Cardiogenic mixing: mechanisms and experimental evidence in dogs

Cardiogenic mixing was studied in seven anaesthetized closed-chest dogs undergoing mechanical ventilation by comparing single-breath washout of two poorly soluble inert gases of widely differing diffusivities (helium (He) and sulphur hexafluoride (SF6)) in normal conditions with the heart beating (control), and during reversible temporary myocardial arrest (heart arrest). Cardiac arrest of approximately 20 s duration was induced repeatedly (8-15 times) by intracoronary injection of acetylcholine (approximately 35 mg) facilitated by a non-occluding 7-French gauge angiographic catheter maintained in the left coronary artery. After equilibration of lung gas with 1% helium and 1% SF6, single breath, constant flow expirograms were recorded in the tracheal tube by mass spectrometry after inspiration of test gas-free air. Series deadspace (VD) and relative alveolar slope (S) (increment of expired partial pressure, normalized to mixed expired-inspired partial pressure difference per increment of expired volume (S = (delta P/(PE-P1]/delta VE litre-1], were determined as indices for intrapulmonary gas mixing. The effects attributable to the action of the heart were quantified by the heart arrest: control ratio of VD and S, which were not significantly different from unity (P greater than 0.05) (VD: 0.95 (SD 0.05) for helium and 0.94 (0.07) for SF6; S:1.03 (0.10) for helium and 1.05 (0.14) for SF6. The He:SF6 ratios of VD and S (0.90 and 0.64, respectively), indicating diffusion dependent separation of gases, also were unaffected by the mechanical action of the heart. The data indicate that convective mixing by the mechanical action of the heart did not significantly enhance intrapulmonary mixing and transport.