The pulmonary physician in critical care c 13: The pulmonary circulation and right ventricular failure in the ITU

The lungs are the only organs that receive the entire cardiac output which is delivered at a mean resting pulmonary arterial pressure of 15 mm Hg. The capacitance pulmonary arteries are larger in calibre and have thinner walls than their systemic counterparts. Moreover, the pulmonary circulation possesses little resting vascular tone and has a large reserve for recruitment of vascular segments that are normally nonperfused. Thus, the pulmonary circulation is a low pressure, low resistance circuit capable of handling large increases in pulmonary blood flow (up to sixfold with strenuous exercise) with only small changes in pressure. The maintenance of a low pulmonary capillary pressure is vital in preserving the function of the blood-gas barrier. In accordance with this low pressure circuit, the right ventricle (RV) is a thin muscle with limited contractile reserve, which has significant implications for both the prognosis associated with severe pulmonary hypertension (PHT) and for the principles underlying the clinical management of PHT and RV failure. Both PHT and RV dysfunction are common complications of the complex medical disorders experienced in the intensive care unit. In most circumstances the PHT is mild or moderate in degree and associated with RV dysfunction rather than frank right heart failure. Occasionally, however, patients do present with life threatening PHT and associated RV failure requiring prompt and appropriate intervention. Right heart dysfunction and PHT of varying severity are commonly encountered in patients with chronic lung disease and left ventricular failure, but these specific entities will not be considered further. This review will rather concentrate on the management of severe PHT in the setting of RV failure. In addition, we will discuss the relevance and treatment of PHT in the context of acute respiratory distress syndrome (ARDS) in adults. Definitions of PHT and calculations for mean pulmonary artery pressure (PAP) are shown in table 1. In the intensive care unit haemodynamics are usually measured using a flow directed, balloon tipped pulmonary artery catheter. Cardiac output is most commonly and conveniently determined by thermodilution techniques. It can also be derived via the Fick principle but this is not used frequently in clinical practice. Transoesophageal echocardiography can also be used to estimate cardiac output using Doppler imaging. The procedure requires sedation, however, and is not therefore usually performed in cases with severe PHT unless the patient is intubated (or undergoing another essential procedure).