Pulmonary circulation

Abstract

Pulmonary circulation is unique compared to systemic circulation because of its flexible structure that is able to meet the needs of gas exchange under different and sometimes extreme conditions. Unique for the pulmonary circulation are its shape, structure and compliance. Pulmonary arteries, veins and capillaries are embedded in and around the alveoli and are able to adapt, while the lung changes its shape or is compressed by the gravity. Small lung arterioles and veins are able to contract if exposed to hypoxia, a mechanism that is essential to divert blood flow away from non-ventilated areas. Although the single determination of the pulmonary vascular resistance (calculated dividing transpulmonary pressure by blood flow) is a good indicator of the state of constriction or dilation of the pulmonary resistive vessels, it is not reliable regarding the evaluation of the functional status of the pulmonary circulation at variable flow rates. The functional properties of the pulmonary circulation are best assessed by the pressure/flow relationship at different levels of cardiac output. Pulmonary hypertension is defined as a mean pulmonary artery pressure equal or above 25 mmHg and is classified as precapillary if pulmonary artery occlusion pressure or wedge pressure is ≤15 mmHg and postcapillary if >15 mmHg. Based on pathophysiological, clinical and therapeutic considerations, the pulmonary hypertension population is divided into five classes. The most frequent classes in the intensive care setting are pulmonary hypertension associated to left heart disease (class 2) and pulmonary hypertension due to lung disease and/or hypoxia (class 3). In the present chapter, we will provide the basic knowledge on the physiology and pathophysiology of the pulmonary circulation from the perspective of the clinician working in the intensive care unit.