The pulmonary circulation


William Harvey reported the experiments which led him to conclude that blood must flow through the lungs in De Motu Cordis in 16281 but it was not until 1661 that Malpighi described the microscopic appearance of the pulmonary capillaries which provided the anatomical link between the right and left heart.2 In 1894 Bradford and Dean3 reported that the pulmonary artery pressure increased during hypoxia, but it was not until 1946 that von Euler and Liljestrand4 concluded that this was the result of hypoxic pulmonary vasoconstriction, and suggested that this mechanism might improve the matching of perfusion to ventilation at the alveolar level. Nissel's subsequent demonstration of hypoxic vasoconstriction in the isolated perfused lung confirmed that this was a local response and not mediated by the autonomic system.5 Meanwhile, the introduction of the cardiac catheter into clinical practice6 in the 1940s had led to studies of the haemodynamics of the heart and lungs, and to the subsequent development of closed and, later, open heart surgery in the 1950s. The subsequent introduction of radioisotope methods for measuring the distribution of ventilation and blood flow, and the development of practical methods of measuring gas and blood gas tensions, resulted in a vast increase in our understanding of the mechanisms governing gas exchange. Later studies have shown that the pulmonary circulation has three other important roles:

1 It has a regulatory function (for example, as part of the renin-angiotensin system).

2 It takes up or metabolises certain drugs [such as propranolol, lignocaine (lidocaine), and noradrenaline (norepinephrine)].

3 It filters out particulate matter (such as platelet or fat emboli).

The pulmonary circulation is also concerned with the generation of surfactant, which maintains alveolar stability, and with the exchange of water. As anaesthesia, mechanical ventilation, and surgery may produce major changes in the pulmonary circulation, it is important that the anaesthetist should have a clear understanding of the factors that govern the distribution of pulmonary blood flow. Readers interested in the pharmacological aspects of the lung are referred to a review by Bakhle.7

The mechanisms affecting the distribution of blood flow in the normal lung will be considered first. This will be followed by a discussion on the effects of posture, haemorrhage, mechanical ventilation, and lung disease. The methods of studying the pulmonary circulation will then be outlined, followed by a brief review of the way in which anaesthetic and related drugs may alter the distribution of blood flow, and so affect the efficiency of gas exchange. Finally, we shall consider the problem of pulmonary hypertension, the effects produced by pulmonary vasodilator drugs, and the pathophysiology of pulmonary oedema.

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