Continuous positive pressure breathing without and with inspiratory pressure support in acute respiratory failure when mean airway pressure is constant.

OBJECTIVE

Mean airway pressure (Pawm) may be a major factor for PaO2, functional residual capacity, and cardiac output in acute respiratory failure (ARF). To clarify effects of inspiratory pressure support (IPS) as a ventilatory mode in ARF, we studied patients in ARF either using IPS or continuous positive pressure breathing (CPAP) at the same level of Pawm, measuring respiratory and circulatory parameters.

METHODS

After consent, 10 patients in ARF of moderate severity (PaO2:FiO2 205 +/- 108 at positive end expiratory pressure (PEEP) 8.7 +/- 3.1 cmH2O; mean +/- SD) were investigated. Measurements were on day 7.4 +/- 8.4 after onset of ARF. IPS was 13.5 +/- 3.9 cmH2O above PEEP. To result in constant Pawm, PEEP was reduced for IPS (Pawm IPS 11.1 +/- 3.6 vs. Pawm CPAP 9.9 +/- 3.3 cmH2O, ns; PEEP IPS 8.7 +/- 3.1 vs. PEEP CPAP 10.6 +/- 4.3 cmH2O, p = 0.04). Inspired concentration of oxygen (FiO2) and the ventilator (Siemens 900 C) were not changed for the individual patient.

RESULTS

For IPS, tidal volume (VT) increased by +31% and respiratory frequency (RF) decreased by -19% (VT IPS 608 +/- 179 vs. VT CPAP 465 +/- 141 ml, p = 0.01; RF IPS 21.6 +/- 7.6 vs. RF CPAP 26.7 +/- 8.3 breaths per minute, p = 0.02). Also, PaCO2 showed a tendency to be lower for IPS, not reaching significance (PaCO2 IPS 44.3 +/- 5 vs. PaCO2 CPAP 47.4 +/- 4.9 mmHg, p = 0.1). All other parameters were unchanged (expiratory minute volume, PaO2, pH, intravascular pressures, cardiac index, stroke volume index (n = 6), systemic and pulmonary vascular resistances, venous admixture, deadspace (n = 3), oxygen consumption and oxygen delivery). WE CONCLUDE: When Pawm remained constant, IPS added to CPAP improved VT and RF without improving oxygenation or deteriorating circulation in patients with ARF of moderate severity. IPS mainly supports the ability to breathe spontaneously in ARF.