Authors Keywords: cardiopulmonary interactions, intrathoracic pressure, spontaneous ventilation, pulmonary vascular resistance Abstract

The goal of the heart and lungs is to deliver oxygen at a tissue level, and this is determined by cardiac output (CO) and the oxygen content of arterial blood (CaO2), DO2 = CO × CaO2.1 The heart and lungs share the same intrathoracic space: mechanically this is akin to having a pump within a pump.2 Cardiopulmonary interactions refer to the relationship between airway pressures, lung volumes, and CO.3 In health, the effect of cardiopulmonary interactions is minimal. It is important to look at each part of the heart separately as phasic changes in intrathoracic pressures (ITP) have differing effects.

In a spontaneously breathing person, a negative ITP is transmitted to the right atrium. The flow of blood to the right atrium is governed by Ohm’s law, flow = △P/R, where △P is the driving pressure to the right atrium and R is the resistance of the capacitance vessels.4 With positive pressure ventilation (PPV) the increased ITP is transmitted to the right atrium, which results in a reduction of the driving pressure and ultimately the venous return (VR). The right ventricle afterload is the pressure that the heart must work against to eject blood during systole, and this is determined by the pulmonary vascular resistance (PVR).

PVR is determined by the extra-alveolar vessels and the intra-alveolar vessels. The total PVR is the addition of the two and lowest at functional residual capacity.5 The left ventricle preload is essentially equal to the right ventricle output. There is a time lag as blood must traverse the pulmonary circulation. Left ventricle afterload can be expressed as left ventricular (LV) wall stress.

Afterload increases as transmural pressure and vascular resistance increase. Positive pressure reduces the transmural pressure needed for the ejection of the stroke volume. With negative ITP there is a pressure pulling outwards as the ventricle attempts to eject its stroke volume, therefore higher transmural pressures are needed.4 Knowing how the heart and lungs work in the normal physiological state will help guide decision-making about heart-lung interactions in critically ill patients therefore mitigating any adverse consequences.

Author Biography M Ngwenya, University of the Witwatersrand

Department of Anaesthesia, School of Clinical Medicine, Faculty of Health Sciences, Charlotte Maxeke Johannesburg Academic Hospital, University of the Witwatersrand, South Africa

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