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The technique of venous catheterization gives only indirect information about lesions of the left side of the heart. Analysis of mitral valve disease was made possible by the discovery that a venous pressure pulse could be obtained from a catheter wedged in a peripheral pulmonary artery (Lagerlof and Werko, 1949;Hellems et al., 1949). The form of the arterial pulse has been used for the indirect evaluation of aortic valve disease (Hancock and Abelman, 1957;, and in recent years several techniques for direct measurement of left heart pressures have been introduced. Puncture of a cardiac chamber is usually necessary, and the risk of these methods is considerably greater than that of the right heart catheterization.Whenever haemodynamic study is thought necessary, the type of investigation to be performed must be decided with regard to the abnormality present and the information required. The value of the data to be obtained must be considered in relation to the morbidity of the procedure. Right heart catheterization alone is often decisive, and left heart pressure measurements should be reserved for situations in which they will provide critical information.In the present study, emphasis is placed on the accurate measurement of the pressure gradient across the mitral or aortic valve, with simultaneous estimation of the cardiac output. Assessment of regurgitant flow by indicator dilution (Carleton et al., 1960; Warner and Toronto, 1958) METHODSThe methods used have been described previously (Davila et al., 1959). Central arterial pressure was recorded continuously using the Seldinger (1953) method. Right heart catheterization was performed for the measurement of pulmonary arterial and wedge pressures. Left atrial pressures were obtained by transbronchial puncture using a slotted bronchoscope which could be removed leaving the needle in place. Left ventricular pressures were recorded by direct puncture through the anterior chest wall (Brock et al., 1956) or by retrograde arterial catheterization using a Cournand catheter inserted in the right brachial artery (Voci and Hamer, 1960).Simultaneous pressure measurements were made on either side of the mitral or aortic valves while cardiac output was estimated by the Fick method. Pressures were recorded with P23Db Statham strain-gauges and a 4-channel direct writing Sanborn Polyviso with optimal damping, and were replotted on the same scale without correction for phase lag. The base-line for all pressure measurements was the middle of the chest at the level of the second costal cartilage . The Ry/v ratio (Owen and Wood, 1955) was calculated from left atrial pressures adjusted to a base-line at the level of the sternal angle. Blood oxygen contents were determined by the Van

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