The most accurate method for measuring pressure in the heart is to utilize a system with the pressure transducer (usually a strain gauge type) located at the exact location of interest. However, while catheters with a pressure transducer at the tip are available, they are too expensive to be used routinely and are generally only used for research purposes. Thus, the most common method of measuring pressure in the cardiac catheterization lab utilizes a system incorporating a fluid-filled catheter connected through a manifold to a pressure transducer. This system, however, has several characteristics that influence its fidelity and its accuracy. Because the pressure waveform is transmitted through fluid until it reaches the transducer outside the body, there is both a time delay and a dampening of the pressure signal that usually filters out the high frequency components. Underdamping of the system can be a problem especially if air bubbles are present in the system (Figure 7-1). Other common sources of error are listed in Table 7-1.

Oximetry measurements are most commonly performed to measure cardiac output utilizing the Fick method (described later in this chapter) and to rule out a left-to-right shunt (described later in this chapter). Oximetry measures the oxygen saturation of blood. The oxygen content of blood can then be calculated.

Oxygen content ˜Hgb(g/dL) × 1.36(mLO₂/g Hgb) × Sat

where Hgb is hemoglobin in grams per deciliter; 1.36 is the oxygen carrying capacity of blood in milliliters of oxygen per gram of hemoglobin; and Sat is the oxygen saturation of the blood. The dissolved oxygen in blood is generally negligible in these calculations and is usually ignored.

A thermistor is mounted at the tip of the pulmonary artery catheter to measure the temperature of the fluid as it passes through the pulmonary artery. Temperature is most commonly used to calculate cardiac output using the thermodilution technique, which is a variant of indicator dilution. Cold saline is injected through an opening in the catheter 25 to 30 cm proximal to the tip. The temperature is measured as a function of time, and temperature change can be used to calculate cardiac output (see “Cardiac Output” section).

See Figures 7-2, 7-3, 7-4, 7-5 and 7-6.

Measuring pressure gradients across stenotic valves is an important process in determining the need for surgical intervention particularly when the hemodynamics as measured by noninvasive means are in question. The valve orifice area can often be estimated by a formula that was developed by Dr. Richard Gorlin if the mean pressure gradient, cardiac output, and the systolic ejection time are known; particularly if the patient is not in a low cardiac output state:

where SEP is the systolic ejection period in aortic stenosis (length of time blood is ejected from LV every beat); DFP is the diastolic filling period in mitral stenosis (length of time blood filling LV every beat); ΔP is the mean pressure gradient; constant (K = 0.85) is added in mitral stenosis.

The Angel correction mandates that the above result be divided by 1.35 for a heart rate <75 beats per minute in the setting of mitral stenosis, or >90 beats per minute in the setting of aortic stenosis.

Caution is advised when using the Hakki formula if coexisting aortic regurgitation or mitral regurgitation is present as this will cause underestimation of the aortic valve area and mitral valve area respectively.

Aortic Stenosis: The normal orifice area of the aortic valve is 3 to 4 cm². The aortic valve can become significantly narrowed prior to the onset of symptoms or even hemodynamic significance.

The most accurate method for measuring aortic valve gradients is by obtaining simultaneous pressure measurements from the left
ventricle and the ascending aorta (Figure 7-7). This method allows the calculation of the mean gradient by direct measurement from both recordings. The easiest way to accomplish this is to use a dual-lumen pigtail catheter, which permits simultaneous measurement of pressures in the LV and ascending aorta.

Alternatively, a long arterial sheath can be placed in the descending thoracic aorta, and pressure measured from the sideport. The femoral artery pressure is also often substituted for this measurement. The peak femoral artery pressure is usually higher than the peak aortic root pressure due to reflected pressure waves seen in the periphery, thus using the femoral artery results in underestimation of the pressure gradient. This can be somewhat compensated by measuring the pressure difference between the catheter at the ascending aorta and the sidearm of the femoral artery sheath, and subtracting the difference.

A more commonly utilized method involves pullback of the catheter from the left ventricle into the ascending aorta. This technique yields a “peak-to-peak” gradient between the maximum aortic pressure and the
maximum left ventricular pressure (Figure 7-8). Each of these peaks occurs at different points in time, however, and this measurement is only an estimate of the mean gradient. In addition, in patients with severe aortic stenosis, the catheter itself may take up a significant fraction of the orifice area, resulting in worsened stenosis and increased gradients.

The Gorlin and Hakki formulas can be used to estimate the valve orifice area, but may be inaccurate in severe aortic stenosis with low-output states. The accuracy of the formula is flow-dependent and will result in small orifice areas, despite low gradients, if the flow across the aortic valve is low. This is frequently observed in patients with severe systolic LV dysfunction.

If maneuvers to increase cardiac output (i.e., exercise, dobutamine, nitroprusside) are performed on this subset of patients and a significant increase in the estimated valve orifice area is observed (usually resulting in a valve area >1 cm2) this is termed “pseudostenosis.” Failure of the estimated valve orifice area to significantly increase with these measures implies either true severe aortic stenosis (increase in aortic valve pressure gradients with maneuvers) or poor left ventricular contractile reserve (no significant increase in aortic valve pressures with maneuvers).

Mitral Stenosis: The normal mitral valve orifice area is 4 to 6 cm². Significant narrowing can occur prior to hemodynamic compromise. When the valve area falls to ˜2.0 cm², the left atrial pressures will start increasing to maintain cardiac output. Valve areas less than 1.0 cm² frequently require some intervention.