Right Heart Hemodynamics During Weaning from Mechanical Ventilation

Abstract

It is estimated that 9-37% of all mechanically ventilated patients experience difficulty during weaning from mechanical ventilation (MV). Weaning may be problematic for individuals with co-existing cardiac dysfunction, since hemodynamic instability is one proposed cause of weaning failure. Hemodynamic instability during weaning may develop with transition from positive intrathoracic pressure (ITP) during inspiration with MV to the negative ITP with spontaneous ventilation. This ITP shift may induce changes in right ventricular preload, afterload and contractility.

The purpose of this study was to describe changes in right heart hemodynamics associated with weaning. Twenty instrumented, anesthetized, canines were randomly assigned to Group 1, normal ventricular function (n = 10), or Group 2, propranolol induced biventricular failure (n = 10). Each subject was randomly exposed to 4 weaning permutations: spontaneous ventilation (T-piece), spontaneous ventilation with a resistor (Resistor), pressure support 5 cm. H$\sb2$O pressure (PS) and continuous positive airway pressure 5 cm. H$\sb2$O pressure (CPAP). Dependent variables were measured on baseline MV and following a 5 minute transition period to each permutation. Data were analyzed with repeated measures ANOVA.

Both groups significantly increased cardiac output with T-piece, Resistor and CPAP. Neither group demonstrated significant changes in heart rate. Stroke volume increased in both groups with T-piece, Resistor and CPAP. Right ventricular end-diastolic volume (preload) increased in both groups with T-piece, Resistor and CPAP. Pulmonary vascular resistance (afterload) was reduced in both groups with T-piece and Resistor, but unchanged with PS and CPAP. Right ventricular stroke work (contractility) increased with T-piece, Resistor and CPAP, but was unchanged with PS.

These data indicate right heart hemodynamics were influenced by weaning from MV using T-piece, Resistor and CPAP. Similar changes in humans, particularly those with preexisting cardiac dysfunction, may induce cardiovascular instability and reduce oxygen delivery. Weaning by PS produced minimal hemodynamic change and may be more suitable for weaning individuals with cardiac dysfunction.