Basics of hemodynamic evaluation – 2

Basics of hemodynamic evaluation – 2

Part 1 Part 3

This is the article in the series on basics of hemodynamic evaluation. As mentioned earlier, pressure measurement in each chamber is an important aspect of hemodynamic evaluation. Two types of catheter based pressure measurements are possible. One is using a catheter tipped manometer, which is more ideal, but expensive. Second, more commonly used method is using fluid filled systems with an external transducer. While using fluid filled systems, care has to be taken to avoid air bubbles in the connecting tubings to avoid damping of pressure wave transmission.

The external transducer is mounted at the mid thoracic level, midway between the anterior and posterior chest walls in the supine position. This is known as phlebostatic axis. This level is used to zero the transducer with the stop cock open to the atmosphere, prior to actual recording. It corresponds to the level of the right atrium typically at the tricuspid valve level. If the transducer position is below the phlebostatic axis, the pressure readings will be erroneously high and vice versa.

Catheter whip artefact is an important problem, especially while measuring pulmonary artery pressure with fluid filled systems. Movement of the catheter during cardiac motion accelerates the fluid within the tubings and can get superimposed on the pressure tracing with a range up to 10 mm Hg.

Though there are a, c and v waves for atrial pressure tracing, usually only a and v waves are measured. Mean pressure is also documented. Generally right atrial a wave is taller than the v wave. Left atrial pattern is the reverse. Right atrial pressure falls during inspiration as the intrathoracic pressure falls and rises during expiration. This pattern is reversed in intermittent positive pressure ventilation. Left atrial pressure is higher than the right atrial pressure normally. But in those with large atrial septal defects, pressures in the atria are equalized.

Right atrial a wave is around 6 mm Hg and v wave 5 mm Hg, with mean of 3 mm Hg. Right atrial pressures will be low if the person is dehydrated and elevated in fluid overload. Left atrial a will be around 10 mm Hg and v around 12 mm Hg, with mean of 8 mm Hg. Regurgitation of the atrioventricular valves produce tall v wave as there is ventricularization of atrial pressure with severe regurgitation. When the ventricular compliance is reduced due to ischemia or hypertrophy, atrial contraction becomes forceful, making the a waves taller. Stenosis of the atrioventricular valves also make atrial contraction more forceful producing taller a waves. Usually the end diastolic pressure in the ventricles is equal to that of the atrial pressures.

But in case of atrioventricular valve stenosis, atrial pressures are higher and a gradient can be measured. The gradients will rise when the stenosis is more severe. It may be noted that a shorter diastole as in tachycardia also increases the gradient as the time for atrial emptying through a stenotic valve is less. If transseptal catheterization is not done, gradient across the mitral valve is taken as the difference between the pulmonary artery wedge pressure and the left ventricular end diastolic pressure. During balloon mitral valvotomy, gradient across the mitral valve is measured both before and after the procedure to document the success of the procedure.

As mentioned earlier, pulmonary capillary wedge pressure (pulmonary arterial wedge pressure) is usually equivalent to left atrial pressure. But they can be different if there is an obstruction in between like pulmonary veno occlusive disease or pulmonary vein stenosis. Pulmonary vein stenosis is a rare but important complication of radiofrequency catheter ablation of atrial fibrillation with pulmonary vein isolation. Pulmonary vein pressures can be measured after trans septal catheterization or through an atrial septal defect when present. There can be a gradient of 15 mm Hg across a stenotic pulmonary vein.