Relevance of Pulmonary Arterial Capacitance in Mechanically Ventilated Critically Ill Trauma and Surgical Patients

Background: Pulmonary arterial capacitance (PAC) accounts for pulsatile right ventricular (RV) afterload. Compared to pulmonary vascular resistance (PVR), PAC better predicts mortality in pulmonary arterial hypertension and correlates with RV function in advanced heart failure. The relationship of PAC and PVR in mechanically ventilated patients under the conditions of acute hemodynamic stress in unknown. We determined the correlation of PAC and PVR and the association of these with RV ejection fraction (EF) and mortality in critically ill mechanically ventilated trauma/surgical patients.
Methods: Thirty two consecutive critically ill and/or injured mechanical ventilated adult surgical patients at a Level I Trauma Center were prospectively enrolled within 6 hr of admission. Invasive hemodynamics were transduced from pulmonary artery catheter every 12 hr for 48 hr. PAC was calculated as the ratio of stroke volume and pulmonary pulse pressure. Spearman’s rank correlation assessed relationship, repeated measures analysis of variance compared survivors and non-survivors and receiver operating characteristic curves examined the association with mortality.
Results: The mean age was 49 ± 20 years (69% males, 84% trauma, 7/32 non-survivors). PAC (range, 0.5-17.6 mL/mmHg) showed strong inverse correlation with PVR (r = -0.62 to -0.78, p <0.001). Wedge pressure was normally distributed (10.1 ± 4.3 to 12.0 ± 3.8 mmHg) and was not related to PAC. Non-survivors had lower RVEF at study initiation and at 48 hr (27 ± 8 vs. 39 ± 11 %, p = 0.036 and 29 ± 8 vs. 36 ± 8 %, p = 0.043 respectively). At 48-hrs, non-survivors had lower PAC (3.1±1.5 mL/mmHg vs. 5.4±1.7 mL/mmHg; p = 0.007) and higher PVR (4.1±1.8 WU vs. 1.8±0.9 WU, p = 0.001). The optimal cut off for PAC at 48-hr to predict mortality was 3.6 mL/mmHg, [sensitivity 71%; specificity 83%; area under the curve (AUC) 84%] and for PVR was 3.25 WU (Sensitivity 71%; specificity 95%; AUC 89%).
Conclusion: In mechanically ventilated trauma/surgical patients during the first 48 hr of resuscitation, assuming the left atrial pressure is constant and not elevated, the pulsatile RV load should be a predictable and a constant proportion of the resistive load.