Basics of hemodynamic evaluation

Basics of hemodynamic evaluation

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Though invasive hemodynamic evaluation was pushed to the backstage with the development of Doppler echocardiography, it is again coming to centre stage with newer therapeutic options being available in pulmonary hypertension and heart failure. A basic knowledge of the principles of hemodynamic evaluation is essential to understand cardiac physiology.

Hemodynamic evaluation usually includes both right and left heart catheterization, though in some cases only one may be done. While writing a report of hemodynamic evaluation, it is customary to note the catheter course as it gives valuable complementary information. Catheter course in a simple right heart study may be written as: Right femoral vein to inferior vena cava, to right atrium and right ventricle and then to pulmonary artery. Superior vena cava may be entered either during the initial part or at the end after returning from the pulmonary artery. When pulmonary artery wedge pressure measurement is needed, an end hole catheter like Cournand has to be taken. But Cournand catheter needs frequent flushing as it can get blocked easily.

Usual routine during catheterization is that which ever chamber you reach, check the pressures, record tracing if possible and measure oxygen saturation. Earlier, in-cathlab hemoximeters were routinely used to measure oxygen saturations. But now samples withdrawn are often analysed using blood gas analyzers. Though blood gas analyzers give more information, the results take a little more time than hemoximeter values. Sampling has to be done fast during oximetry runs to avoid fluctuations in values. In a simple right heart study, to get rapid sequence sampling, reach pulmonary artery first and start sampling back to right ventricle, right atrium and the vena cavae. Before each sample is withdrawn, about 1 ml which represents the catheter capacity has to be discarded to get the actual value at the catheter tip. This may be difficult in case of pulmonary artery wedge sampling. Avoid contamination of the sample with room air by side leak between the piston and barrel of the sampling syringe. This is more likely to occur with a true pulmonary artery wedge sampling. In fact, if the ‘wedge’ sample is coming quickly, it is unlikely to be a true wedge!

Some basic aspects of oxygen saturation have to be known while evaluating oxygen saturations. Actual oxygen content has to be calculated for quantification of  intracardiac shunts, though a quick assessment can be obtained from the step up or step down in oxygen saturations. Oxygen saturation in inferior vena cava is higher than that of the superior vena cava due to the contribution from renal veins with highest oxygen levels among the systemic veins. Two inferior vena caval samples, one above and another below the renal veins, named high and low IVC are taken sometimes. Similarly two superior vena caval samples are also taken – high and low SVC, especially when a sinus venosus type of atrial septal defect is suspected. Three samples are taken in the right atrium – high, mid and low RA. As the right atrium is the mixing chamber for superior and inferior vena caval blood with quite different oxygen content, RA saturation has a wide range between these locations. In addition RA also receives blood from the coronary sinus with the lowest oxygen saturation in the body.

In the right ventricle, samples can be taken from the inflow, body and outflow if a shunt at the ventricular level is suspected. Pulmonary artery can be sampled from the main pulmonary artery, branch pulmonary arteries and wedge positions, depending on the clinical situation. When there is difficulty in getting a true wedge pressure, pulmonary artery diastolic pressure is sometimes taken as a surrogate of wedge pressure. In pulmonary artery, systolic, diastolic and mean pressures are noted. Right ventricular systolic and end diastolic pressures are documented. In case of the right atrium, both ‘a’ and ‘v’ wave pressures can be noted from the tracing and the electronic mean recorded. If the left heart is catheterized, aortic and left ventricular pressures are noted akin to their right sided counterparts. Left atrial pressures are easily measured when there is an atrial septal defect. Otherwise pulmonary artery wedge pressures are taken as surrogates of left atrial pressure.

To get accurate pressure measurements, tracings with corresponding ECG is needed for timing. When there is a pressure difference between chambers, a pull back tracing is obtained to assess the gradient. Both peak gradient and mean gradient are assessed. In this case, it should be noted that Doppler echocardiography measures peak instantaneous gradient while cath measures peak to peak gradient.

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