Stress echocardiography using transesophageal atrial pacing in rats

Cardiac remodeling rather than disturbed myocardial energy metabolism is associated with cardiac dysfunction in diabetic rats

BACKGROUND

Diabetes mellitus (DM) alters the energy substrate metabolism in the heart and the early sign of diabetic cardiomyopathy is the diastolic dysfunction. Although it is known that the extracellular matrix must be altered in the presence of diabetes, its local regulation has not been fully elucidated. Our aim was to evaluate in vivo left ventricular (LV) structure; function and bioenergetics in streptozotocin (STZ) induced diabetes mellitus.

METHODS

Cardiac function was evaluated using echocardiography in anesthetized Sprague–Dawley rats 12 weeks after injection of STZ and in age-matched control rats before and after atrial pacing. In vivo ³¹P magnetic resonance spectroscopy was done to measure the phosphocreatine (PCr) to ATP ratio. Myocardial protein expression of metalloproteinases MMP-2, -9, tissue inhibitor TIMP-1, -2 and collagen was measured using Western blot.

RESULTS

Bodyweight (BW) was decreased in diabetic rats. Heart weight/BW and LV mass/BW ratios were higher in diabetic animals compared to controls (2.3 ± 08 vs 2.1 ± 08 mg/g p <0.05). Heart rate was lower in diabetic rats (293 ± 20 vs 394 ± 36 bpm p <0.05). The velocity of circumferential shortening and peak aortic velocity were lower in diabetic animals and were more pronounced during atrial pacing. The basal PCr/ATP ratio was not different in the two groups. Total collagen was higher in diabetic rats (3.8 ± 0.3 vs 2.9 ± 01 mg/g, p <0.05). Protein expression of MMP-2 was significantly diminished in diabetic rats by ≈ 60%, while MMP-9, TIMP-1 and -2 were unchanged.

CONCLUSION

Streptozotocin induced diabetes led to increased LV/bodyweight, increased collagen content, and diminished MMP-2 with no change in PCr/ATP. Therefore, remodeling rather than disturbed energetics may underlie diabetic cardiomyopathy.

In vivo effects of myocardial creatine depletion on left ventricular function, morphology, and energy metabolism--consequences in acute myocardial infarction

BACKGROUND

The failing heart is characterized by disturbed myocardial energy metabolism and creatine (Cr) depletion. The aims of this study were to in vivo evaluate the effects of Cr depletion on: a) left ventricular (LV) function and morphology during rest and stress, b) LV energy metabolism, c) catecholamine in LV and plasma content, and d) incidence of malignant ventricular arrhythmias (MVA) during acute myocardial infarction (MI).

METHODS AND RESULTS

Male rats weighing approximately 200 g were used. Two groups were studied: the rats treated with Cr analogue beta-guanidinopropionic acid (BGP) (n = 25) and controls (n = 23). BGP (1 M) was administered by subcutaneously implanted osmotic minipumps over 4 weeks. The rats (BGP n = 9, control n = 12) were than examined with transthoracic echocardiography at basal and at stress conditions induced by transesophageal pacing. In vivo (31)P magnetic resonance spectroscopy (MRS) was used for evaluation of myocardial energy status (BGP n = 7, control n = 12). (31)P MRS, echocardiography and high-performance liquid chromatography analysis of myocardial Cr, total adenine nucleotides and catecholamines in myocardium and plasma were performed on noninfarcted hearts. Myocardial infarction was induced in a subgroup of animals (BGP n = 15, control n = 15) by ligation of the left coronary artery resulting in a large ( approximately 50%) anterolateral MI and acute HF. A computerized electrocardiogram tracing was obtained continuously before induction of MI and up to 60 minutes postinfarction. Qualitative and quantitative variables of ventricular arrhythmias were analyzed using arrhythmia score. Body weight (BW) was lower (P < .01), whereas LV/BW was higher (P < .01) in the BGP group. Total myocardial Cr pool was decreased for at least 50% (P < .01) compared with the controls. There was no difference in total nucleotide pool. Phosphocreatine/adenosine-3-phosphate ratio was lower in the BGP group (P < .01). LV systolic function was disturbed during rest and stress (P < .05). Similarly, LV dimensions were increased in the BGP group (P < .05). Induction of acute MI resulted in markedly increased incidence of MVA and higher mortality in the BGP group (P < .01).

CONCLUSIONS

Myocardial Cr depletion results in functional and structural LV alterations associated with lower myocardial energy reserve. Intact myocardial Cr metabolism is important for normal LV function during basal and stress conditions. Acute MI in the setting of myocardial Cr depletion leads to excessive mortality from ventricular arrhythmias and progressive heart failure.

Immediate Functional Effects of Left Ventricular Reduction: A Doppler Echocardiographic Study in the Rat

BACKGROUND

Immediate functional effects of left ventricle reduction (LVR) are not yet fully defined. Those effects have been studied in the experimental model of myocardial infarction scar plication (MISP) in the rat.

METHODS AND RESULTS

A Doppler echocardiogram was performed immediately before and after MISP in 20 rats with infarction of the left ventricle (LV) larger than 40%. LV diastolic volume reduction (475 +/- 114 versus 185 +/- 65 muL) was accompanied by heart rate decrease (230 +/- 25 versus 166 +/- 27 beats/min) and increase of ejection fraction (37 +/- 7 versus 67 +/- 12%), fractional shortening (18 +/- 3 versus 46 +/- 8%) and posterior wall shortening velocity (1.50 +/- 0.62 versus 2.01 +/- 0.46 cm/s). LV diastolic volume/stroke volume slope was steeper after LVR, suggesting enhancement of the Frank-Starling mechanism. Restrictive pattern of left atrial emptying was alleviated after LVR (E wave: 101 +/- 15 versus 66 +/- 14 cm/s; E/A ratio: 6.8 +/- 2.9 versus 5.0 +/- 2.2; E wave deceleration time: 36 +/- 6 versus 51 +/- 10 msec) even though left atrial diameter (0.69 +/- 0.07 versus 0.66 +/- 0.06 cm) and A wave (18.0 +/- 9.4 versus 15.8 +/- 7.8 cm/s) did not vary. Additionally, a pulmonary flow profile suggesting pulmonary hypertension was observed in 12 of 17 animals before, and in only 3 after, LVR.

CONCLUSION

LVR favors cardiac function not only by reducing afterload. The present data are in consonance with previous suggestions that the Frank-Starling mechanism is enhanced after MISP and, in addition to LV ejection function improvement, the unprecedented facilitation of left atrial emptying after LVR was particularly noteworthy. Even though LVR restricts ventricular distensibility, atrial emptying can be facilitated, probably on account of LV ejection improvement.

Pacing stress echocardiography

Background

High-rate pacing is a valid stress test to be used in conjunction with echocardiography; it is independent of physical exercise and does not require drug administration. There are two main applications of pacing stress in the echo lab: the noninvasive detection of coronary artery disease through induction of a regional transient dysfunction; and the assessment of contractile reserve through peak systolic pressure/ end-systolic volume relationship at increasing heart rates to assess global left ventricular contractility.

Methods

The pathophysiologic rationale of pacing stress for noninvasive detection of coronary artery disease is obvious, with the stress determined by a controlled increase in heart rate, which is a major determinant of myocardial oxygen demand, and thereby tachycardia may exceed a fixed coronary flow reserve in the presence of hemodynamically significant coronary artery disease. The use of pacing stress echo to assess left ventricular contractile reserve is less established, but promising. Positive inotropic interventions are mirrored by smaller end-systolic volumes and higher end-systolic pressures. An increased heart rate progressively increases the force of ventricular contraction (Bowditch treppe or staircase phenomenon). To build the force-frequency relationship, the force is determined at different heart rate steps as the ratio of the systolic pressure (cuff sphygmomanometer)/end-systolic volume index (biplane Simpson rule). The heart rate is determined from ECG.

Conclusion

Two-dimensional echocardiography during pacing is a useful tool in the detection of coronary artery disease. Because of its safety and ease of repeatability noninvasive pacing stress echo can be the first-line stress test in patients with permanent pacemaker.

The force-frequency can be defined as up- sloping (normal) when the peak stress pacing systolic pressure/end-systolic volume index is higher than baseline and intermediate stress values, biphasic with an initial up- sloping followed by a later down-sloping trend, or flat or negative when peak stress pacing systolic pressure/end-systolic volume index is equal or lower than baseline stress values. This approach is certainly highly feasible and allows a conceptually immaculate definition of contractility with prognostic usefulness, but its therapeutic implications remains to be established. Bowditch treppe, assessed with pacing stress, can be used to assess the optimal stimulation frequency and to optimise the patient's chronotropic response in programming rate-adaptive pacemakers.