Differentiation of physiologically significant coronary artery lesions by coronary blood flow measurements during isoproterenol infusion

Pre-angioplasty instantaneous wave-free ratio pullback provides virtual intervention and predicts hemodynamic outcome for serial lesions and diffuse coronary artery disease

OBJECTIVES

The aim of this study was to perform hemodynamic mapping of the entire vessel using motorized pullback of a pressure guidewire with continuous instantaneous wave-free ratio (iFR) measurement.

BACKGROUND

Serial stenoses or diffuse vessel narrowing hamper pressure wire-guided management of coronary stenoses. Characterization of functional relevance of individual stenoses or narrowed segments constitutes an unmet need in ischemia-driven percutaneous revascularization.

METHODS

The study was performed in 32 coronary arteries with tandem and/or diffusely diseased vessels. An automated iFR physiological map, integrating pullback speed and physiological information, was built using dedicated software to calculate physiological stenosis severity, length, and intensity (ΔiFR/mm). This map was used to predict the best-case post-percutaneous coronary intervention (PCI) iFR (iFRexp) according to the stented location, and this was compared with the observed iFR post-PCI (iFRobs).

RESULTS

After successful PCI, the mean difference between iFRexp and iFRobs was small (mean difference: 0.016 ± 0.004) with a strong relationship between ΔiFRexp and ΔiFRobs (r = 0.97, p < 0.001). By identifying differing iFR intensities, it was possible to identify functional stenosis length and quantify the contribution of each individual stenosis or narrowed segment to overall vessel stenotic burden. Physiological lesion length was shorter than anatomic length (12.6 ± 1.5 vs. 23.3 ± 1.3, p < 0.001), and targeting regions with the highest iFR intensity predicted significant improvement post-PCI (r = 0.86, p < 0.001).

CONCLUSIONS

iFR measurements during continuous resting pressure wire pullback provide a physiological map of the entire coronary vessel. Before a PCI, the iFR pullback can predict the hemodynamic consequences of stenting specific stenoses and thereby may facilitate the intervention and stenting strategy.

Relation between graded, subcritical impairments of coronary flow reserve and regional myocardial dysfunction induced by isoproterenol infusion in dogs

Isoproterenol has been used experimentally and clinically to elicit ischemia. The usefulness of this approach, however, in eliciting regional dysfunction in the presence of mild to moderate single-vessel coronary disease quantitated on the basis of coronary flow reserve measurements has not been previously defined. Open-chest, anesthetized dogs were instrumented with an electromagnetic flow probe, high-fidelity micromanometers, and subendocardial ultrasonic crystals. A rigid, screw occluder was used to produce five subcritical coronary stenoses in each dog associated with varying impairment of postocclusion reactive hyperemia at rest but no impairment of resting coronary blood flow. Regional function at rest and in response to the isoproterenol challenge (0.25 micrograms/kg/min) in nonstenotic and stenotic conditions was assessed. Relative regional function was maintained during the infusion until nearly total loss of coronary flow reserve. With this near-critical stenosis, function was lower than in the nonstenotic state but remained greater than resting control values. Moderate impairments of coronary flow reserve were not associated with isoproterenol-induced deterioration of regional function. In conclusion, detection of impaired coronary flow reserve at rest is a more sensitive index of the severity of a coronary stenosis than is detection of regional dysfunction during isoproterenol challenge. Failure to maintain the expected isoproterenol-induced increase in regional function is manifested only when stenoses are associated with nearly total loss of resting coronary flow reserve. This suggests that the clinical use of isoproterenol challenge is not effective in eliciting regional dysfunction when mild coronary disease is present.

Sequential changes in regional coronary flow during pacing-induced angina pectoris: coronary flow limitation precedes angina

To investigate the sequence of changes in regional myocardial perfusion which precedes stress-induced angina, we measured great cardiac vein flow (GCVF), draining the anterior left ventricle, during incremental atrial pacing in 10 patients with normal anterior perfusion (group I) and in 11 patients with greater than or equal to 50% diameter stenosis of the left main or proximal left anterior descending coronary artery (group II). Pacing produced angina in 11 of 11 and regional lactate production in 9 of 11 group II patients. Both groups had comparable resting GCVF (group I = 62 +/- 7 ml/min vs group II = 76 +/- 9 ml/min; p = NS) and both exhibited progressive increases in GCVF with pacing. However, the entire flow-demand relationship was displaced downward in group II, as evidenced by a reduction in the percent increase in GCVF both following the first 20-beat pacing increment (group I = 46 +/- 6% vs group II = 16 +/- 4%; p less than 0.001) and at angina (group I = 113 +/- 16% vs group II = 44 +/- 9%; p less than 0.001). The first 20-beat pacing increment increased the heart rate to only 77 +/- 2 bpm in group II whereas angina and ECG changes did not occur until a pacing rate of 117 +/- 6 bpm. These data indicate that regional flow abnormalities precede the onset of pacing-induced angina in patients with coronary disease (CAD) and that these flow abnormalities frequently are detectable at heart rates substantially below the anginal threshold.

Functional significance of coronary collateral vessels in patients with coronary artery disease

Regional myocardial perfusion was measured in 12 normal subjects and in 34 patients with coronary artery disease (CAD) at rest and during infusion of isoproterenol. Increments in regional flows normalized for change in heart rate-systolic product were 91 +/- 28% (S.D.) in normals, 71 +/- 44% in normal regions in diseased hearts, 58 +/- 33% in regions supplied by 50% to 70% stenosed arteries, 35 +/- 32% in regions supplied by arteries stenosed by more than 70%, and 16 +/- 27% in regions supplied by collateral vessels only. In 3 of 14 regions perfused entirely via collateral pathways, regional perfusion decreased with isoproterenol. Therefore the extent to which coronary flow reserve estimated with isoproterenol was compromised varied directly with the severity of anatomic coronary artery lesions and, in some regions entirely perfused by collateral vessels, an increase in heart rate-systolic pressure product was accompanied by a decrease in perfusion below resting levels. It is concluded that collateral vessels effectively maintain flow rates at normal levels at rest but tend to be inefficient at delivering blood when myocardial oxygen demand is increased.

Changes in coronary artery disease five years after coronary bypass surgery

Seventy-nine patients underwent repeat coronary angiographies five years after coronary bypass surgery. Ninety-eight of 122 inserted grafts (80%) were patent. Significant coronary obstruction (greater than 50% reduction of luminal diameter) developed in 43/79 patients (54%) and was associated with a longer duration of angina before surgery and a lower diastolic blood pressure at the five-year follow-up, but significantly related to such factors as age, sex, type of angina, previous myocardial infarction, hypertension, hyperlipaemia, diabetes or smoking. The total number of significant obstructions increased from 230 to 308 (34%). Progression of pre-existing changes to occlusion was common and the number of occlusions increased 95% in non-grafted arteries compared with 48% in grafted arteries until the five-year evaluation. Fifty-seven of 81 new significant obstructions (70%) were found in non-grafted coronary arteries. The proximal part of the right coronary artery was most commonly affected with 19/57 (33%) of these new obstructions. A significant stenosis regressed in three patients. At the five-year follow up, 74/79 patients (94%) had less symptoms than before operation and 27/79 patients (34%) were asymptomatic. Nine patients had no angina, despite non-bypassed significant obstructions. All grafts were patent in 25/27 asymptomatic patients (93%) and in 38/52 (73%) of those with angina. Two patients had no anginal symptoms, despite occluded grafts. One had sustained a myocardial infarction and the other had symptoms of left ventricular failure. Well-developed collateral vessels were observed in 15/27 asymptomatic patients (56%) and in 45/52 (87%) of those with angina. Recurrence of symptoms was related to progressive coronary disease, graft occlusions, obstruction of anastomoses, non-bypassed obstruction or combinations of these changes.

Effect of isoproterenol on regional myocardial perfusion and tissue oxygenation in acute myocardial infarction

The effect of isoproterenol infusion on regional myocardial perfusion and tissue oxygenation during acute myocardial infarction was investigated in anesthetized dogs. Measurements of regional flow with radioactive microspheres and myocardial lactate and adenosine triphosphate from analysis of myocardial biopsies were compared in normal, marginal, and infarcted tissue in dogs with a ligated coronary artery. After 10 minutes of isoproterenol 0.15 microgram/Kg./minute, flow was unchanged in the marginal and infarcted regions, and, although rises occurred in most dogs, changes were inconsistent in the normal regions. In the marginal regions, tissue lactate rose by 5.6 mumoles/g (97 per cent) and adenosine triphosphate fell by 2.4 mumoles/g (46 per cent) after isoproterenol. No consistent changes occurred in the normal or infarcted regions of the dogs given isoproternol or in any regions of control dogs given saline. It is concluded that beta-adrenergic stimulation with isoproterenol increases tissue ischemia in experimental acute myocardial infarction.

Assessment of coronary stenoses with myocardial perfusion imaging during pharmacologic coronary vasodilatation. IV. Limits of detection of stenosis with idealized experimental cross-sectional myocardial imaging

Because atherosclerosis may be reversible, a routine noninvasive screening test for the reliable diagnosis of mild coronary arterial lesions would allow potential prevention of coronary events in specific patients through intensive dietary management, drug therapy and physical training. To determine the minimal coronary stenosis detectable with myocardial perfusion imaging techniques, standardized stenoses ranging from 31.4 to 72.5 percent diameter narrowing were applied to the left circumflex coronary artery of 12 open chest dogs. Indium-113m-labeled human albumin microspheres were injected into the left atrium under control conditions and technetium-99m human albumin microspheres during maximal coronary vasodilatation induced with intravenous dipyridamole. Hearts were removed, sliced into 1 cm thick cross sections and imaged under a gamma camera. The results demonstrate that 40 percent diameter coronary stenoses can be identified by imaging relative subendocardial underperfusion during pharmacologic coronary vasodilatation. An imaging technique sensitive enough to identify mild coronary lesions for diagnostic screening purposes requires (1) a potent stimulus for coronary vasodilatation, such as intravenous dipyridamole; (2) an imaging agent taken up by the myocardium in proportion to coronary flow at flow rates up to four or more times resting coronary flow so that differences in regional maximal flows caused by mild stenoses can be identified; and (3) cross-sectional tomographic myocardial imaging to visualize relative endocardial-epicardial perfusion, the most sensitive indicator of the hemodynamic effects of coronary stenoses, and to exclude from the imaging field the interfering activity of lung, background and overlying heart structures.

Low serum dopamine beta-hydroxylase activity. A marker of congestive heart failure

To gain information about the nature of disturbances in sympathetic nervous system control in congestive heart failure, serum dopamine beta-hydroxylase (DBH) activity was measured in 30 patients with heart failure of diverse etiologies and 29 healthy normotensive controls. The heart failure patients had been symptomatic for at least 6 wk and had elevated filling pressures, low cardiac indices, low ejection fractions, and wide arteriovenous oxygen differences. DBH activity was 47.1+/-4.7 (mean+/-SE) for the controls and 14.4+/-2.7 IU for the heart failure patients (P < 0.001). Sera from some patients with heart failure had potent inhibitory effects on DBH activity of normal sera. The inhibitor was heat stable and dialyzable and could be demonstrated despite presence of N-ethylmaleimide or Cu(++) in the reaction mixture. However, some inhibitory activity was also present in sera of normal patients; this inhibitory property was not demonstrable in unheated normal serum, but was unmasked when DBH was heat inactivated. It is proposed that although the inhibitor may have been a factor in low serum DBH activity in some patients with heart failure, the major cause of the low activity in the heart failure group was a reduced rate of synthesis or release of the enzyme by sympathetic nerves. This may reflect a dissociation between rates of neural release of norepinephrine and release of DBH in chronic, severe heart failure. The observation of low serum DBH levels in patients with heart failure suggests that measurement of DBH levels may serve as a useful indicator of cardiac dysfunction.

Studies of regional myocardial blood flow: results in patients with left anterior descending coronary artery disease

Measurements of regional myocardial blood flow have been performed rapidly and safely in man at the time of coronary arteriography. Xenon-133 was injected into the coronary artery and estimates of myocardial capillary perfusion were made by computer analysis of the multiple precordial tracer washout curves recorded with a scintillation camera. Blood flow rates/100 g/min in different areas of the heart were calculated by the Schmidt-Kety formula, using an assumed partition coefficient, and were related to the coronary vascular lesions by landmarks provided by the arteriograms. The flow significance of lesions was assessed by making measurements at rest and during interventions that increase coronary blood flow. Perfusion was also estimated in areas supplied by collateral vessels, and regions of scar were detected from the initial distribution of peak tracer activity. In patients with less than 50% obstructions apparent on coronary arteriograms and in those with greater than 50% isolated left anterior descending lesions average mean left ventricular (LV) myocardial perfusion values were not reduced below values found in patients with normal coronary arteries and normal cardiac function. However, in patients with significant two-vessel disease (LAD + right, LAD + circ) mean LV perfusion was significantly reduced. Average regional myocardial perfusion distal to significant lesions was not selectively reduced below the remainder of the ventricle unless the lesions were 100% obstructions. However, both total and distal regional myocardial perfusion increased less in 12 patients with greater than 50% lesions than in 12 others with normal arteriograms or less than 50% lesions. Subnormal flow responses in all of the former group were associated with angina pectoris compared with no angina in the latter. The studies indicate the potential utility of the measurements of regional myocardial blood flow in the evaluation of patients with ischemic heart disease who are potential candidates for myocardial revascularization procedures.

Influence of dobutamine on hemodynamics and coronary blood flow in patients with and without coronary artery disease

The influence of dobutamine on hemodynamics and coronary blood flow was studied in patients after routine cardiac catheterization. The data demonstrated that dobutamine is a powerful inotropic agent at a dose that has a relatively small influence on heart rate. In patients without coronary artery disease dobutamine greatly increased coronary arterial perfusion. In patients with severe coronary artery diseases dobutamine resulted in a much smaller increase in coronary perfusion, and the pattern of perfusion became more inhomogeneous. The results suggest that dobutamine has a potential inotropic value but raise concern about its influence on regional myocardial perfusion in patients with serious coronary artery disease.

The relationship between regional myocardial perfusion at rest and arteriographic lesions in patients with coronary atherosclerosis

Measurements of mean left ventricular (LV) and regional myocardial blood flow rates were made at rest in 161 patients with 133Xe and a multiplecrystal scintillation camera. Myocardial perfusion rates were correlated with assessments of the degree of coronary artery disease made from the arteriograms obtained during the same studies. In patients with normal coronary arteries without heart failure, the presence of hypertension, aortic stenosis, or aortic insufficiency was not associated with changes in mean LV perfusion from the control value of 61+/-7 ml/100 g-min. However, mean LV perfusion was significantly reduced in patients with normal coronary arteries who had cariomyopathy and impaired ventricular performance. Mean LV perfusion was not significantly different from control values in patients with "mild" coronary artery disease (less than 50% obstruction) or in patients with significant isolated disease (greater than 50% obstruction) of the left anterior descending (lad) artery. Significant reductions in mean LV perfusion were found in patients with greater than 50% obstruction of two coronary arteries (LAD + right or LAD + circumflex) and in patients with triple-vessel disease. The average perfusion rate for regions distal to LAD obstructions in patients with isolated LAD disease was not lower than the LAD perfusion in control patients, but was significantly reduced in patients with LAD + right coronary artery disease (43+/-14 ml/100 g-min). In the latter group average perfusion distal to the LAD lesion was significantly lower than the average regional perfusion rate for the remainder of the LV. However, the mean blood flow rate for the remainder of the LV was also significantly lower than control values despite the lack of significant circumflex disease. The data demonstrate that the presence of radiographically "mild" or significant isolated LAD coronary disease is not associated with reductions in mean LV perfusion at rest, but that mean LV perfusion is reduced in the presence of significant disease of two or three coronary artieries. None of the patients experienced angina during the resting studies and most had clinical evidence of ventricular failure. The observation of depressed LV perfusion in this group, as in the patients with cardiomyopathy, raises the possibility that a lowered resting blood supply may be adequate for a reduced level of performance of a diseased ventricle. The lack of selective reductions of regional perfusion at rest in the majority of the patients with LAD lesions suggests that regional myocardial blood flow must be measured during an intervention which increases myocardial oxygen consumption in order to assess the physiological significance of lesions which are observed at coronary arteriography.

[Value and limitations of coronary blood flow measurement in man (author's transl)]

Attempts to measure coronary blood flow in man have made considerable progress during the last 25 years. The major techniques are based on the direct or indirect Fick principle; coronary flow is calculated from the arterio-coronary venous difference of inert gases or from the precordial recorded disappearance curve of radioactive gases or substances. The accuracy of the techniques depends upon the properties or the indicators used and the precision of their determination. All techniques applied hitherto are intricate and unsuitable for general use. -A lot of information is obtained about coronary circulation in health and disease by coronary flow measurements in man. Further studies in this field may influence pathophysiological and clinical concepts especially concerning coronary heart disease.

Myocardial imaging with indium- 113m- and technetium-99m-macroaggregated albumin. New procedure for identification of stress-induced regional ischemia

Regional coronary blood flow distribution was studied by myocardial imaging after intracoronary injection of technetium-99m- and indium-113m-macroaggregated albumin at rest and during coronary hyperemia induced by intracoronary injection of Hypaque-M, 75 percent. The four- to five-fold increase in coronary flow after injection of radiographic contrast material was similar in magnitude to that occurring with maximal exercise stress. Experimentally, resting coronary blood flow and regional distribution of radioactive particles remains normal in spite of coronary stenosis of up to 85 percent. Less severe stenosis causes flow and distribution abnormalities only during periods of increased flow, and the degree of maldistribution is directly related to the physiologic severity of the stenosis. Of 49 patients with suspected coronary artery disease, 10 had no significant lesions by coronary arteriography and all had normal images at rest and during coronary hyperemia. Thirty-seven of 39 patients with significant obstructive coronary artery disease had abnormal images at rest or during contrast agent-induced hyperemia, or both. Patients with significant coronary artery disease without previous infarction usually demonstrated abnormalities in flow distribution only during coronary hyperemia. Patients with previous infarction demonstrated resting perfusion abnormalities that often became more abnormal during hyperemia. This technique provides a new method for assessing the physiologic effects of coronary stenosis in conjuntion with coronary arteriography.