Effects of cardiac contraction and cavity pressure on myocardial blood flow

Regional impairment of cardiac contraction uncouples force generation from left ventricular pressure (LVP) and may alter the determinants of the phasic pattern and transmural distribution of coronary flow. In anesthetized, open-chest dogs with maximal coronary vasodilation, we studied the effects of abolishing local contraction and changing cavity pressure on phasic myocardial inflow and net transmural flow in a region of left ventricular free wall. With contraction present, the normalized amplitude of distal phasic coronary velocity (NAmp) was not significantly different at normal vs. low LVP (1.00 vs. 0.92 +/- 0.09, respectively, intracoronary lidocaine, however, NAmp varied with LVP (1.62 +/- 0.25 at normal LVP, 0.85 +/- 0.22 at low LVP, P < 0.0001). With contraction present, inner-to-outer flow ratio was not consistently different at normal vs. low LVP (0.47 +/- 0.15 vs. 0.64 +/- 0.28, respectively, P = NS) but was consistently higher at low than at normal LVP with contraction absent (1.01 +/- 0.30 vs. 1.84 +/- 0.38, respectively, P < 0.0001). During uniform global function, contraction is the main determinant of phasic amplitude and transmural distribution of myocardial flow. When regional contraction is abolished, allowing passive deformation of the wall during systole, LVP assumes a powerful role.

Alterations of phasic coronary artery flow velocity in humans during percutaneous coronary angioplasty

BACKGROUND AND OBJECTIVES

Studies using Doppler catheters to assess blood flow velocity and vasodilator reserve in proximal coronary arteries have failed to demonstrate significant improvement immediately after coronary angioplasty. Measurement of blood flow velocity, flow reserve and phasic diastolic/systolic velocity ratio performed distal to a coronary stenosis may provide important information concerning the physiologic significance of coronary artery stenosis. This study was designed to measure these blood flow velocity variables both proximal and distal to a significant coronary artery stenosis in patients undergoing coronary angioplasty.

METHODS

A low profile (0.018-in.) (0.046-cm) Doppler angioplasty guide wire capable of providing spectral flow velocity data was used to measure blood flow velocity, flow reserve and diastolic/systolic velocity ratio both proximal and distal to left anterior descending or left circumflex coronary artery stenosis. These measurements were made in 38 patients undergoing coronary angioplasty and in 12 patients without significant coronary artery disease.

RESULTS

Significant improvement in mean time average peak velocity was noted in distal coronary arteries after angioplasty (before 19 +/- 12 cm/s; after 35 +/- 16 cm/s; p less than 0.01). Increases in proximal average peak velocity after angioplasty were less remarkable (before 34 +/- 18 cm/s; after 41 +/- 14 cm/s; p = 0.04). Mean flow reserve remained unchanged after angioplasty both proximal (1.5 +/- 0.5 vs. 1.6 +/- 1; p greater than 0.10) and distal (1.6 +/- 1 vs. 1.5 +/- 0.8; p greater than 0.10) to a coronary stenosis. Before angioplasty, mean diastolic/systolic velocity ratio measured distal to a significant stenosis was decreased compared with that in normal vessels (1.3 +/- 0.5 vs. 1.8 +/- 0.5; p less than 0.01). After angioplasty, distal abnormal phasic velocity patterns generally returned to normal, with a significant increase in mean diastolic/systolic velocity ratio (1.3 +/- 0.5 vs. 1.9 +/- 0.6; p less than 0.01). Phasic velocity patterns and mean diastolic/systolic velocity ratio measured proximal to a coronary stenosis were not statistically different from values in normal vessels (1.8 +/- 0.8 vs. 1.8 +/- 0.5; p greater than 0.10) and did not change significantly after angioplasty (1.8 +/- 0.8 vs. 2.13 +/- 0.9; p greater than 0.10).

CONCLUSIONS

Flow velocity measurements may be performed distal to a coronary stenosis with the Doppler guide wire. Phasic velocity measurements made proximal to a coronary stenosis differed from those in the distal coronary artery. Both proximal and distal flow reserve measurements made immediately after angioplasty were of limited utility. Changes in distal flow velocity patterns and diastolic/systolic velocity ratio appeared to be more relevant than the hyperemic response in assessing the immediate physiologic outcome of coronary angioplasty.

Does systolic subepicardial perfusion come from retrograde subendocardial flow?

To examine the influence of cardiac contraction on systolic coronary flow and transmural blood flow distribution, we measured phasic blood flow velocity in distal extramural coronary arteries by Doppler velocimeter and regional myocardial blood flow by radiolabeled microspheres while the heart was beating and during prolonged diastoles in 12 dogs. A servo-controlled coronary perfusion circuit allowed mean coronary pressure to be selected and maintained during beating and diastolic conditions. In epicardial arteries just proximal to their entrance into the myocardium, blood flow was either negligible or reverse in direction during systole. When the heart was beating, subepicardial blood flow was 24.2 +/- 12.3% higher than during asystole (5.05 +/- 0.91 and 4.11 +/- 0.79 ml.min-1.g-1 for beating and prolonged diastoles, respectively; P less than 0.01). In the subendocardium, flow was 49.8 +/- 14.7% lower in the beating condition than during prolonged diastoles (4.23 +/- 1.46 and 8.26 +/- 1.71 ml.min-1.g-1 for beating and asystole, respectively; P less than 0.01). When heart rate was increased stepwise from 60 to 150 beats/min, subendocardial flow fell approximately linearly; flow to the superficial layer was relatively unaffected. In beating hearts, lowering mean left main coronary artery (LMCA) pressure from 80 to 50 mmHg resulted in more systolic reverse flow and a fall in inner-to-outer flow ratio from 0.82 +/- 0.18 to 0.66 +/- 0.34 (P less than 0.05). Because mean LMCA pressure was held constant when the heart was stopped, differences in regional blood flow between beating and diastolic conditions were primarily due to cardiac contraction. Because little or no blood entered the myocardium from the extramural arteries during systole, we conclude that the decrease in subendocardial flow and the increase in subepicardial flow were caused by retrograde pumping of blood from the deep layer to the superficial layer of the left ventricle. Systolic retrograde flow to the subepicardium may help explain this layer's relative protection from ischemia.

Effect of intracoronary nitroglycerin administration on phasic pattern and transmural distribution of flow during coronary artery stenosis

BACKGROUND

Nitroglycerin is effective in relieving myocardial ischemia; however, intracoronary nitroglycerin often fails to relieve angina and has been reported to have deleterious effects on subendocardial blood flow. To understand the mechanisms involved, we evaluated the direct effect of nitroglycerin on coronary circulation of the ischemic hearts.

METHODS AND RESULTS

We measured the phasic pattern of intramyocardial coronary arterial flow with an 80-channel, 20-MHz pulsed Doppler ultrasound flowmeter under moderate to severe coronary artery stenosis (distal perfusion pressure approximately 45 mm Hg group 1, n = 6) and transmyocardial blood flow distribution using radioactive microspheres while maintaining coronary pressure at a low constant level (40 mm Hg, group 2, n = 6). In anesthetized open-chest dogs, the left main coronary artery was perfused directly from the right carotid or femoral artery. In this bypass circuit, pressure was controlled with an occluder or a reservoir was connected to the circuit. In group 1, the systolic and diastolic pressures distal to the stenosis decreased significantly after intracoronary administration of nitroglycerin at maximal coronary flow from 66.5 +/- 18.5 to 56.5 +/- 13.8 mm Hg (p less than 0.01) and from 36.6 +/- 14.4 to 27.5 +/- 8.9 mm Hg (p less than 0.01), respectively. The phasic pattern of the septal artery flow was predominantly diastolic and was characterized by systolic reverse flow even in the absence of stenosis. Coronary stenosis increased systolic reverse flow. Nitroglycerin increased diastolic forward flow (p less than 0.05) but augmented systolic reverse flow markedly (p less than 0.001). In group 2, nitroglycerin increased subepicardial flow (p less than 0.05) but failed to increase subendocardial flow. With the administration of nitroglycerin, the subendocardial-to-subepicardial flow ratio decreased significantly from 0.73 +/- 0.19 to 0.32 +/- 0.14 (p less than 0.01).

CONCLUSIONS

The increased systolic reverse flow after intracoronary administration of nitroglycerin may be closely related to failure of subendocardial blood flow to increase with increase subepicardial flow.

Validation of a Doppler guide wire for intravascular measurement of coronary artery flow velocity

BACKGROUND

An improved intravascular ultrasonic Doppler device could aid the clinical assessment of coronary hemodynamics. We evaluated a new device consisting of a 12-MHz piezoelectric transducer integrated onto the tip of a 0.018-in. flexible, steerable angioplasty guide wire.

METHODS AND RESULTS

Doppler spectra were recorded in model tubes with pulsatile blood flow and in-line electromagnetic flowmeter. In four straight tubes (i.d., 0.79-4.76 mm), the time average of spectral peak velocity (APV) was linearly related to blood flow (QEMF) (r2 greater than or equal to 0.98 for each tube). A Doppler-derived quantitative flow estimate (QD) was calculated as the product of vessel cross-sectional area and mean velocity, with mean velocity estimated as 0.5 x APV. The slope of QD versus QEMF for the four tubes was near unity. APV was less accurate in a 7.94-mm straight tube and in tortuous segments. In four dogs, the left circumflex coronary artery (LCx) was perfused from the femoral artery via a cannula with in-line electromagnetic flowmeter. Good-quality signals were obtained in proximal and distal LCx vessels 3.3-1.2 mm in diameter. APV varied linearly with QEMF (r2 greater than or equal to 0.99 in the cannula, r2 = 0.93-0.99 in proximal LCx, and r2 = 0.86-0.99 in distal LCx). QD was calculated by quantitative angiography to determine proximal LCx diameter. For all dogs combined, the slope of QD versus QEMF was 0.95 in the cannula and 0.85 in the proximal LCx.

CONCLUSIONS

The Doppler guide wire measures phasic flow velocity patterns and linearly tracks changes in flow rate in small, straight coronary arteries. It should facilitate measurement of phasic coronary flow velocity during coronary angiography and angioplasty.

Cardiac contraction affects deep myocardial vessels predominantly

To evaluate the roles of intramyocardial forces and systolic ventricular pressure in myocardial flow in the different layers separately, we measured myocardial flow in rabbit hearts during stable systolic contracture with left ventricular pressures of 60 (n = 5) and 0 mmHg (n = 5) and during stable diastolic arrest (n = 5). We also measured the number and size of the intramyocardial vessels after perfusion fixation (systolic arrest, n = 5; diastolic arrest, n = 5). In 25 rabbits, hearts were excised and perfused from the aortic root. Systolic arrest was achieved by perfusion of a low-Ca2+ Tyrode solution containing 2.0 mM Ba2+. Diastolic arrest was achieved by intraventricular injection of 700-1,000 mg pentobarbital sodium and was maintained by perfusion with St. Thomas cardioplegic solution. At perfusion pressure of 100 mmHg, subendocardial flow was lower than subepicardial flow during systolic arrest regardless of left ventricular pressure, whereas during diastolic arrest, subendocardial flow was higher than subepicardial flow. Subendocardial-to-subepicardial flow ratios for a physiological range of perfusion pressures were lower during systolic arrest with low rather than with high left ventricular pressure. Small arteriolar and capillary densities showed no difference between subendocardium and subepicardium. During systolic arrest, diameters of subendocardial terminal arterioles (4.6 +/- 1.3 microns) and capillaries (4.0 +/- 1.3 microns) were smaller than those in the subepicardium (8.8 +/- 1.7 and 7.1 +/- 1.6 microns, respectively; P less than 0.0001), whereas during diastolic arrest, diameters of subendocardial terminal arterioles (10.1 +/- 2.0 microns) and capillaries (7.6 +/- 1.8 microns) were slightly larger than those in the subepicardium (9.5 +/- 1.5 and 6.7 +/- 1.0 microns, respectively; P less than 0.01). We conclude that cardiac contraction predominantly affects subendocardial vessels and impedes subendocardial flow more than subepicardial flow regardless of left ventricular pressure.

Nonuniform blood flow in the canine left ventricle

In order to investigate the relationship between coronary perfusion pressure and blood flow distribution in the left ventricle (LV), we measured myocardial blood flow in small regions using radioactive microspheres in six anesthetized, open-chest dogs. Mean coronary perfusion pressure (CPP) was controlled with a femoral artery to left main coronary artery shunt which included a pressurized, servo-controlled blood reservoir. In each dog, we measured flow in 192 regions of the LV free wall (mean weight per region = 206 +/- 38 mg) at different perfusion pressures. At CPP = 80 mm Hg, blood flow to individual regions varied fourfold (0.30 to 1.18 ml/min/g; relative dispersion (RD) = 21.8 +/- 2.3%). At CPP = 50 mm Hg, flow varied over sevenfold (0.08 to 0.60 ml/min/g; RD = 42.8 +/- 10%; P less than 0.01 vs 80 mm Hg). This relationship between flow variability and CPP was present within individual LV layers as well between layers and is much higher than the error associated with the microsphere technique. We conclude that blood flow to small regions of the LV is markedly nonuniform. This heterogeneity becomes more profound at lower CPP. These findings suggest that (1) global measurements of coronary flow must be interpreted with caution, and (2) even in hearts with normal coronary arteries some regions of the LV are more susceptible to ischemia than others. In addition, these findings may help explain the patchy nature of myocardial damage that occurs following periods of low coronary pressure or inadequate myocardial protection during cardiopulmonary bypass.