Influence of propranolol, enalaprilat, verapamil, and caffeine on adenosine A(2A)-receptor-mediated coronary vasodilation

The influence of clopidogrel on ischemia diagnosed by myocardial perfusion stress testing

BACKGROUND

Clopidogrel is a platelet adenosine receptor antagonist which can influence coronary vascular tone and thus can potentially interfere with myocardial perfusion imaging. We investigated whether clopidogrel can hamper the diagnosis of ischemia in patients undergoing myocardial perfusion testing.

METHODS

Data from a database of 6349 myocardial perfusion stress tests were analyzed. Using a propensity analysis, patients who were taking clopidogrel were compared with patients not taking clopidogrel for the presence of reversible perfusion defects on myocardial single-photon emission computed tomography scans.

RESULTS

Of the 6349 tests, the stress technique was adenosine in 2713 patients and exercise in 3636. At the time of the stress test, 277 (4.3%) of the patients were taking clopidogrel. The odds ratio (OR) for patients taking clopidogrel to have a reversible perfusion defect was 2.75 (95% confidence interval [CI] 2.09-3.62; P < .01). After adjusting for the propensity to take clopidogrel, the OR was 1.06 (CI 0.76-1.49; P = .73) for patients undergoing adenosine stress tests and 1.60 (CI 0.85-3.00; P = .14) for patients undergoing exercise stress tests.

CONCLUSIONS

We found no evidence that the use of clopidogrel decreases the likelihood of ischemia on adenosine or exercise stress myocardial perfusion scans.

Caffeine Enhances Heart Rate Variability in Middle-Aged Healthy, But Not Heart Failure Subjects

BACKGROUND

In chronic heart failure (CHF) due to left ventricular dysfunction, diminished heart rate variability (HRV) is an independent predictor of poor prognosis. Caffeine has been shown to increase HRV in young healthy subjects. Such an increase may be of potential benefit to patients with CHF.

OBJECTIVE

We hypothesized that intravenous infusion of caffeine would increase HRV in CHF, and in age-matched healthy control subjects.

METHODS

On two separate days, 11 patients (1F) with CHF (age=51.3±4.6 years; left ventricular ejection fraction=18.6±2.7%; mean±standard error) and 10 healthy control subjects (age=48.0±4.0) according to a double-blind randomization design, received either saline or caffeine (4 mg/kg) infusion. We assessed HRV over 7 minutes of supine rest (fast Fourier Transform analysis) to determine total spectral power as well as its high-frequency (HF) (0.15-0.50 Hz) and low-frequency (LF) (0.05-0.15 Hz) components, and recorded muscle sympathetic nerve activity (MSNA) directly from the peroneal nerve (microneurography).

RESULTS

In healthy control subjects, compared with saline, caffeine reduced both heart rate and sympathetic nerve traffic (p≤0.003) and increased the ratio of HF/total power (p≤0.05). Baseline LF power and the ratio LF/HF were significantly lower in CHF compared with controls (p=0.02), but caffeine had no effect on any element of HRV.

CONCLUSIONS

Caffeine increases cardiac vagal heart rate modulation and reduces MSNA in middle-aged healthy subjects, but not in those with CHF.

The CAM-LDPI method: a novel platform for the assessment of drug absorption

OBJECTIVES

This study aimed to explore the use of the chicken chorioallantoic membrane (CAM) with laser doppler perfusion imaging (LDPI) as a platform to assess absorption of vasoactive drugs.

METHODS

The optimal age of the CAM to be employed in the test and the indicator of vasoactivity were first established. Test substances that included common solvents and vasoactive drugs were tested on the CAM surface to determine their irritancy and blood perfusion effects.

KEY FINDINGS

Insignificant changes in blood perfusion were observed with deionized water, 0.9% w/v soldium chloride and 5% w/v glucose monohydrate, as well as theophylline and glucagon. Complex changes in blood perfusion were detected with ethanol, N-methyl-2-pyrrolidone, glycerin and propranolol. Both caffeine and glyceryl trinitrate resulted in a drop in blood perfusion.

CONCLUSIONS

It was concluded that the LDPI offers a rapid and non-invasive method to measure blood perfusion in the CAM. The latter provides a potentially useful platform in formulation studies to evaluate the effects of additives on drug absorption using caffeine or glyceryl trinitrate as model drugs.

Comparison of positron emission tomography measurement of adenosine-stimulated absolute myocardial blood flow versus relative myocardial tracer content for physiological assessment of coronary artery stenosis severity and location

OBJECTIVES

This study tests the hypothesis that absolute measurement of adenosine (Ado)-stimulated myocardial blood flow (MBFado) is superior to measurement of relative tracer uptake for identification of hemodynamically significant coronary artery disease (CAD).

BACKGROUND

Positron emission tomography measurement of absolute myocardial blood flow (MBF) ((13)N-ammonia) with Ado has the capability to more accurately assess hemodynamic severity of CAD than measurement of relative tracer content (TC) (nCi/ml) during Ado, which by definition depends on at least 1 normal zone to which others are compared.

METHODS

A total of 27 patients (20 male, 58 +/- 11 years, mean +/- SD) with known or suspected CAD and 21 normal subjects (13 male, 38 +/- 10 years) were studied. Parametric (K1) MBF images and TC sum images were analyzed. A stenosis > or =70% defined significant CAD. The receiver-operator characteristic curve (ROC) analysis area under the curve (AUC) compared MBF and TC results. Cut-point analysis for sensitivity, specificity, and accuracy showed the best MBF criteria for CAD as MBFado <1.85 ml/min/g and the best TC as <70% maximum. The myocardial blood flow reserve ratio (MBFR) (optimal <2.0x) also was studied.

RESULTS

The ROC analysis of PET parameters showed that MBFado was superior to <70% maximum uptake for CAD detection (n = 144 vessels; AUC 0.900 vs. 0.690, respectively, p < 0.0001) and was marginally greater than MBFR (0.856; p = 0.10). For CAD cut-point analysis, MBFado accuracy exceeded TC (0.84 vs. 0.72, respectively, p = 0.005), as did sensitivity (0.81 vs. 0.48, respectively; p = 0.001). Specificity of MBFado for CAD classification (0.85) was comparable to TC (0.82; p = NS). Sensitivity, specificity, and predictive accuracy for MBFR were 0.62, 0.85, and 0.79, respectively. The difference in specificity was not significant versus MBFado. However, MBFado was more sensitive than MBFR (p = 0.01). The difference in predictive accuracy was borderline (p = 0.06) in favor of MBFado.

CONCLUSIONS

Measurement of Ado-stimulated absolute MBF is superior to relative measurement of myocardial tracer retention for identification of CAD and can be accomplished with a single MBFado measurement.

High-dose adenosine overcomes the attenuation of myocardial perfusion reserve caused by caffeine

OBJECTIVES

We studied whether an increase in adenosine dose overcomes caffeine antagonism on adenosine-mediated coronary vasodilation.

BACKGROUND

Caffeine is a competitive antagonist at the adenosine receptors, but it is unclear whether caffeine in coffee alters the actions of exogenous adenosine, and whether the antagonism can be surmounted by increasing the adenosine dose.

METHODS

Myocardial perfusion scintigraphy (MPS) was used to assess adenosine-induced hyperemia in 30 patients before (baseline) and after coffee ingestion (caffeine). At baseline, patients received 140 microg/kg/min of adenosine combined with low-level exercise. For the caffeine study, 12 patients received 140 microg/kg/min of adenosine (standard) and 18 patients received 210 microg/kg/min (high dose) after caffeine intake (200 mg). Myocardial perfusion was assessed semiquantitatively and quantitatively, and perfusion defect was characterized according to the presence of reversibility.

RESULTS

Caffeine reduced the magnitude of perfusion abnormality induced by standard adenosine as measured by the summed difference score (SDS) (12.0 +/- 4.4 at baseline vs. 4.1 +/- 2.1 after caffeine, p < 0.001) as well as defect size (18% [3% to 38%] vs. 8% [0% to 22%], p < 0.01), whereas it had no effect on the abnormalities caused by high-dose adenosine (SDS, 7.7 +/- 4.0 at baseline vs. 7.8 +/- 4.2 after caffeine, p = 0.7). There was good agreement between baseline and caffeine studies for segmental defect category (kappa = 0.72, 95% confidence interval: 0.65 to 0.79) in the high-dose group. An increase in adenosine after caffeine intake was well tolerated.

CONCLUSIONS

Caffeine in coffee attenuates adenosine-induced coronary hyperemia and, consequently, the detection of perfusion abnormality by adenosine MPS. This can be overcome by increasing the adenosine dose without compromising test tolerability.

Caffeine Attenuates the Duration of Coronary Vasodilation and Changes in Hemodynamics Induced by Regadenoson (CVT-3146), a Novel Adenosine A2A Receptor Agonist

Effects of caffeine on regadenoson-induced coronary vasodilation and changes in hemodynamics were examined in conscious dogs. Sixteen dogs were chronically instrumented for measurements of coronary blood flow (CBF), mean arterial pressure (MAP), and heart rate (HR). Regadenoson (5 microg/kg, IV) increased CBF from 34 +/- 2 to 191 +/- 7 mL/min. The duration of the 2-fold increase in CBF was 515 +/- 71 seconds. Regadenoson decreased MAP by 15 +/- 2% and increased HR by 114 +/- 14%. Regadenoson-induced maximum increases in CBF were not significantly lower in the presence of caffeine at 1, 2, 4, and 10 mg/kg (2 +/- 3, 0.7 +/- 3, 16 +/- 5, and 13 +/- 8%, respectively; all P > 0.05). Caffeine at 1, 2, 4, and 10 mg/kg significantly decreased the duration of the 2-fold increase in CBF induced by regadenoson by 17% +/- 4%, 48% +/- 8%, 62% +/- 5%, and 82% +/- 5%, respectively (all P < 0.05). Caffeine at 4 and 10 mg/kg significantly attenuated the effects of regadenoson on MAP and HR. The results indicate that 1 to 10 mg/kg caffeine dose-dependently reduced the duration, but not the peak increase of CBF caused by 5 microg/kg regadenoson.

Effect of Caffeine on Ischemia Detection by Adenosine Single-Photon Emission Computed Tomography Perfusion Imaging

OBJECTIVES

The purpose of this research was to study the effect of one cup of coffee taken 1 h before adenosine stress on the results of myocardial perfusion imaging.

BACKGROUND

Caffeine is believed to attenuate the coronary hyperemic response to adenosine by competitive blockade of the A2a receptor. Caffeine is commonly withheld before adenosine single-photon emission computed tomography (SPECT) perfusion imaging so as not to mask ischemia detection.

METHODS

We studied the effect of one 8-oz cup of coffee taken 1 h before adenosine stress in patients who had demonstrable reversible defects on adenosine SPECT perfusion imaging performed while off caffeine.

RESULTS

There were 22 men and 8 women, age 64 +/- 9 years. The blood level of caffeine 1 h after intake was 3.1 +/- 1.6 mg/l. There were two patients with ST-segment depression before and one after caffeine intake (p = NS). The summed stress score (SSS) based on 17 segments (scale of 0 to 3, 3 being normal) was 44 +/- 5 before and 45 +/- 5 after caffeine (p = NS). The summed difference score was 3.8 +/- 1.9 before and. 3.9 +/- 2.3 after caffeine (p = NS), reflecting that around 50% of the perfusion abnormality was reversible before and after caffeine. Using polar maps, the perfusion abnormality was 12 +/- 10% at baseline and 12 +/- 10% after caffeine (p = NS) in agreement with SSS. The left ventricular ejection fraction by gated SPECT was 50 +/- 13% at baseline and 51 +/- 13 % after caffeine (p = NS).

CONCLUSIONS

A cup of coffee does not mask the presence or severity of reversible defects induced by adenosine SPECT imaging.

Adenosine receptor agonists: from basic medicinal chemistry to clinical development

Adenosine is a physiological nucleoside which acts as an autocoid and activates G protein-coupled membrane receptors, designated A(1), A(2A), A(2B) and A(3). Adenosine plays an important role in many (patho)physiological conditions in the CNS as well as in peripheral organs and tissues. Adenosine receptors are present on virtually every cell. However, receptor subtype distribution and densities vary greatly. Adenosine itself is used as a therapeutic agent for the treatment of supraventricular paroxysmal tachycardia and arrhythmias and as a vasodilatatory agent in cardiac imaging. During the past 20 years, a number of selective agonists for A(1), A(2A) and A(3) adenosine receptors have been developed, all of them structurally derived from adenosine. Several such compounds are currently undergoing clinical trials for the treatment of cardiovascular diseases (A(1)and A(2A)), pain (A(1)), wound healing (A(2A)), diabetic foot ulcers (A(2A)), colorectal cancer (A(3)) and rheumatoid arthritis (A(3)). Clinical evaluation of some A(1) and A(2A) adenosine receptor agonists has been discontinued. Major problems include side effects due to the wide distribution of adenosine receptors; low brain penetration, which is important for the targeting of CNS diseases; short half-lifes of compounds; or a lack of effects, in some cases perhaps due to receptor desensitisation or to low receptor density in the targeted tissue. Partial agonists, inhibitors of adenosine metabolism (adenosine kinase and deaminase inhibitors) or allosteric activators of adenosine receptors may be advantageous for certain indications, as they may exhibit fewer side effects.

Building a Better Fluid for Emergency Resuscitation of Traumatic Brain Injury

Hextend (HEX) is a colloid solution that is FDA-approved for volume expansion during surgery. ATL-146e is a novel adenosine A2A receptor agonist that has anti-inflammatory, neuroprotective, and coronary vasodilator properties. Three series of experiments were designed to evaluate the therapeutic potential of HEX+/-ATL-146e for emergency resuscitation from traumatic brain injury (TBI) + hemorrhagic hypotension.

METHODS

In the first two studies in vivo, anesthetized, ventilated pigs (30-45 kg) received a fluid percussion TBI, 45% arterial hemorrhage, and 30 minutes shock period. In Series 1, resuscitation consisted of unlimited crystalloid (n = 8) or HEX (n = 8) to correct systolic arterial pressure >100 mm Hg and heart rate <100 bpm for the first 60 minutes ("emergency phase"), and then maintain cerebral perfusion pressure (CPP) > 70 mm Hg for 60-240 minutes. In Series 2 (n = 31), resuscitation consisted of a 1 L bolus of HEX + ATL-146e (10 ng/kg/min, n = 10) or HEX +placebo (n = 10) followed by crystalloid to the same endpoints. In Series 3 in vivo, the hemodynamic response evoked by 0, 10, 50, or 100 ng/kg/min ATL-146e was measured before or 60 minutes after HEX resuscitation from 45% hemorrhage.

RESULTS

Following TBI+hemorrhage, there were 4/22 deaths in series 1 and 11/31 deaths in series 2. In those alive at 30 minutes, mean arterial pressure, cardiac index, mixed venous O2 saturation, and cerebral venous O2 saturation were all reduced by 40-60%, while heart rate and lactate were increased 2-5 fold. With no resuscitation (n = 2), there was minimal hemodynamic compensation and progressive acidosis. Upon resuscitation, these values corrected but intracranial pressure progressively rose from <5 mm Hg to 15-20 mm Hg. Series 1: With HEX (n = 8) versus crystalloid (n = 8), CPP was less labile, acid/base was maintained, and the fluid requirement was reduced by 60% (all p < 0.05) Series 2: With ATL-146e (n = 10) versus placebo (n = 10), stroke volume and cardiac output were improved by 40-60%, and the fluid requirement was reduced by 30% (all p < 0.05). Series 3: ATL-146e caused a dose-related increase (p < 0.05) in stroke volume after, but not before, hemorrhage. The effects on pre-load, afterload, and heart rate were similar before and after hemorrhage.

CONCLUSIONS

HEX alone is a safe and efficacious low volume alternative to initial crystalloid resuscitation after TBI. An adenosine A2A agonist combined with 1 L of HEX safely and effectively counteracted a decrease in cardiac performance noted after TBI+hemorrhage without causing hypotension or bradycardia.

Black tea increases coronary flow velocity reserve in healthy male subjects

Epidemiologic studies suggest that tea consumption decreases the risk for cardiovascular events. However, there has been no clinical report examining the effects of tea consumption on coronary circulation. The purpose of this study was to evaluate the effects of black tea on coronary flow velocity reserve (CFVR) using transthoracic Doppler echocardiography (TTDE). This was a double-blind crossover study of 10 healthy male volunteers conducted to compare the effects of black tea and caffeine on coronary circulation. The coronary flow velocity of the left anterior descending coronary artery was measured at baseline and at hyperemia during adenosine triphosphate infusion by TTDE to determine CFVR. The CFVR ratio was defined as the ratio of CFVR after beverage consumption to CFVR before beverage consumption. All data were divided into 2 groups according to beverage type: group T (black tea) and group C (caffeine). Two-way analysis of variance showed a significant group effect and interaction in CFVR before and after beverage consumption (p = 0.001). CFVR significantly increased after tea consumption in group T (4.5 +/- 0.9 vs 5.2 +/- 0.9, p <0.0001). The CFVR ratio of group T was larger than that of group C (1.18 +/- 0.07 vs 1.04 +/- 0.08, p = 0.002). Acute black tea consumption improves coronary vessel function, as determined by CFVR.

First annual Mario S. Verani, MD, Memorial lecture: clinical value of myocardial perfusion imaging in coronary artery disease

I am honored to give this lecture in memory of Dr Mario S. Verani, a friend and a respected colleague in the field of nuclear cardiology. Dr Verani made many immense contributions to the field of nuclear cardiology during his lifetime. He was a pioneer in the field of pharmacologic stress imaging and played a leading role in validating the utility of intravenous adenosine infusion for stress imaging in detecting significant coronary stenoses and assessing prognosis. He and his colleagues at Baylor reported that pharmacologic stress imaging could separate high- and low-risk patient subsets after acute myocardial infarction, particularly among patients who underwent thrombolytic therapy. He informed us that certain drugs that patients took could influence perfusion defect severity. This work led to a clinical study showing that ischemic defect size could be substantially reduced with medical therapy in patients who had recently had an acute infarction. An outcome of this observation was the launching of the INSPIRE (adenosINe technetium-99m Sestamibi single-photon emission computed tomography Post-InfaRction Evaluation) trial in which postinfarction patients are randomized to medical therapy or revascularization. Dr Verani was an able debater when assigned the position for advocating for stress radionuclide perfusion imaging against stress echocardiography. He engaged in these debates with substantial knowledge but always with a touch of humor. Dr Verani's courage was an inspiration to all of us during his illness. His legacy as a pioneer will endure as the field of nuclear cardiology continues to mature and expand.

The role of pharmacology in spinal cord protection during thoracic aortic reconstruction

Surgery of the thoracic aorta continues to have a significant risk of neurologic complication. Several strategies to minimize this risk are emerging. Pharmacologic protection from these complications continues to be researched, but at this point few medications are being used clinically. This article reviews the pathophysiology of ischemic spinal cord injury and summarizes the investigational pharmacology that may prevent these serious complications.