Impact of Chest Wall on Respiratory Mechanics, Ventilation and Ventilation-Perfusion Matching During Cardiac Surgery.

Abstract

Objective: To evaluate how sternotomy and subsequent sternal closure affect respiratory mechanics, ventilation heterogeneity, ventilation-perfusion (V/Q) matching, and oxygenation in patients undergoing cardiac surgery with cardiopulmonary bypass (CPB).

Design: Prospective consecutive self-controlled clinical investigation.

Setting: A university hospital.

Participants: 53 patients undergoing elective cardiac surgery.

Interventions: Measurements performed at 4 protocol stages: before sternotomy, immediately after sternotomy, after weaning from CPB, and after sternal closure.

Measurements and main results: Respiratory mechanics (airway resistance, tissue damping, and elastance) were assessed using forced oscillations, and ventilation heterogeneity was evaluated via capnographic slopes (phase 2 and phase 3). Physiologic dead spaces (Bohr and Enghoff fractions), V/Q matching, and oxygenation (partial pressure of oxygen [PaO₂]/fraction of inspired oxygen [FiO₂] ratio, intrapulmonary shunt fraction) were analyzed as well. Sternotomy significantly improved airway resistance (-30%), tissue damping (-25%), and elastance (-35%), thereby enhancing ventilation homogeneity (phase 3 slope reduced by 20%; p < 0.001). Conversely, CPB markedly deteriorated respiratory mechanics and ventilation efficiency (airway resistance, +45%; elastance, +50%; p < 0.001). Sternal closure led to further worsening in respiratory mechanics and ventilation heterogeneity, although without any additional significant impact on V/Q matching or oxygenation parameters (PaO₂/FiO₂ were unchanged after closure).

Conclusions: Reestablishing chest wall integrity significantly deteriorates respiratory mechanics and ventilation homogeneity, mirroring improvements initially gained from sternotomy. The absence of additional V/Q mismatch underscores the protective role of pulmonary vascular regulation.