High-frequency oscillatory ventilation for adult acute respiratory distress syndrome: a decade of progress

Oscillatory ventilation redux: alternative perspectives on ventilator-induced lung injury in the acute respiratory distress syndrome

For patients with the acute respiratory distress syndrome (ARDS), ventilation strategies that limit end-expiratory derecruitment and end-inspiratory overdistension are the only interventions to have significantly reduced the morbidity and mortality. For this reason, the use of high-frequency oscillatory ventilation (HFOV) was considered to be an ideal protective strategy, given its reliance on very low tidal volumes cycled at very high rates. However, results from clinical trials in adults with ARDS have demonstrated that HFOV does not improve clinical outcomes. Recent experimental and computational studies have shown that oscillation of a mechanically heterogeneous lung with multiple simultaneous frequencies can reduce parenchymal strain, improve gas exchange, and maintain lung recruitment at lower distending pressures compared to traditional ‘single-frequency’ HFOV. This review will discuss the theoretical rationale for the use of multiple oscillatory frequencies in ARDS, as well as the mechanisms by which it may reduce the risk for ventilator-induced lung injury.

In vitro performance comparison of the Sensormedics 3100A and B high-frequency oscillatory ventilators

OBJECTIVE

The Sensormedics 3100A and 3100B are widely used to provide high-frequency oscillatory ventilation in clinical practice. Infants and children <35 kg are typically oscillated with the 3100A and >35 kg with the 3100B. This study compares the effect of ventilator and patient parameters on delivered tidal volume during high-frequency oscillatory ventilation of a test lung with these devices.

DESIGN

Laboratory-based study.

SUBJECTS

Test lung and Sensormedics 3100A and 3100B high-frequency oscillators.

INTERVENTIONS

A previously validated hot-wire flowmeter (Florian) was placed in series with either a 3100A (n = 3) or 3100B (n = 3) ventilator and a Michigan test lung. Tidal volumes were measured over a range of mean airway pressure, inspiratory:expiratory ratio, frequency, pressure amplitude, and endotracheal tube internal diameter.

MEASUREMENTS AND MAIN RESULTS

The 3100A and 3100B delivered similar tidal volumes across a range of ventilator parameters for an inspiratory:expiratory ratio of 1:1, differing by <10%. However, at an inspiratory:expiratory ratio of 1:2, there was a statistically significant decrease in tidal volume for the 3100B compared with the 3100A at lower frequencies, which was partially mitigated by increasing pressure amplitude. The difference in the generated pressure and flow waveforms may account for the observed tidal volume differences between the high-frequency oscillatory ventilation models. Delivered tidal volume was highly dependent on endotracheal tube size.

CONCLUSIONS

Multiple variables contribute to the delivered tidal volume during high-frequency oscillatory ventilation, including ventilator model selection and endotracheal tube size. It is possible that real-time, clinical monitoring of delivered tidal volume during high-frequency oscillatory ventilation would allow better titration and maximize performance of these ventilators in caring for critically ill patients.

Demand flow facilitates spontaneous breathing during high-frequency oscillatory ventilation in a pig model

OBJECTIVE

Maintenance breathing is advocated in mechanical ventilation, which is difficult for the high-frequency oscillatory (HFO) ventilation. To facilitate spontaneous breathing during HFO ventilation, a demand flow system (DFS) was designed. The aim of the present study was to evaluate the system.

DESIGN

Animal experiment.

SETTING

: University animal laboratory.

SUBJECTS

Eight pigs (47-64 kg).

INTERVENTIONS

Lung injury was induced by lung lavage with normal saline. After spontaneous breathing was restored HFO ventilation was applied, in runs of 30 minutes, with continuous fresh gas flow (CF) or the DFS operated in two different setups. Pressure to regulate the DFS was sampled directly at the Y-piece of the ventilator circuit (DFS) or between the endotracheal tube and measurement equipment at the proximal end of the endotracheal tube. In the end, animals were paralyzed. Breathing pattern, work of breathing, and gas exchange were evaluated.

MEASUREMENTS AND MAIN RESULTS

HFO ventilation with demand flow decreased breathing frequency and increased tidal volume compared with CF. Comparing HFO modes CF, DFS, and DFSPROX, total pressure-time product (PTP) was 66 cm H2O x sec x min (interquartile range 59-74), 64 cm H2O x sec x min (50-72), and 51 cm H2O x sec x min (41-63). Ventilator PTP was 36 cm H2O x sec x min (32-42), 8.6 cm H2O x sec x min (7.4-10), and 1 cm H2O x sec x min (-1.0 to 2.8). Oxygenation, evaluated by Pao2, was preserved when spontaneous breathing was maintained and deteriorated when pigs were paralyzed. Ventilation, evaluated by Paco2, improved with demand flow. Paco2 increased when using continuous flow and during muscular paralysis.

CONCLUSIONS

In moderately lung-injured anesthetized pigs during HFO ventilation, demand flow facilitated spontaneous breathing and augmented gas exchange. Demand flow decreased total breathing effort as quantified by PTP. Imposed work caused by the HFO ventilator appeared totally reduced by demand flow.

High-frequency oscillatory ventilation in adults: clinical considerations and management priorities

Recently, there has been renewed interest in high-frequency oscillatory ventilation (HFOV) as a lung-protective strategy in adults. It limits overdistension and prevents cyclic collapse by maintaining end-expiratory lung volume. Studies have shown that HFOV is safely tolerated in the adult population and may offer more benefit if applied early in the course of disease. These findings have implications for clinicians as the use of HFOV may increase in the coming decade. Gas transport mechanisms, ventilator settings, patient monitoring, and clinical considerations for HFOV are substantially different from conventional mechanical ventilation. This article reviews management strategies and monitoring priorities currently recommended for management of adults receiving HFOV.

Tidal volume delivery during high-frequency oscillatory ventilation in adults with acute respiratory distress syndrome

OBJECTIVE

a) Characterize how ventilator and patient variables affect tidal volume during high-frequency oscillatory ventilation; and b) measure tidal volumes in adults with acute respiratory distress syndrome during high-frequency oscillatory ventilation.

DESIGN

Observational study.

SETTING

Research laboratory and medical intensive care unit.

PATIENTS

Test lung and patients with acute respiratory distress syndrome.

INTERVENTIONS

Using a previously validated hot wire anemometer placed in series with a Sensormedics 3100B high-frequency ventilator, an endotracheal tube, and a test lung, tidal volume was measured at different combinations of frequency (4, 6, 8, 10, and 12 Hz), pressure amplitude (50, 60, 70, 80, and 90 cm H2O), mean airway pressure (20, 30, and 40 cm H2O), test lung compliance (10, 30, and 50 mL/cm H2O), endotracheal tube internal diameter (6, 7, and 8 mm), bias flow (20, 30, and 40 L/min), and inspiratory/expiratory ratio (1:2 and 1:1). In patients, tidal volume was measured at baseline ventilator settings and at baseline frequency +/-2 Hz and baseline pressure amplitude +/-10 cm H2O.

MEASUREMENTS AND MAIN RESULTS

Measured tidal volumes were 23-225 mL during high-frequency oscillatory ventilation of the test lung. A 2-Hz increase in frequency and a 10-cm H2O increase in pressure amplitude caused a 21.3% +/- 4.1% decrease and 21.4% +/- 3.4% increase in tidal volume, respectively. Decreasing endotracheal tube internal diameter from 8 mm to 7 mm and from 7 mm to 6 mm caused a 15.3% +/- 1.7% and 18.9% +/- 2.1% reduction in tidal volume, respectively. Increasing bias flow from 20 L/min to 30 L/min increased tidal volume by 11.2% +/- 3.9%. Further increases in bias flow, changes in compliance, and changes in mean airway pressure had little effect. Tidal volumes measured in acute respiratory distress syndrome patients were 44-210 mL. A 2-Hz increase in frequency was associated with a 23.1% +/- 6.3% decrease in tidal volume. In contrast to the test lung data, a 10-cm H2O increase in pressure amplitude resulted in only a 5.6% +/- 4.5% increase in tidal volume.

CONCLUSIONS

Tidal volumes are not uniformly small during high-frequency oscillatory ventilation. The primary determinant of tidal volume in adults with acute respiratory distress syndrome during high-frequency oscillatory ventilation with the Sensormedics 3100B is frequency. Test lung findings suggest that endotracheal tube internal diameter is also an important determinant of tidal volume.

Systematic review of determinants of mortality in high frequency oscillatory ventilation in acute respiratory distress syndrome

INTRODUCTION

Mechanical ventilation has been shown to cause lung injury and to have a significant impact on mortality in acute respiratory distress syndrome. Theoretically, high frequency oscillatory ventilation seems an ideal lung protective ventilation mode. This review evaluates determinants of mortality during use of high frequency oscillatory ventilation.

METHODS

PubMed was searched for literature reporting randomized trials and cohort studies of high frequency ventilation in adult patients with acute respiratory distress syndrome. Data on mortality and determinants were extracted for patients treated with high frequency oscillatory ventilation. Linear regression analyses were conducted to produce graphical representations of adjusted effects of determinants of mortality.

RESULTS

Cohorts of patients treated with high frequency oscillatory ventilation from two randomized trials and seven observational studies were included. Data from cohorts comparing survivors with non-survivors showed differences in age (42.3 versus 51.2 years), prior time on conventional mechanical ventilation (4.0 versus 6.2 days), APACHE II score (22.4 versus 26.1), pH (7.33 versus 7.26) and oxygenation index (26 versus 34). Each extra day on conventional ventilation was associated with a 20% higher mortality adjusted for age and APACHE II score (relative risk (RR) 1.20, 95% confidence interval (CI) 1.15-1.25). However, this association was confounded by differences in pH (pH adjusted RR 1.03, 95% CI 0.73-1.46). Oxygenation index seemed to have an independent effect on mortality (RR 1.10, 95% CI 0.95-1.28).

CONCLUSION

Prolonged ventilation on conventional mechanical ventilation prior to high frequency oscillatory ventilation was not related to mortality. Oxygenation index was a determinant of mortality independent of other disease severity markers.

Burn wound infections

Burns are one of the most common and devastating forms of trauma. Patients with serious thermal injury require immediate specialized care in order to minimize morbidity and mortality. Significant thermal injuries induce a state of immunosuppression that predisposes burn patients to infectious complications. A current summary of the classifications of burn wound infections, including their diagnosis, treatment, and prevention, is given. Early excision of the eschar has substantially decreased the incidence of invasive burn wound infection and secondary sepsis, but most deaths in severely burn-injured patients are still due to burn wound sepsis or complications due to inhalation injury. Burn patients are also at risk for developing sepsis secondary to pneumonia, catheter-related infections, and suppurative thrombophlebitis. The introduction of silver-impregnated devices (e.g., central lines and Foley urinary catheters) may reduce the incidence of nosocomial infections due to prolonged placement of these devices. Improved outcomes for severely burned patients have been attributed to medical advances in fluid resuscitation, nutritional support, pulmonary and burn wound care, and infection control practices.

Burn wound infections

Burns are one of the most common and devastating forms of trauma. Patients with serious thermal injury require immediate specialized care in order to minimize morbidity and mortality. Significant thermal injuries induce a state of immunosuppression that predisposes burn patients to infectious complications. A current summary of the classifications of burn wound infections, including their diagnosis, treatment, and prevention, is given. Early excision of the eschar has substantially decreased the incidence of invasive burn wound infection and secondary sepsis, but most deaths in severely burn-injured patients are still due to burn wound sepsis or complications due to inhalation injury. Burn patients are also at risk for developing sepsis secondary to pneumonia, catheter-related infections, and suppurative thrombophlebitis. The introduction of silver-impregnated devices (e.g., central lines and Foley urinary catheters) may reduce the incidence of nosocomial infections due to prolonged placement of these devices. Improved outcomes for severely burned patients have been attributed to medical advances in fluid resuscitation, nutritional support, pulmonary and burn wound care, and infection control practices.

Acute respiratory distress syndrome in pregnancy

OBJECTIVE

To summarize the pathophysiology and treatment of acute lung injury and acute respiratory distress syndrome (ARDS) during pregnancy.

DATA SOURCE

Review of select articles from MEDLINE, including published abstracts, case reports, observational studies, controlled trials, review articles, and institutional experience.

DATA SUMMARY

ARDS occurs in pregnancy and may have unique causes. Despite extensive clinical research to improve the management of ARDS, mortality remains high, and few strategies have shown a mortality benefit. Furthermore, in most published studies, pregnancy is an exclusionary criterion, and thus, few treatments have been adequately evaluated in obstetric populations. The treatment of ARDS in pregnancy is extrapolated from studies performed in the general ARDS patient population, with consideration given to the normal physiologic changes of pregnancy. In general, the best support of the fetus is support of the mother. From the age of viability (24-26 wks at most institutions) until full term, decisions regarding delivery should be made based primarily on the standard obstetric indications.

CONCLUSIONS

Little evidence exists regarding the management of ARDS specifically in pregnancy, and thus, treatment approaches must be drawn from studies performed in a general patient population. A multidisciplinary approach involving maternal-fetal medicine, neonatology, anesthesiology, and intensivist clinicians is essential to optimizing maternal and fetal outcomes.