Critical assessment of haemodynamic data

Loading conditions for individualmyocytes vary with their position in the myocardium. Unfortunately, the loading conditions of the intact ventricle are difficult to determine. Therefore, assessment of ventricular load involves compromise. The best approximation for ventricular wall stress uses the Law of Laplace, modified for thick walled structures. This is expressed as Lamé’s equation: s / ðP1 P2Þ R=w ð1Þ s 1⁄4 wall stress; P1 1⁄4 intraventricular pressure; P2 1⁄4 extraventricular pressure; R 1⁄4 ventricular radius; w 1⁄4 wall thickness. P1 for each ventricle can be estimated from central venous pressure (CVP) or pulmonary artery occlusion pressure (PAOP). R andw can be estimated fromM-mode echocardiography. Using this equation, preload and afterload can be predicted from measures of circumferential wall stress using data from M-mode echocardiography and invasive pressure monitoring. The calculation is impractical as a clinical tool but demonstrates the importance of positiveend-expiratorypressure (PEEP),which influences, but is not equal to, P2. Arterial blood pressure

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