Comparison of two ventilation modes in post-cardiac surgical patients

Abstract

Background: The cardiopulmonary bypass (CPB)-associated atelectasis accounted for most of the marked post-CPB increase in shunt and hypoxemia. We hypothesized that pressure-regulated volume-control (PRVC) modes having a distinct theoretical advantage over pressure-controlled ventilation (PCV) by providing the target tidal volume at the minimum available pressure may prove advantageous while ventilating these atelactic lungs.

Methods: In this prospective study, 36 post-cardiac surgical patients with a PaO(2)/FiO(2) (arterial oxygen tension/Fractional inspired oxygen) < 300 after arrival to intensive care unit (ICU), (n = 34) were randomized to receive either PRVC or PCV. Air way pressure (P(aw)) and arterial blood gases (ABG) were measured at four time points [T1: After induction of anesthesia, T2: after CPB (in the ICU), T3: 1 h after intervention mode, T4: 1 h after T3]. Oxygenation index (OI) = [PaO(2)/ {FiO(2) × mean airway pressure (P(mean))}] was calculated for each set of data and used as an indirect estimation for intrapulmonary shunt.

Results: There is a steady and significant improvement in OI in both the groups at first hour [PCV, 27.5(3.6) to 43.0(7.5); PRVC, 26.7(2.8) to 47.6(8.2) (P = 0.001)] and second hour [PCV, 53.8(6.4); PRVC, 65.8(7.4) (P = 0.001)] of ventilation. However, the improvement in OI was more marked in PRVC at second hour of ventilation owing to significant low mean air way pressure compared to the PCV group [PCV, 8.6(0.8); PRVC, 7.7(0.5), P = 0.001].

Conclusions: PRVC may be useful in a certain group of patients to reduce intrapulmonary shunt and improve oxygenation after cardiopulmonary bypass-induced perfusion mismatch.

Keywords: Atelectasis; mechanical ventilation; pressure-controlled ventilation; pressure-regulated volume-controlled ventilation.

Figure 1

Consort diagram

Figure 2

Oxygenation index [arterial oxygen tension (PaO₂)/{fraction of inspired oxygen (FiO₂) ȕ mean airway pressure (P_mean)}] at different time interval among the groups. T1 = before commencement of cardiopulmonary bypass, T2 = after cardio pulmonary bypass (in ICU), T3 = one hour after intervention mode, T4 = 2 h after intervention mode. ^*P < 0.05 versus to T1; ^†P = 0.001 versus T2; ^‡P = 0.03 versus PCV