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Hemodynamics for the structural interventionalist
Introduction
Invasive hemodynamic assessment is a critical part of structural intervention. Imaging and hemodynamics are complementary, and both need to be mastered for effectiveness in structural heart disease (SHD) interventions.
Indications for diagnostic invasive assessment include:
- 1.
The diagnosis remains ambiguous after noninvasive testing.
- 2.
There is discrepancy in the reported severity of valvular heart disease by different noninvasive measures.
- 3.
There is suspicion for pulmonary vascular disease or precapillary pulmonary arterial hypertension.
- 4.
A high-output state or shunt disease is suspected or of unclear severity.
Invasive hemodynamic assessment remains the gold-standard direct measurement of absolute pressures in various chambers of the heart, which can only be indirectly estimated by Doppler echocardiography. In addition, cardiac catheterization allows assessment of cardiac output (CO) using the Fick and thermodilution principles, as we will discuss later.
In practical terms, cardiac catheterization is useful when there is discordance between the clinical evaluation and diagnostic testing. Because cardiac catheterization is only performed for those patients in whom there is a discrepancy, it takes a very meticulous approach to assure that reliable data are obtained that answers the clinical question. This chapter will focus primarily on the interpretation of data obtained and common pitfalls in interpretation and measurement of hemodynamics of SHD. We will not discuss the actual technical performance of the left and right heart catheterization, which will be well known to structural interventionalists.
Procedural considerations
Setting the zero value at the phlebostatic axis
The standard right or left heart catheterization involves the use of fluid-filled catheters, which measure absolute time-varying pressure in reference to an externally determined zero reference point. By convention, the zero value is set at atmospheric pressure at the level of the right atrium. In a supine patient, this typically corresponds to the phlebostatic axis (the intersection between the midaxillary line and the fourth intercostal space).
Careful attention to zeroing to room air at the phlebostatic axis is important, as errors in setting the pressure gauge at the correct phlebostatic axis height can lead to miscalibration in absolute pressures recorded. For every 10-cm error in height of the pressure gauge, there is a 7.6-mmHg error in absolute pressure recorded. Therefore, if the pressure gauge is incorrectly set 10 cm higher than the true phlebostatic axis, there will be a falsely low absolute pressure by 7.6 mmHg (although relative changes in pressure over time, such as pulse pressure or gradients, will remain accurate).
Choice of right heart catheter
There are typically two choices for right heart catheter use: (1) an end-hole balloon wedge catheter or (2) a Swan-Ganz catheter with thermodilution CO capabilities. The balloon wedge catheter has a larger lumen due to a single end hole, with the entire internal diameter utilized for pressure transmission. Therefore, when accurate pressure waveform measurements are required (such as for gradient measurement or constriction/restriction studies), a balloon wedge catheter should be the catheter of choice. This then requires measurement of CO by the Fick principle (which will be described later) using pulmonary and arterial saturations. When multiple CO measurements are required throughout a case with different loading conditions, this then necessitates continuous oxygen consumption by a metabolic cart. When this is not available, a thermodilution-capable Swan-Ganz catheter may be preferable, although the pressure waveforms will often be damped compared with a balloon wedge catheter due to the narrower internal luminal diameter. It is also important to remember situations when the thermodilution CO may be inaccurate (described later), in which case the balloon wedge catheter with direct Fick CO should be preferred.
Left heart catheterization: Transeptal versus retrograde access
Measurement of left ventricular pressures requires either (1) retrograde entry into the left ventricle across the aortic valve or (2) transseptal entry into the left atrium (LA) via a Mullens sheath with a balloon angioplasty catheter introduced through the sheath across the mitral valve and into the left ventricle. The technique and complications of transseptal access will be discussed in Chapter 5 , but transseptal access, even in experienced hands, does carry a small but increased risk of complications compared with retrograde left ventricular access. Therefore it should be reserved for situations when the information obtained by transseptal access justifies the risk, including: (1) accurate measurement of mitral valve gradients for indeterminate severity of mitral stenosis; (2) localization and measurement of intraventricular and outflow tract gradient in suspected obstructive hypertrophic cardiomyopathy; (3) when high-quality left ventriculography for mitral regurgitation severity is required (because ectopy is decreased via transseptal left ventriculogram compared with retrograde ventriculography); (4) localization of left heart pathology when trying to differentiate pulmonary vein stenosis, stiff left atrial syndrome, or heart failure with preserved ejection fraction where direct left atrial pressure is necessary; or (5) left heart pressure measurement in the presence of a mechanical aortic valve, where retrograde access is not possible. In choosing an appropriate left heart catheter for hemodynamic evaluation, end-hole catheters should not be used in the left ventricle. The chosen catheter should have multiple side holes, such as in a Berman catheter, pigtail catheter, or multipurpose catheter.
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