulmonary capillary wedge pressure increased (CPPV and PEEP, 12 +/- 5 mm Hg; CPAP, 14 +/- 7 mm Hg) (p less than 0.01), suggesting the possibility of a decrease in left ventricular compliance with the spontaneous ventilatory mode. This study suggests that in the absence of ventilatory failure, spontaneous ventilation provides for better systemic oxygen transport and overall tissue perfusion than either controlled ventilation or IMV; however, this benefit of enhanced oxygen delivery with spontaneous ventilation may potentially be offset by a decrease in left ventricular compliance.

Two simple assemblies for the application of intermittent mandatory ventilation with positive end expiratory pressure

Two cheap disposable systems for delivering intermittent mandatory ventilation with positive end expiratory pressure (IMV-PEEP) with high voluntary inspiratory gas flow are described. These designs can be used in ventilators with either an internal or an external PEEP valve.

High-frequency jet ventilation: theoretical considerations and clinical observations

High-frequency jet ventilation (HFJV) described a technique of mechanical respiratory support based on the delivery of gases under conditions of constant flow and low pressure. Among the benefits ascribed to HFJV are lessened interference with hemodynamic function and reduced danger of barotrauma. The theoretical and technical aspects of HFJV are discussed and the clinical experience with 39 patients in respiratory failure reported. Synchronization of HFMV with heart rate was attempted in three patients. Cardiac output and ejection fraction increased in all of them. At present, results suggest that HFJV may be the ideal form of support for patients with major airway disruption. The available data also indicated that extensive clinical trials are warranted to define advantages and limits of this form of ventilation.

Efficacy of continuous positive airway pressure administered by face mask

The efficacy of administering continuous positive airway pressure (CPAP) by face mask was evaluated in 40 consecutive patients treated with 10 cm of water pressure or greater. Thirty-five patients were treated for progressive hypoxemia with all patients improving their PaO2/FIO2 ratio within the first hour of therapy. Oxygen delivery, when measured, also improved in each patient, although five patients ultimately required endotracheal intubation and higher CPAP levels to further improve their arterial hypoxemia. Five other patients were treated for atelectasis unresponsive to the usual therapeutic measures, with three patients demonstrating roentgenographic improvement. Face mask CPAP proved to be a safe and effective method for treating hypoxemia associated with early progressive respiratory distress in alert, spontaneously breathing patients.

Central haemodynamics and oxygen transport during CPAP with and without mandatory ventilations

Ten patients, subjected 16 h earlier to open-heart surgery (aortocoronary bypass and/or aortic valve replacement), were studied during the weaning period after postoperative mechanical ventilation. Central haemodynamics and oxygen transport were assessed along with total oxygen consumption during continuous positive airway pressure with four mandatory ventilations per minute (mode CPAP + IMV) and, subsequently, during CPAP alone. During the two modes of ventilation, airway pressure was adjusted to be equal during the spontaneous inspiratory phases. All parameters of haemodynamics, oxygenation and oxygen consumption were found to be essentially satisfactory and unchanged during both modes of ventilation. Our observations suggest that, as the parameters studied were unaltered with the change from CPAP + IMV to CPAP, the use of ventilatory support for these patients during the weaning period (in the form of four mandatory ventilations per minute) appears, in terms of central haemodynamics and oxygen transport, to be well tolerated in cases where adequate spontaneous ventilation is in doubt.

Central haemodynamics and oxygen transport with and without continuous positive pressure ventilation after open-heart surgery

Twelve patients, subjected 20 h earlier to coronary artery bypass surgery, were studied on discontinuation of the postoperative mechanical ventilation employing PEEP+5 cmH2O (CMV+PEEP). Compared to the values obtained during CMV+PEEP, cardiac index and mixed venous blood oxygen tension increased with change to spontaneous ventilation at ambient pressure, employing a 28% O2 Ventimask for the intubated patient. There was a slight decrease in both the arterial oxygen content and oxygen tension, but the increase in cardiac output compensated well for the lowering in the arterial oxygen content, and consequently the systemic oxygen transport was statistically unaltered. Oxygen consumption, pulmonary shunt fraction and arteriovenous oxygen content difference also remained unaltered. The observations suggest that after open-heart surgery, CMV using a PEEP as low as +5 cmH2O may exert, in comparison to controlled oxygen therapy ;during spontaneous breathing, a significant lowering effect on the already compromised cardiac performance. This necessitates continuous weighing of the beneficial effects obtained by employing postoperative CMV+PEEP, against the adverse haemodynamic effects, although the alterations in cardiac output may partly ensue from the changes in metabolism, muscular effort and oxygen consumption during the two modes of ventilation, although there was no significant increase in oxygen consumption.

Effects of the level of CPAP on central haemodynamics and oxygen transport

The effects of different levels of continuous positive airway pressure (CPAP) on central haemodynamics and oxygen transport were studied in ten spontaneously breathing male patients who had undergone aortocoronary bypass graft operation 18 h earlier. With increasing CPAP levels ranging from 5 cmH2O(0.49kPa)(CPAP 5) to 15 cmH2O(1.47kPa)(CPAP 15), the cardiac index was found to decrease significantly, while the intraluminal pulmonary capillary wedge and right atrial pressures increased simultaneously. The mean systemic arterial pressure remained unaltered, while the mean pulmonary arterial pressure increased with increasing CPAP. Systemic oxygen transport changed concomitantly with the changes in cardiac output, since arterial blood oxygen content was not altered. The mixed venous blood oxygen tension decreased with increasing CPAP, as did the cardiac output. No changes in the total oxygen consumption or in the arteriovenous oxygen content difference were found at the various CPAP levels. Pulmonary vascular resistance was significantly higher during hypoxia caused by closure of the small airways as a consequence of a reduction in the functional residual capacity during CPAP 0. Thus, low level CPAP might be beneficial in maintaining proper lung volume in an intubated patient after aortocoronary bypass surgery. The observations also suggest that, in these patients, CPAP levels exceeding 10 cmH2O bring about cardiac depression leading to an undesirable reduction in systemic oxygen transport. Mixed venous blood oxygen tension may offer information useful in the adjustment of the level of CPAP.

Hemodynamics and renal function during low frequency positive pressure ventilation with extracorporeal CO2 removal. A comparison with continuous positive pressure ventilation

Six lambs were anesthetized and connected venovenous mode to a Membrane Lung for Extracorporeal CO2 removal. The animals underwent several hours periods of continuous positive pressure ventilation (CPPV), at 5 cmH2O positive end expiratory pressure (PEEP), alternated with several hours periods of low frequency positive pressure ventilation (5 cmH2O PEEP, 2 b.p.m.) with extracorporeal CO2 removal (LFPPV-ECCO2R). During LFPPV-ECCO2R compared with CPPV, cardiac output increased by 26%, pulmonary vascular resistances and systemic vascular resistances decreased by 28% and 22% respectively. The renal function improved significantly during LFPPV-ECCO2R compared with CPPV, i.e. urinary flow, creatinine clearance and osmolar clearance increased by 50%, 37% and 52% respectively. In these experiments LFPPV-ECCO2R, a form of completely artificial ventilation, seems to prevent hemodynamic and renal complications of CPPV.

Measuring exhaled volume with continuous positive airway pressure and intermittent mandatory ventilation. Techniques and rationale

When patients breathe spontaneously through a ventilator circuit, a fall in airway pressure during the inspiratory cycle may increase inspiratory effort. A system of delivery which incorporates a distensible reservoir bag and delivers a constant flow of gas that is two or three times the patient's minute volume will prevent a significant drop in inspiratory airway pressure. Unfortunately, the constant flow of gas mixes with the patient's exhaled gas and makes continuous monitoring of exhaled volumes difficult. Two modifications of circuits are described which allow accurate continuous measurement of volume. One of these circuits allows analysis of the concentrations of expired gases. When spontaneous ventilation occurs, tidal volume and minute ventilation demonstrate an intact connection between the ventilator and the patient, continuously indicate the patient's ability to sustain independent ventilation, and give early warning of a change in respiratory status.

Instantaneous blood flow responses to positive end-expiratory pressure with spontaneous ventilation

Variable hemodynamic responses to positive end-expiratory pressure (PEEP) with spontaneous ventilation have been reported. To clarify these responses, 15 awake patients were studied using a catheter-tip velocity transducer to record phasic aortic root blood flow continuously before, during and after PEEP (10 cm H2O) applied with a face mask. Central blood volume and effective ventricular filling pressures were measured. Phasic pulmonary artery blood flow was also simultaneously recorded in three of these patients. PEEP produced an acute aortic blood flow reduction, detected within one respiratory cycle. Stroke volume decreased 12%, and since heart rate was unchanged, cardiac output also declined (p less than 0.05). Inspiratory-to-expiratory aortic flow changes were less during PEEP. In contrast, inspiratory-to-expiratory pulmonary artery flow alterations were exaggerated due to a marked flow decline during expiration. Central blood volume and effective left ventricular filling pressure decreased 9% and 19%, respectively (p less than 0.05 in all patients). The decrease in pulmonary artery flow was associated with a decrease in central blood volume in the three patients in whom pulmonary flow was measured. PEEP promptly reduces cardiac output during spontaneous ventilation, related to a decrease in pulmonary flow in expiration.