Negative- versus positive-pressure ventilation in intubated patients with acute respiratory distress syndrome

Abstract

Introduction: Recent experimental data suggest that continuous external negative-pressure ventilation (CENPV) results in better oxygenation and less lung injury than continuous positive-pressure ventilation (CPPV). The effects of CENPV on patients with acute respiratory distress syndrome (ARDS) remain unknown.

Methods: We compared 2 h CENPV in a tankrespirator ("iron lung") with 2 h CPPV. The six intubated patients developed ARDS after pulmonary thrombectomy (n = 1), aspiration (n = 3), sepsis (n = 1) or both (n = 1). We used a tidal volume of 6 ml/kg predicted body weight and matched lung volumes at end expiration. Haemodynamics were assessed using the pulse contour cardiac output (PiCCO) system, and pressure measurements were referenced to atmospheric pressure.

Results: CENPV resulted in better oxygenation compared to CPPV (median ratio of arterial oxygen pressure to fraction of inspired oxygen of 345 mmHg (minimum-maximum 183 to 438 mmHg) vs 256 mmHg (minimum-maximum 123 to 419 mmHg) (P < 0.05). Tank pressures were -32.5 cmH2O (minimum-maximum -30 to -43) at end inspiration and -15 cmH2O (minimum-maximum -15 to -19 cmH2O) at end expiration. NO Inspiratory transpulmonary pressures decreased (P = 0.04) and airway pressures were considerably lower at inspiration (-1.5 cmH2O (minimum-maximum -3 to 0 cmH2O) vs 34.5 cmH2O (minimum-maximum 30 to 47 cmH2O), P = 0.03) and expiration (4.5 cmH2O (minimum-maximum 2 to 5) vs 16 cmH2O (minimum-maximum 16 to 23), P =0.03). During CENPV, intraabdominal pressures decreased from 20.5 mmHg (12 to 30 mmHg) to 1 mmHg (minimum-maximum -7 to 5 mmHg) (P = 0.03). Arterial pressures decreased by approximately 10 mmHg and central venous pressures by 18 mmHg. Intrathoracic blood volume indices and cardiac indices increased at the initiation of CENPV by 15% and 20% (P < 0.05), respectively. Heart rate and extravascular lung water indices remained unchanged.

Conclusions: CENPV with a tank respirator improved gas exchange in patients with ARDS at lower transpulmonary, airway and intraabdominal pressures and, at least initially improving haemodynamics. Our observations encourage the consideration of further studies on the physiological effects and the clinical effectiveness of CENPV in patients with ARDS.

Figure 1

The transparent plastic tank respirator during ventilation of the second patient (Table 1). The tank covered the whole patient, including the head. This setting improves the practicability of continuous external negative-pressure ventilation in an intubated patient in whom flow is delivered from the conventional mechanical ventilator through the endotracheal tube. Apertures in the bottom, below the wooden frame, were used to lead out all connections to the patient, and trimmed-to-fit sponge rubbers were used to seal these apertures. (The shoes were put on this patient to prevent contractions.).

Figure 2

The course of arterial oxygen-to-fraction of inspired oxygen pressure ratio (PaO₂/FiO₂), arterial carbon dioxide partial pressure (PaCO₂) and pH immediately before lung recruitment and during continuous positive-pressure ventilation (CPPV) and continuous external negative-pressure ventilation (CENPV). Measurements were taken at time 0 (5 minutes after the recruitment manoeuvre) immediately after starting the 2-hour ventilatory period of CPPV or CENPV. *P < 0.05 compared to corresponding values at 1 or 2 hours during CPPV.

Figure 3

Original polygraph recordings during a change from continuous positive-pressure ventilation (CPPV) to continuous negative-pressure ventilation (CENPV) in patient 6. The pressure-time profiles of endotracheal pressure (AWP) during CPPV and tank pressure were similar during inspiration and expiration. During CENPV, endotracheal airway pressure increased during inspiration and decreased after a short initial peak. This patient had high intraabdominal pressure despite an open abdomen that decreased impressively during CENPV. (To convert pressure values from millimetres of mercury to centimetres of water, multiply by 1.33.) ECG, electrocardiogram; AWP, airway pressure (measured in the trachea); CVP, central venous pressure; AP, arterial pressure; IAP, intraabdominal pressure, exp. CO₂, expired carbon dioxide.

Figure 4

Haemodynamics during continuous positive-pressure ventilation (CPPV) and continuous external negative-pressure ventilation (CENPV). Measurements were taken at time 0 (5 minutes after the recruitment manoeuvre) immediately after starting the 2-hour ventilatory period of CPPV or CENPV. The changes in intravascular pressure effects were more permanent in contrast to the more transient effects on intrathoracic blood volume and cardiac index. *P < 0.05 compared to corresponding values at 1 or 2 hours during CPPV.