Effect of tidal volume and positive end-expiratory pressure on compliance during mechanical ventilation.

In 12 patients requiring therapy with mechanical ventilation for acute respiratory failure, total static compliance (Cst) increased from 29 +/- 4 ml/cm H2O at a tidal volume (TV) of 5 ml/kg to 42 +/- 7 ml/cm H2O at a TV of 15 ml/kg. Similarly, Cst increased from 42 +/- 7 ml/cm H2O to 52 +/- 8 ml/cm H2O between 0 and 6 cm H2O of positive end-expiratory pressure (PEEP). At high levels of pulmonary inflation (ie, high PEEP and large TV) compliance decreased. The changes of total respiratory compliance with TV were mainly due to changes in pulmonary compliance. With PEEP, the functional residual capacity increased, and specific compliance did not change. Two mechanisms may be responsible for the changes in compliance. First, varying TV or PEEP will alter the position of tidal ventilation on the pressure-volume curve, resulting in an increase in compliance with increasing TV and PEEP up to a point, where overdistention occurs and compliance decreases. Secondly, the function of the surface-lowering substance may be altered in acute pulmonary parenchymal disease, thus disturbing the regulation of surface tension over the range of pulmonary inflation studied.

Age, chronic obstructive pulmonary disease, and Innovar induced ventilatory depression during regional anesthesia.

The effect of Innovar on ventilatory response to CO2 was studied in 35 patients undergoing peripheral surgery with regional anesthesia. The dosage schedule (per 70 kg body weight) was 2 ml intramuscularly, prior to the block, and 1 ml intravenously, after the block. The decrease in mean CO2 response slope (15 percent decrease from control 30 minutes after the first dose) was not statistically significant. Control slope varied inversely with age (r = 0.41, p less than 0.05), and (in 22 patients) directly with the FEV1/FVC ratio (r = 0.54, p less than 0.02) and with the combined variables (FEV1/FVC)/age (r = 0.58, p less than 0.01). Depression of CO2 response slope following Innovar did not vary with age or FEV1. We conclude that, in otherwise normal patients, these doses of innovar cause only minor depression of ventilatory response to CO2. However, in those patients who already have a depressed response (the elderly and those with a decreased FEV1/FVC ratio), this additional depression occasionally may be clinically important.

Optimum end-expiratory airway pressure in patients with acute pulmonary failure.

To determine whether in the management of pulmonary failure, the maximum compliance produced by positive end-expiratory pressure coincides with optimum lung function, 15 normovolemic patients requiring mechanical ventilation for acute pulmonary failure were studied. The end-expiratory pressure resulting in maximum oxygen transport (cardiac output times arterial oxygen content) and the lowest dead-space fraction both resulted in the greatest total static compliance. This end-expiratory pressure varied between 0 and 15 cm of water and correlated inversely with functional residual capacity at zero end-expiratory pressure (r equal -0.72, p less than or equal to 0.005). Mixed venous oxygen tension increased between zero end-expiratory pressure and the end-expiratory pressure resulting in maximum oxygen transport, but then decreased at higher end-expiratory pressures. When measurements of cardiac output or of true mixed venous blood are not available, compliance may be used to indicate the end-expiratory pressure likely to result in optimum cardiopulmonary function.