In most cases in patients with normal lungs, positive pressure ventilation (PPV) with positive end expiratory pressure (PEEP) reduces cardiac output by decreasing venous return. We hypothesized that in patients receiving PPV, continuous negative pressure (CNP) applied to the chest via a cuirass would counter the negative effects of positive pressure in the chest, yet result in lung expansion similar to PEEP.
20 patients with normal ejection fractions were studied 2 hours post coronary artery bypass graft (CABG) surgery with the pericardium left open. The patients were ventilated on pressure support ventilation (PSV) and synchronized intermittent mandatory ventilation (SIMV) with PEEP. Parameters were studied during 4 modes: A (baseline-1)= SIMV, PSV and PEEP, B=SIMV & PSV without PEEP, C=SIMV & PSV without PEEP and with CNP applied to the thorax at -20 cm H2O for 15 minutes, D (baseline-2)= SIMV, PSV and PEEP. Hemodynamic measurements were performed in the supine position after a 15 minutes period of equilibration between each mode using a standard pulmonary artery catheter and arterial line. No patient was treated with additional fluids or inotropes during the study. All comparisons were done using repeated ANOVA and the results of the 2 baselines were averaged, mean baseline (MB).
All patients were hemodynamically stable during the trial; HR, BP and gas exchange were not affected. The removal of PEEP (5 cm H2O) did not affect CVP, WP or CO significantly MB vs B and B vs C. However, CNP significantly lowered both wedge and CVP (p< .05) compared to before and after CNP MB vs C. The cardiac index (CI) and stroke volume index (SVI) also improved significantly with CNP compared to baselines MB vs C.
The use of CNP applied to the thorax in the postoperative period appears to improve cardiovascular hemodynamics possibly by increased venous return or by improving the contractility of the myocardium.
CNP may be beneficial as a novel treatment modality for postoperative low output syndrome.
Rakesh Chaturvedi, None.