Matthew Coggins

Noninvasive quantification of coronary blood flow reserve in humans using myocardial contrast echocardiography

Background—We hypothesized that coronary blood flow (CBF) reserve could be quantified noninvasively in humans using myocardial contrast echocardiography (MCE). Methods and Results—Eleven patients with normal epicardial coronary arteries (group I) and 19 with single-vessel coronary stenosis (group II) underwent quantitative coronary angiography, MCE, and CBF velocity measurements at rest and during intravenous adenosine infusion. In group I patients, MCE-derived myocardial blood flow (MBF) velocity reserve (2.4±0.08) was similar to CBF velocity reserve using a Doppler flow wire (2.4±1.1). Patients with a single risk factor had a significantly higher MBF reserve (3.0±0.89) than those with ≥2 risk factors (1.7±0.22). In group II patients, significant differences were found in MBF velocity reserve in patients with mild (<50%), moderate (50% to 75%), or severe (>75%) stenoses (2.2±0.40, 1.6±0.65, and 0.55±0.19, respectively;P =0.005). A linear relation was found between flow velocity reserve determined using the 2 methods (r =0.76, P <0.001), and a curvilinear relation was noted between the percent coronary stenosis measured using quantitative coronary angiography and velocity reserve using both methods. Conclusions—CBF reserve can be measured in humans using MCE. This method may allow the noninvasive assessment of coronary stenosis severity and the detection of microvascular dysfunction.

Quantification of renal blood flow with contrast-enhanced ultrasound

OBJECTIVES The goal of this study was to determine the ability of contrast-enhanced ultrasound (CEU) to quantify renal tissue perfusion. BACKGROUND The kinetics of tracers used to assess renal perfusion are often complicated by countercurrent exchange, tubular transport or glomerular filtration. We hypothesized that, because gas-filled microbubbles are pure intravascular tracers with a rheology similar to that of red blood cells, CEU could be used to quantify renal tissue perfusion. METHODS During a continuous venous infusion of microbubbles (SonoVue), regional renal perfusion was quantified in nine dogs using CEU by destroying microbubbles and measuring their tissue replenishment with intermittent harmonic imaging. Both renal blood volume fraction and microbubble velocity were derived from pulsing-interval versus video-intensity plots. The product of the two was used to calculate renal nutrient blood flow. Renal arterial blood flow was independently measured with ultrasonic flow probes placed directly on the renal artery and was increased using dopamine and decreased by placement of a renal artery stenosis. RESULTS An excellent correlation was found between cortical nutrient blood flow using microbubbles and ultrasonic flow probe-derived renal blood flow (r = 0.82, p < 0.001) over a wide range (2.5 fold) of flows. CONCLUSIONS Ultrasound examination during microbubble infusion can be used to quantify total organ as well as regional nutrient blood flow to the kidney.

Noninvasive Imaging of Inflammation by Ultrasound Detection of Phagocytosed Microbubbles

BACKGROUND We have previously shown that microbubbles adhere to leukocytes in regions of inflammation. We hypothesized that these microbubbles are phagocytosed by neutrophils and monocytes and remain acoustically active, permitting their detection in inflamed tissue. METHODS AND RESULTS In vitro studies were performed in which activated leukocytes were incubated with albumin or lipid microbubbles and observed under microscopy. Microbubbles attached to the surface of activated neutrophils and monocytes, were phagocytosed, and remained intact for up to 30 minutes. The rate of destruction of the phagocytosed microbubbles on exposure to ultrasound was less (P</=0.05) than that of free microbubbles at all acoustic pressures applied. Intravital microscopy and simultaneous ultrasound imaging of the cremaster muscle was performed in 6 mice to determine whether phagocytosed microbubbles could be detected in vivo. Fifteen minutes after intravenous injection of fluorescein-labeled microbubbles, when the blood-pool concentration was negligible, the number of phagocytosed/attached microbubbles within venules was 7-fold greater in tumor necrosis factor-alpha (TNF-alpha)-treated animals than in control animals (P<0.01). This increase in retained microbubbles resulted in a 5- to 6-fold-greater (P<0.01) degree of ultrasound contrast enhancement than in controls. CONCLUSIONS After attaching to activated neutrophils and monocytes, microbubbles are phagocytosed intact. Despite viscoelastic damping, phagocytosed microbubbles remain responsive to ultrasound and can be detected by ultrasound in vivo after clearance of freely circulating microbubbles from the blood pool. Thus, contrast ultrasound has potential for imaging sites of inflammation.

Microbubble persistence in the microcirculation during ischemia/reperfusion and inflammation is caused by integrin- and complement-mediated adherence to activated leukocytes

BACKGROUND Albumin microbubbles that are used for contrast echocardiography persist within the myocardial microcirculation after ischemia/reperfusion (I-R). The mechanism responsible for this phenomenon is unknown. METHODS AND RESULTS Intravital microscopy of the microcirculation of exteriorized cremaster muscle was performed in 12 wild-type mice during intravenous injections of fluorescein-labeled microbubbles composed of albumin, anionic lipids, or cationic lipids. Injections were performed at baseline and after 30 to 90 minutes of I-R in 8 mice and 2 hours after intrascrotal tumor necrosis factor-alpha (TNF-alpha) in 4 mice. Microbubble adherence at baseline was uncommon (<2/50 high-power fields). After I-R, adherence increased (P<0.05) to 9+/-5 and 5+/-4 per 50 high-power fields for albumin and anionic lipid microbubbles, respectively, due to their attachment to leukocytes adherent to the venular endothelium. TNF-alpha produced even greater microbubble binding, regardless of the microbubble shell composition. The degree of microbubble attachment correlated (r=0.84 to 0.91) with the number of adhered leukocytes. Flow cytometry revealed that microbubbles preferentially attached to activated leukocytes. Albumin microbubble attachment was inhibited by blocking the leukocyte beta(2)-integrin Mac-1, whereas lipid microbubble binding was inhibited when incubations were performed in complement-depleted or heat-inactivated serum rather than control serum. CONCLUSIONS Microvascular attachment of albumin and lipid microbubbles in the setting of I-R and TNF-alpha-induced inflammation is due to their beta(2)-integrin- and complement-mediated binding to activated leukocytes adherent to the venular wall. Thus, microbubble persistence on contrast ultrasonography may be useful for the detection and monitoring of leukocyte adhesion in inflammatory diseases.

Role of capillaries in determining CBF reserve: new insights using myocardial contrast echocardiography

To define the role of capillaries in the control of coronary blood flow (CBF) reserve, we developed a model of the coronary circulation and evaluated experimental data in its context. Our model comprised three compartments connected in series (arterial, capillary, and venous), each with its own resistance. The resistance in each vascular compartment was derived from the model based on hemodynamic data obtained in nine dogs during baseline and stenosis, both at rest and during hyperemia. The capillary hydrostatic pressure was assumed to be constant in all stages. Although in the absence of stenosis, the contribution of capillaries to total myocardial vascular resistance was only 25 +/- 5% at rest, it increased to 75 +/- 14% during hyperemia, despite the total myocardial vascular resistance decreasing by 51 +/- 13%. In the presence of a noncritical stenosis, total myocardial vascular resistance decreased by 22 +/- 10% at rest, with no change in capillary resistance. During hyperemia, total myocardial vascular resistance increased by 58 +/- 50% in the presence of the noncritical stenosis. In this situation, because arteriolar and venular resistances were already minimal, the increase in myocardial vascular resistance was due to increased capillary resistance, making it the predominant source (84 +/- 8%) of total myocardial vascular resistance. Myocardial video intensity (VI) on myocardial contrast echocardiography (MCE), which reflects capillary blood volume, decreased distal to the stenosis during hyperemia. In the presence of a flow-limiting stenosis at rest, myocardial VI also decreased, indicating that decrease in CBF was associated with an increase in capillary resistance. Our findings also provide an alternative explanation for the critical coronary closing pressure. Thus, contrary to previously held notions, capillaries play a vital role in the regulation of CBF.

Nitric Oxide in the Pulmonary Vasculature

Homeostasis in the pulmonary vasculature is maintained by the actions of vasoactive compounds, including nitric oxide (NO). NO is critical for normal development of the pulmonary vasculature and continues to mediate normal vasoregulation in adulthood. Loss of NO bioavailability is one component of the endothelial dysfunction and vascular pathology found in pulmonary hypertension (PH). A broad research effort continues to expand our understanding of the control of NO production and NO signaling and has generated novel theories on the importance of pulmonary NO production in the control of the systemic vasculature. This understanding has led to exciting developments in our ability to treat PH, including inhaled NO and phosphodiesterase inhibitors, and to several promising directions for future therapies using nitric oxide-donor compounds, stimulators of soluble guanylate cyclase, progenitor cells expressing NO synthase (NOS), and NOS gene manipulation.

The Fire Within: Cardiac Inflammatory Signaling in Health and Disease

Inflammatory mediators are operative in the pathogenesis of the most common forms of heart disease. Although in most cases the induction of these pathways is maladaptive and deleterious, there are notable exceptions when inflammatory pathways participate in healing or limiting the extent of injury. The appreciation of the role of these mechanisms in myocardial homeostasis and pathophysiology has led to increased efforts to elucidate the specific signaling pathways most relevant to the heart. Our goal in this introductory overview is to provide context for the five detailed reviews that follow by introducing the major relevant stimuli, the receptors, and pathways that mediate inflammatory signaling in the heart. We try to impart a sense of the scope and complexity of these pathways, as well as their interactions with signaling pathways regulating cell survival and metabolism. These complexities underscore the potential challenges of therapeutically targeting inflammatory mechanisms in heart disease and may help explain the mostly disappointing results of this approach to date.

Direct effects of dobutamine on the coronary microcirculation: Comparison with adenosine using myocardial contrast echocardiography

The direct effects of dobutamine on capillary blood volume (VOL) and blood flow velocity (VEL) are not known. We hypothesized that these would be more similar to that of adenosine because of its effects on the beta(2) receptors on the coronary circulation. A total of 9 open-chest anesthetized dogs were studied after placement of 2 noncritical stenoses at rest and during separate intracoronary administrations of 5 microg/kg(-1)/min(-1) of adenosine and 2 microg/kg(-1)/min(-1) of dobutamine. VOL and VEL were measured using myocardial contrast echocardiography, wall thickening with 2-dimensional echocardiography, and myocardial blood flow (MBF) with radiolabeled microspheres. Dobutamine increased the rate-pressure product significantly, whereas adenosine had no effect on the rate-pressure product. In the normal myocardium, adenosine had no effect on VOL and increases in MBF were all a result of increases in VEL. Dobutamine also caused mostly an increase in VEL and only a 30% increase in VOL indicating modest capillary recruitment. In the bed with stenosis both drugs attenuated increase in MBF by the same amount, which was associated with an attenuation in the increase in VEL secondary to a 15% increase in capillary resistance because of capillary derecruitment. The MBF-wall thickening relation was described for both drugs by the same function: y = 1 - exp(x) with wall thickening being significantly higher for dobutamine compared with adenosine for each level of MBF. We conclude that the increase in MBF in the normal myocardium with intracoronary dobutamine occurs mostly from an increase in VEL rather than from an increase in VOL. In the bed with a noncritical stenosis, the increases in MBF and VEL are similar for both drugs. Similar to intracoronary adenosine, intracoronary dobutamine also caused capillary derecruitment distal to a noncritical coronary stenosis.

Mechanism of Inducible Regional Dysfunction During Dipyridamole Stress

Background—We hypothesized that increased myocardial oxygen demand resulting from hypotension and reflex tachycardia unmasking a reduced endocardial myocardial blood flow (MBF) reserve is the mechanism of dipyridamole-induced regional dysfunction in chronic coronary artery disease. Methods and Results—Ameroid constrictors were placed around the proximal coronary arteries and their major branches in 15 dogs to create chronic coronary stenosis. Seven days later, radiolabeled microsphere–derived MBF and 2-dimensional echocardiography–derived percent wall thickening (%WT) were measured at rest and after 0.56 mg/kg dipyridamole. Dipyridamole caused an increase (mean, 21%) in the rate-pressure product secondary to reflex tachycardia resulting from mild systemic hypotension. %WT in myocardial segments with an endocardial MBF reserve (dipyridamole/resting MBF) of 1.5 to 2.5 (n=35) did not change after dipyridamole, whereas it decreased in segments with an endocardial MBF reserve of <1.5 (n=30) and increased in those with an endocardial MBF reserve of ≥2.5 (n=45) (P <0.05). Most (80%) segments with endocardial MBF reserve of <1.5 and 14% with an endocardial MBF reserve of 1.5 to 2.5 showed inducible dysfunction after dipyridamole, whereas none of the segments with an endocardial MBF reserve of ≥2.5 showed this finding. A sigmoid relation (y =−6.74/[1+exp (19.9 · [x −1.84])]+1.35 ·x, r =0.93, P <0.0001) was noted between endocardial MBF reserve and &Dgr;%WT. In contrast, neither the epicardial MBF reserve nor the endocardial/epicardial MBF ratio during hyperemia was associated with inducible regional dysfunction. Conclusions—Increased myocardial oxygen demand resulting from hypotension and reflex tachycardia unmasking a reduced endocardial MBF reserve is the primary mechanism of dipyridamole-induced regional dysfunction in chronic coronary artery disease.

Changes in myocardial blood volume over a wide range of coronary driving pressures: role of capillaries beyond the autoregulatory range

Objective: To determine whether, when the vasomotor capacity of the coronary arterioles is exhausted at rest, myocardial blood volume decreases in order to maintain a normal capillary hydrostatic pressure, even at the expense of myocardial oxygen delivery. Methods: 18 dogs were studied. In group 1 (n  =  9), coronary driving pressure (CDP) was reduced by 10–80 mm Hg below normal by a stenosis; in group 2 (n  =  9), it was increased 20–80 mm Hg above baseline by increasing aortic pressure with phenylephrine. Myocardial contrast echocardiography (MCE) was undertaken to measure the myocardial blood volume fraction and myocardial blood flow (MBF). Results: In group 1 dogs, as CDP was reduced, both coronary blood flow (CBF) and MBF decreased. Myocardial blood volume fraction also decreased and myocardial vascular resistance increased, while coronary sinus Po2 decreased. In group 2 dogs, as CDP was increased, epicardial CBF increased but MBF remained unchanged because of a decrease in myocardial blood volume fraction. Myocardial vascular resistance decreased, however, implying the presence of coronary arteriovenous shunting, which was supported by a progressive increase in the coronary sinus Po2. Conclusions: When arteriolar tone is exhausted so that CBF becomes dependent on CDP, myocardial blood volume decreases in order to maintain a constant capillary hydrostatic pressure, which takes precedence over myocardial oxygen delivery. These novel findings implicate capillaries in the regulation of CBF beyond the autoregulatory range.