Positive end-expiratory pressure may alter breathing cardiovascular variability and baroreflex gain in mechanically ventilated patients

The place of positive end expiratory pressure in ventilator-induced lung injury generation

PURPOSE OF REVIEW

Describe the rationale for concern and accumulating pathophysiologic evidence regarding the adverse effects of high-level positive end expiratory pressure (PEEP) on excessive mechanical stress and ventilator-induced lung injury (VILI).

RECENT FINDINGS

Although the inclusion of PEEP in numerical estimates of mechanical power may be theoretically debated, its potential to increase stress, strain, and mean airway pressure are not. Recent laboratory data in a variety of animal models demonstrate that higher levels of PEEP coupled with additional fluids needed to offset its impediment of hemodynamic function are associated with increased VILI. Moreover, counteracting end-tidal hyperinflation by external chest wall pressure may paradoxically improve respiratory mechanics, indicating that lower PEEP helps protect the small 'baby lung' of advanced acute respiratory distress syndrome (ARDS).

SUMMARY

The potentially adverse effects of PEEP on VILI can be considered in three broad categories. First, the contribution of PEEP to total mechanical energy expressed through mechanical power, raised mean airway pressure, and end-tidal hyperinflation; second, the hemodynamic consequences of altered cardiac loading, heightened pulmonary vascular stress and total lung water; and third, the ventilatory consequences of compromised carbon dioxide eliminating efficiency. Minimizing ventilation demands, optimized body positioning and care to avoid unnecessary PEEP are central to lung protection in all stages of ARDS.

Cardiac arrhythmias associated with volume-assured pressure support mode in a patient with autonomic dysfunction and mitochondrial disease

NONE

A 15-year-old boy with autonomic dysfunction and mitochondrial disease was diagnosed with sleep-related hypoventilation at 6 years of age and treated with bilevel positive airway pressure therapy. At 12 years of age, treatment was transitioned to volume-assured pressure support (VAPS) due to clinical evidence of respiratory muscle weakness. Subsequent titration polysomnogram revealed the emergence of cardiac arrhythmia (isolated premature ventricular contractions, bigeminy, and trigeminy) while on VAPS mode that improved after transition to bilevel positive airway pressure therapy. During the titration study, higher tidal volumes correlated with increased pressures and the presence of arrhythmia. Prior to initiation of VAPS therapy, the patient had normal electrocardiogram evaluations. This case highlights the potential relationship between VAPS therapy and cardiac arrhythmias, especially in patients with underlying conditions with associated cardiac abnormalities, such as autonomic dysfunction and mitochondrial disease. While using VAPS mode, patients should be closely monitored for cardiac rhythm abnormalities.

High-frequency power of heart rate variability can predict the outcome of thoracic surgical patients with acute respiratory distress syndrome on admission to the intensive care unit: a prospective, single-centric, case-controlled study

BACKGROUND

The morbidity and mortality of acute respiratory distress syndrome (ARDS) remains high, and the strategic focus of ARDS research has shifted toward identifying patients at high risk of mortality early in the course of illness. This study intended to identify the heart rate variability (HRV) measure that can predict the outcome of patients with ARDS on admission to the surgical intensive care unit (SICU).

METHODS

Patients who had lung or esophageal cancer surgery were included either in the ARDS group (n = 21) if they developed ARDS after surgery or in the control group (n = 11) if they did not. The ARDS patients were further stratified into survivors and non-survivors subgroups according to their outcomes. HRV measures of the patients were used for statistical analysis.

RESULTS

The mean RR interval (mRRI), high-frequency power (HFP) and product of low-/high-frequency power ratio tidal volume and tidal volume (LHR*VT) were significantly lower (p < 0.05), while the normalized HFP to VT ratio (nHFP/VT) was significantly higher in the ARDS patients (p = 0.011). The total power (TP), low-frequency power (LFP), HFP and HFP/VT were all significantly higher in the non-survived ARDS patients, whereas Richmond Agitation-Sedation Scale (RASS) was significantly lower in the non-survived ARDS patients. After adjustment for RASS, age and gender, firth logistic regression analysis identified the HFP, TP as the significant independent predictors of mortality for ARDS patients.

CONCLUSIONS

The vagal modulation of thoracic surgical patients with ARDS was enhanced as compared to that of non-ARDS patients, and the non-survived ARDS patients had higher vagal activity than those of survived ARDS patients. The vagal modulation-related parameters such as TP and HFP were independent predictors of mortality in patients with ARDS on admission to the SICU, and the HFP was found to be the best predictor of mortality for those ARDS patients. Increased vagal modulation might be an indicator for poor prognosis in critically ill patients following thoracic surgery.

Effort-adapted modes of assisted breathing

PURPOSE OF REVIEW

New developments in mechanical ventilation have focused on increasing the patient's control of the ventilator by implementing information on lung mechanics and respiratory drive. Effort-adapted modes of assisted breathing are presented and their potential advantages are discussed.

RECENT FINDINGS

Adaptive support ventilation, proportional assist ventilation with load adjustable gain factors and neurally adjusted ventilatory assist are ventilatory modes that follow the concept of adapting the assist to a defined target, instantaneous changes in respiratory drive or lung mechanics. Improved patient ventilator interaction, sufficient unloading of the respiratory muscles and increased comfort have been recently associated with these ventilator modalities. There are, however, scarce data with regard to outcome improvement, such as length of mechanical ventilation, ICU stay or mortality (commonly accepted targets to demonstrate clinical superiority).

SUMMARY

Within recent years, a major step forward in the evolution of assisted (effort-adapted) modes of mechanical ventilation was accomplished. There is growing evidence that supports the physiological concept of closed-loop effort-adapted assisted modes of mechanical ventilation. However, at present, the translation into a clear outcome benefit remains to be proven. In order to fill the knowledge gap that impedes the broader application, larger randomized controlled trials are urgently needed. However, with clearly proven drawbacks of conventional assisted modes such as pressure support ventilation, it is probably about time to leave these modes introduced decades ago behind.