Effects of preload, afterload and inotropy on dynamics of ischemic segmental wall motion

Understanding Vulnerable Plaques: Current Status and Future Directions

The main cause of acute myocardial infarction is plaque rupture accompanied by superimposed coronary thrombosis. Thin-cap fibroatheromas (TCFAs) have been suggested as a type of lesion with a vulnerability that can cause plaque rupture. However, not only the existence of a TCFA but also the fine and complex interactions of other anatomical and hemodynamic factors, such as microcalcification in the fibrous cap, cholesterol crystal-induced inflammasome activation, the apoptosis of intraplaque macrophages, and endothelial shear stress distribution should precede a clinical event caused by plaque rupture. Recent studies are being conducted to identify these mechanisms through molecular imaging and hemodynamic assessment using computational fluid dynamics, which will result in better clinical results through selective coronary interventions.

Exaggerated coronary vasoconstriction limits muscle metaboreflex-induced increases in ventricular performance in hypertension

Increases in myocardial oxygen consumption during exercise mainly occur via increases in coronary blood flow (CBF) as cardiac oxygen extraction is high even at rest. However, sympathetic coronary constrictor tone can limit increases in CBF. Increased sympathetic nerve activity (SNA) during exercise likely occurs via the action of and interaction among activation of skeletal muscle afferents, central command, and resetting of the arterial baroreflex. As SNA is heightened even at rest in subjects with hypertension (HTN), we tested whether HTN causes exaggerated coronary vasoconstriction in canines during mild treadmill exercise with muscle metaboreflex activation (MMA; elicited by reducing hindlimb blood flow by ~60%) thereby limiting increases in CBF and ventricular performance. Experiments were repeated after α₁-adrenergic blockade (prazosin; 75 µg/kg) and in the same animals following induction of HTN (modified Goldblatt 2K1C model). HTN increased mean arterial pressure from 97.1 ± 2.6 to 132.1 ± 5.6 mmHg at rest and MMA-induced increases in CBF, left ventricular dP/dt_max, and cardiac output were markedly reduced to only 32 ± 13, 26 ± 11, and 28 ± 12% of the changes observed in control. In HTN, α₁-adrenergic blockade restored the coronary vasodilation and increased in ventricular function to the levels observed when normotensive. We conclude that exaggerated MMA-induced increases in SNA functionally vasoconstrict the coronary vasculature impairing increases in CBF, which limits oxygen delivery and ventricular performance in HTN.

NEW & NOTEWORTHY

We found that metaboreflex-induced increases in coronary blood flow and ventricular contractility are attenuated in hypertension. α₁-Adrenergic blockade restored these parameters toward normal levels. These findings indicate that the primary mechanism mediating impaired metaboreflex-induced increases in ventricular function in hypertension is accentuated coronary vasoconstriction.

Transient Ischemic Dilatation during Stress Echocardiography: An Additional Marker of Significant Myocardial Ischemia

AIM

Left ventricular (LV) transient ischemic dilatation (TID) is not clear how it relates to inducible myocardial ischemia during stress echocardiography (SE).

METHODS AND RESULTS

Eighty-eight SEs were examined from the site certification phase of the ISCHEMIA Trial. LV end-diastolic volume (EDV) and end-systolic volume (ESV) were measured at rest and peak stages and the percent change calculated. Moderate or greater ischemia was defined as ≥3 segments with stress-induced severe hypokinesis or akinesis. Optimum cut points in stress-induced percent EDV and ESV change that identified moderate or greater myocardial ischemia were analyzed. Analysis from percentage distribution identified a > 13% LV volume increase in EDV or a > 9% LV volume increase in ESV as the optimum cutoff points for moderate or greater ischemia. Using these definitions for TID, there were 27 (31%) with TIDESV and 12 (14%) with TIDEDV . By logistic regression analysis and receiver operating characteristic curves, the percent change in ESV had a stronger association with moderate or greater myocardial ischemia than that of EDV change. Compared to those without TIDESV , cases with TIDESV had larger extent of inducible wall-motion abnormalities, lower peak stress LVEF, and higher likelihood of moderate or grater ischemia. For moderate or greater myocardial ischemia detection, TIDESV had a sensitivity of 46%, specificity of 83%, positive predictive value of 70%, and negative predictive value of 64%.

CONCLUSION

Transient ischemic dilatation by SE is a marker of extensive myocardial ischemia and can be used as an additional marker of higher risk.

Assessing left ventricular systolic dysfunction after myocardial infarction: are ejection fraction and dP/dt(max) complementary or redundant?

Among the various cardiac contractility parameters, left ventricular (LV) ejection fraction (EF) and maximum dP/dt (dP/dt(max)) are the simplest and most used. However, these parameters are often reported together, and it is not clear if they are complementary or redundant. We sought to compare the discriminative value of EF and dP/dt(max) in assessing systolic dysfunction after myocardial infarction (MI) in swine. A total of 220 measurements were obtained. All measurements included LV volumes and EF analysis by left ventriculography, invasive ventricular pressure tracings, and echocardiography. Baseline measurements were performed in 132 pigs, and 88 measurements were obtained at different time points after MI creation. Receiver operator characteristic (ROC) curves to distinguish the presence or absence of an MI revealed a good predictive value for EF [area under the curve (AUC): 0.998] but not by dP/dt(max) (AUC: 0.69, P < 0.001 vs. EF). Dividing dP/dt(max) by LV end-diastolic pressure and heart rate (HR) significantly increased the AUC to 0.87 (P < 0.001 vs. dP/dt(max) and P < 0.001 vs. EF). In naïve pigs, the coefficient of variation of dP/dt(max) was twice than that of EF (22.5% vs. 9.5%, respectively). Furthermore, in n = 19 pigs, dP/dt(max) increased after MI. However, echocardiographic strain analysis of 23 pigs with EF ranging only from 36% to 40% after MI revealed significant correlations between dP/dt(max) and strain parameters in the noninfarcted area (circumferential strain: r = 0.42, P = 0.05; radial strain: r = 0.71, P < 0.001). In conclusion, EF is a more accurate measure of systolic dysfunction than dP/dt(max) in a swine model of MI. Despite the variability of dP/dt(max) both in naïve pigs and after MI, it may sensitively reflect the small changes of myocardial contractility.

Association of increased myocardial contractility and elevated end-diastolic wall tension with short-term myocardial ischemia: a pressure-volume analysis

OBJECTIVE

Critical myocardial oxygen imbalance as indicated by elevated interstitial lactate levels may occur in cases of rapidly elevated end-diastolic myocardial wall tension during elevated myocardial contractility in the intact myocardium. Simultaneous administration of beta-adrenergic receptor agonist and antagonist reliably allows for investigating the myocardial response.

DESIGN

Experimental using an in vivo animal model.

SETTING

Research institution.

PARTICIPANTS

Animal model.

INTERVENTIONS

Not applicable.

MEASUREMENTS AND RESULTS

Fifteen minipigs were investigated in an open-chest model. After a midline sternotomy, a thin dialysis tube was implanted into the LV midmyocardium. Extracellular lactate in perfusate was analyzed every 5 minutes. End-systolic time-varying elastance and end-diastolic wall tension were measured. After a stable period, dobutamine (10 microg/kg/min) was administered to 8 animals. After 20 minutes, esmolol (0.5-mg/kg bolus, repetitively) was added until heart rate decreased to <100 beats/min. For 20 minutes, esmolol was infused at a rate of 3 mg/kg/h, and then dobutamine alone was continued for 10 minutes. With dobutamine, the lactate level did not change, but wall tension decreased and contractility increased. Simultaneous esmolol initially (in the first 10 minutes) increased lactate, whereas LV end-diastolic wall tension and contractility both increased; but after 10 minutes, lactate and contractility decreased significantly. Lactate again increased within 10 minutes after stopping esmolol. A group of 7 animals received esmolol for 20 minutes and showed no changes in lactate; myocardial wall tension increased and contractility decreased.

CONCLUSION

Results suggest that oxygen demand/supply is balanced until both end-diastolic wall tension and myocardial contractility are elevated to critical levels.

Peri-operative beta-blockade and haemodynamic optimisation in patients with coronary artery disease and decreasing exercise capacity presenting for major noncardiac surgery

Patients with coronary artery disease presenting for major noncardiac surgery may have indications for both peri-operative beta-blockade and haemodynamic optimisation. The combination of peri-operative cardiorespiratory failure and myocardial ischaemia has a grave prognosis. Recent investigations have shown that in patients with coronary artery disease, beta-blockade does not depress cardiac output as much as originally thought. There may, therefore, be a place for both peri-operative beta-blockade and haemodynamic optimisation. The indications for peri-operative beta-blockade and haemodynamic optimisation, the effect of acute beta-blockade on cardiac output in patients with coronary artery disease, and the interaction of peri-operative beta-blockade and haemodynamic optimisation are discussed.

Myocardial viability independently influences left ventricular diastolic function in the early phase after acute myocardial infarction

BACKGROUND

After acute myocardial infarction, a broad range of left ventricular (LV) end-diastolic pressure (LVEDP) is expected because of chamber remodeling. However, intrinsic characteristics of the infarcted tissue (necrosis or viability) may also play a role. We aimed to evaluate whether myocardial viability (Mviab) has an influence on LVEDP.

METHODS

One hundred twenty-three consecutive patients with acute myocardial infarction underwent low-dose dobutamine echocardiography (5-10 microg/kg/min) to assess Mviab. Mviab was quantitatively evaluated by the variation of Delta wall motion score index. Patients underwent left heart catheterization with recording of LVEDP and a complete echocardiographic examination with measurement of LV volumes, ejection fraction, and mass.

RESULTS

The overall population (81% male; mean age 58 +/- 10 years) was divided into 2 groups according to the presence (group 1; 66 patients) or absence (group 2; 57 patients) of Mviab. LVEDP was higher in patients without Mviab (16 +/- 8 vs 20 +/- 7 mm Hg; P =.02). The multivariate analysis showed that Delta wall motion score index correlated with LVEDP (P =.01) independent of wall motion score index and LV end-systolic volume.

CONCLUSIONS

After acute myocardial infarction, LVEDP shows wide variability and is independently associated with Mviab.

Does dobutamine improve ventricular function in dogs with regional myocardial dysfunction?

We studied the effect of systemic dobutamine infusion (4 microg. kg(-1). min(-1) IV) on regional wall motion abnormalities (RWMAs) in eight anesthetized open-chested dogs. We hypothesized that infusion of small doses of dobutamine would reduce RWMAs and improve global ventricular function. Apical RWMAs were induced by local intracoronary boluses of 9.0 mg esmolol. Phase angles, effective stroke volume (SV), maximum SV, stroke work, and segmental shortening were compared among four left ventricular (LV) regions (apical, papillary, chordal, and basal) during baseline, dobutamine, esmolol, and dobutamine-esmolol treatments. The minimal global LV volume was designated as 0 degrees, and the cardiac cycle was divided into 360 intervals. Regional phase angles were defined as the distance (in degrees) that regional minimum volume differed from global minimal LV volume (end-systole). RWMA decreased blood pressure (92 +/- 2 mm Hg to 84 +/- 3 mm Hg) and increased LV end-diastolic pressure (1.8 +/- 0.5 mm Hg to 4.2 +/- 0.8 mm Hg). RWMA delayed regional contraction (-2.9 degrees +/- 1.6 degrees to 52.3 degrees +/- 1.5 degrees ) and decreased effective SV (2.3 +/- 0.4 mL to 1.6 +/- 0.3 mL) in the affected apical region but did not decrease maximal SV. Systemic infusion of dobutamine restored global LV function but failed to eliminate RWMA, as evidenced by decreased apical synchrony, effective SV, and stroke work. We concluded that systemic dobutamine restored global LV function but failed to correct RWMA.

IMPLICATIONS

We examined the effect of systemic dobutamine on regional wall motion abnormalities (RWMAs) induced by intracoronary esmolol infusion in eight anesthetized dogs. Esmolol dilated the heart and decreased regional synchrony of contraction. Dobutamine restored cardiac function but failed to correct the asynchrony of regional contraction caused by esmolol-induced RWMAs.