Alveolar recruitment improves arterial oxygenation in responders to prone position

Introduction

Although prone position is known as a simple method to improve arterial oxygenation in patients with acute respiratory distress syndrome (ARDS), the underlying physiological mechanisms remain poorly understood. This study was performed to show the effect of prone position on alveolar recruitment.

Methods

After approval by the local ethics committee of the medical faculty 12 patients with ARDS diagnosed according to the criteria of the American–European Consensus Conference were included. Patients were ventilated in volume controlled mode and the ventilatory settings were kept unchanged throughout the whole period of measurements. Patients were kept in prone position for 8 hours. Arterial partial pressure of oxygen (PaO₂), airway pressure, gas flow and functional residual capacity (FRC) were measured (AMIS 2001 Intensive Care Monitoring System; INNOVISION, Odense Denmark) and intrapulmonary shunt (Qs/Qt) was calculated from arterial and mixed venous blood gas analyses. Measurements were performed in supine position (Ts0), immediately after turning to prone position (Tp0), after 1, 2, 4 and 8 hours in prone position (Tp1, Tp2, Tp4, Tp8) and immediately after turning back to supine (Ts1). Patients were defined as responders to prone position if the oxygenation quotient (PaO₂/FiO₂) increased more than 30%. Individual pressure–volume curves (pv-curves) of the respiratory system were constructed by means of FRC measurements and dynamic compliances which were calculated from gas flow and airway pressure measurements. Then alveolar recruitment during prone position was identified as volume increase between pv-curves at a predefined airway pressure of 20 cmH₂O.

Results

Seven of 12 patients showed a sustained increase of oxygenation quotient greater than 30% after prone therapy and were defined as responders (+100% vs +10% in nonresponders). There was no statistical difference in biometric data and severity of ARDS between the two groups. Responders showed a continuous increase of recruited lung volume during prone position. Total alveolar recruitment was significantly greater in responders than in non-responders (+800 ± 200 ml vs -40 ± 180 ml; P < 0.0001). Time course of the alveolar recruitment and time of maximal recruitment differs in all patients. A good correlation was found between total recruited volume and decrease of intrapulmonary shunt (R² = 0.72).

Conclusion

The present results show that alveolar recruitment increases in responders to prone therapy. An individual time course of alveolar recruitment was found, indicating that the duration of prone position has to be selected according to the specific requirements of each patient. The good correlation between increased lung volume and decrease of intrapulmonary shunt indicates that the recruited lung spaces are capable of participating in gas exchange and are not caused by overdistension or dead space increase.