A Cardiac Catheterization Laboratory Is Not Just an Angiographic Suite

Abstract

When I began my cardiology training in 1965, my instructors in the cardiac catheterization laboratory made it clear to me that many things other than anatomic assessment by angiography could be accomplished in the cardiac catheterization laboratory. For example, at that time cardiac output, pressure gradients, valvar insufficiency, shunts, assessment of contractility, assessment of myocardial blood flow using radioactive techniques, lactate metabolism, and assessment of ventricular function were the principal physiologic parameters measured. Copyright © 2010 Wiley Periodicals, Inc.

What Can Be Measured in 2010

Much has evolved since that time that has allowed more precise measurement of coronary blood flow (TIMI flow and TIMI myocardial perfusion grade), physiologic significance of coronary stenoses (FFR), assessment of the vascular wall (IVUS, OCT), and assessment of vascular reactivity (nitrates, acetylcholine, ergonovine).

Fellowship Training in 2010

For various reasons, trainees in 2010 and their trainers are heavily focused on the angiographic determination of coronary artery stenoses and their potential for angioplasty intervention. I think there are two major reasons. First, the volume of patients coming into the catheterization laboratory has markedly increased since the 1960s. In the early days, most active laboratories were performing 5 to 10 studies per week. Many of these studies were associated with research investigations and therefore were somewhat longer than the usual studies performed in 2010. In 2010, 10 cases/day is not unusual. Thus, there is little time to perform any sort of sophisticated investigation in the catheterization laboratory.

Second, the modern catheterization laboratory is not just a diagnostic facility. Now therapy is being done in the cardiac catheterization laboratory whereas this was not the case until the mid-1970s.

The Donut and the Hole

Several years ago, I wrote an editorial entitled “Vascular Disease: 'The Donut and the Hole.”1 I use the term donut and the hole to make the point that there is an interrelationship between the vascular wall (the donut) and the vascular lumen (the hole). Obviously, if nothing happens to the lumen, for example thrombosis or severe stenosis, then the end organ, the myocardium, does not suffer an insult such as an ischemia or a necrosis. However, stenosis or occlusion of the lumen does not occur unless there are some abnormalities in the vascular wall.

Over the last several years, investigation has focused on the lumen, e.g. percent stenosis, occlusion, minimal irregularities, collaterals and very little on the vessel wall. One of the reasons is that finding problems in the vascular wall is not a simple matter. However, many things have been learned over the years. For example, vascular reactivity in the coronary circulation has been studied using nitroglycerin to dilate the vessel, ergonovine to constrict the vessel, and acetylcholine to investigate nitric-oxide-dependent vascular constriction and relaxation. Intracoronary ultrasound studies provide information about the anatomy and pathology of the vessel wall, including dissection, expanding hematomas, neo-intimal and smooth muscle proliferation, plaque thickness or thinness, plaque eccentricity, calcification (location and extent), stent deployment, and so forth.

Catheter-Based Assessment of the Vessel Wall

Based on several sophisticated autopsy studies of the coronary arteries, it is now believed that progression of blood vessel stenosis and/or occlusion is related to disruption of a vascular plaque consisting of a thin cap covering a lipid pool which is mainly oxidized LDL. Since the prevention of lumen occlusion is ultimately related to the prevention of disease in the vessel wall, efforts are being directed toward identifying vessel wall abnormalities to separate the high-risk patient from the low-risk patient.

Recent studies using virtual histology intravascular ultrasound have provided some startling observations that identify plaques with a thin cap and a lipid pool that are not necessarily high-grade stenoses but are thought to be plaques that are prone to rupture in the future.

Left heart catheterization and angiography provide excellent anatomic information, but it also has the added value of providing physiologic information. From the anatomic standpoint, several devices can be used during left heart catheterization including intracoronary ultrasound (virtual histology), optical coherence tomography, and, in the future, infrared spectroscopy. All of these techniques help to assess the presence or absence of plaques that may be prone to rupture. Doppler velocity wires assess coronary flow reserve, which is an indication of the physiology of the entire epicardial coronary vessel plus the microcirculation. In contrast, fractional flow reserve which measures pressure proximal to and distal to a coronary stenosis estimates the physiologic significance of that particular epicardial coronary stenosis.

Myocardial hibernation and viability can be inferred if collaterals are present or if there is a myocardial contrast blush and the ventricle improves contraction after nitrates, dobutamine, or after a premature ventricular complex.

Hemodynamic Assessment of Coronary Stenosis Severity

Feldman et al, working in the animal laboratory, demonstrated that resting coronary blood flow and resting distal coronary pressure were decreased by short snare stenoses which reduced the diameter of the branches studied more than 80%. In contrast, during peak reactive hyperemia, coronary blood flow was decreased by a 60% stenosis of the coronary artery, and peak reactive hyperemic coronary blood flow responses decreased further as length of a coronary artery stenosis was increased. These data indicate that several anatomic variables of coronary stenoses affect the physiologic response of the coronary circulation. The variables include:

• 1.
  Reduction in lumen diameter;
• 2.
  Length of stenosis;
• 3.
  Number of stenoses in one vessel.2

The animal studies of Feldman, et al. provide a rationale for fractional flow reserve used in human coronary arteries.

Human Studies

In 2009, investigators using fractional flow reserve measurements in humans reported the FAME trial. Patients with multivessel disease were randomized to either routine angiography, guided PCI, or fractional flow reserve guided PCI with stenting of only those lesions with a fractional flow reserve of < 0.8. Outcomes were compared at one year and revealed that fractional flow reserve guided PCI is associated with a lower incidence of MACE compared with angiographic guided PCI in patients with multivessel disease.3

Conclusion

Thus, the combination of anatomy, physiology, and pharmacology are important tools in the overall assessment of cardiac patients in the catheterization laboratory. In my opinion, training of cardiovascular fellows should not be limited to angiography if advances in our understanding of the coronary circulation are to continue in the 21st century.