Transmural fibre direction in the anterior wall of the feline left ventricle: theoretical considerations with regard to uniformity of contraction

Equations for estimating muscle fiber stress in the left ventricular wall

Left ventricular muscle fiber stress is an important parameter in cardiac energetics. Hence, we developed equations for estimating regional fiber stresses in rotationally symmetric chambers, and equatorial and apical fiber stresses in prolate spheroidal chambers. The myocardium was modeled as a soft incompressible material embedding muscle fibers that support forces only in their longitudinal direction. A thin layer of muscle fibers then contributes with a pressure increment determined by the fiber stress and curvature. The fiber curvature depends on the orientation of the fibers, which varies continuously across the wall. However, by assuming rotational symmetry about the long axis of the ventricle and including a longitudinal force balance, we obtained equations where fiber stress is completely determined by the principal curvatures of the middle wall surface, wall thickness, and cavity pressure. The equations were validated against idealized prolate spheroidal chambers, whose wall thicknesses are such that the fiber stress is uniform from the equator to the apex. Because the apex is free to rotate, the resultant moment about the long axis of the LV must be zero. By using this constraint together with our fiber-stress equations, we were able to estimate a muscle fiber orientation distribution across the wall that was in qualitative agreement with published measurements.

Carvedilol improves function and reduces infarct size in the feline myocardium by protecting against lethal reperfusion injury

This study examined the effect of carvedilol, a vasodilating beta-adrenoceptor antagonist and antioxidant, on lethal reperfusion injury in feline hearts subjected to 40 min of regional ischemia and 180 min of reperfusion. 30 open chest anaesthetized cats were randomized into three groups. A control (n = 10) was compared with a group given carvedilol before coronary artery occlusions (n = 10) and a group given carvedilol immediately before and during early reperfusion (n = 10). Regional myocardial function was measured by sonomicrometry. Infarct size was determined by staining the left ventricle with triphenyl tetrazolium chloride. Myocardial blood flow was measured by radiolabeled microspheres. Tissue levels of glutathione were measured after reperfusion. Infarct size was significantly reduced compared to control both when carvedilol was given before ischemia (0.2 +/- 0.1 vs. 17.6 +/- 3.6%, P < 0.05). and when given immediately before reperfusion (3.7 +/- 1.3 vs. 17.6 +/- 3.6%, P < 0.05). Regional shortening improved significantly and the incidence of ventricular fibrillation during early reperfusion was reduced in both groups treated with carvedilol compared to control. Oxidized glutathione did not differ between groups in the post-ischaemic myocardium. This study supports that lethal reperfusion injury is a significant phenomenon. Furthermore, carvedilol reduces infarct size and reperfusion arrhythmias, and improves post-ischaemic regional myocardial function by protecting against both ischaemic and lethal reperfusion injury. The present study does not answer whether it is the non-selective beta- or alpha 1-receptor antagonism, the antiarrhythmic or the antioxidant actions of carvedilol that is responsible for the protective effect.

Carvedilol protects against lethal reperfusion injury through antiadrenergic mechanisms

We examined the effect of carvedilol as compared with that of a combination of propranolol and doxazosin on lethal reperfusion injury in 21 feline hearts subjected to 40-min regional ischemia and 180-min reperfusion. A control group (n = 7) was compared with one group given carvedilol, a nonselective beta - and alpha 1-adrenoceptor blocker and antioxidant (n = 7) and another group given nonselective beta - and alpha 1-adrenoceptor blockade with propranolol and doxazosin (n = 7) during initial reperfusion. Infarct size (IS: percent of area at risk, AAR) determined by staining the myocardium with triphenyl tetrazolium chloride (TTC), was reduced both in the carvedilol-treated group (median 1.8%, p < 0.05) and in the group given propranolol/doxazosin (median 6.5%, p < 0.05) as compared with controls (median 14.4%). Treatment with carvedilol reduced IS more than did treatment with propranolol/doxazosin (p < 0.05). Longitudinal segment shortening measured with sonomicrometry, improved in both treatment groups as compared with control (p < 0.05), but to a greater extent in the group treated with carvedilol. In circumferential segments, only carvedilol significantly improved segment shortening. The incidence of ventricular fibrillation (VF) after reperfusion was reduced in both treatment groups as compared with control. Oxidized glutathione and thiobarbituric acid-reactive substances (TBARS) measured at the end of reperfusion did not differ between groups. Carvedilol protected against lethal reperfusion injury mainly through blockade of adrenoceptors.

Effects of endurance training on left ventricular performance: a study in anaesthetized rabbits

Endurance training is known to increase ventricular performance during exercise and to decrease resting heart rate. The aim of this study was to evaluate a model for endurance training in rabbits and to study the effects of endurance training on local myocardial performance in the left ventricle during resting conditions. One group of rabbits underwent a 10-week exercise training programme. The rabbits trained 5 days a week on a treadmill. Training periods increased gradually from 15 min to 1 h with increments in speed from 0.5 to 1.2 km h-1. After the training programme the rabbits were anaesthetized and studied as acute open-chest preparations. A micro-tip pressure transducer was introduced via apex to the left ventricle and two pairs of ultrasonic crystals were implanted in the left anterior wall to measure segment lengths. One pair measured shortening in the circumferential direction whereas the other pair measured shortening in the longitudinal direction. Heart rate was lower in the trained group (n = 5), 172 +/- 9 beats min-1 (mean +/- SEM), compared with 235 +/- 19 beats min-1 in the control group (n = 8) (P < 0.02). Stroke volume, measured by radio-nuclidelabelled microspheres, was greater in the trained rabbits compared with controls (P < 0.03). Shortening in both segments was of similar magnitude for the trained and control groups. End-systolic pressure-length relations (ESPLR) obtained by occlusion of the descending aorta (balloon catheter) showed reduced slopes for longitudinal segments in the trained group compared with the control group (P < 0.05). We conclude that this endurance training programme in rabbits can be used to study myocardial effects of endurance training. Furthermore, the less steep slope of ESPLRs for the longitudinal segment in the trained animals might indicate a structural myocardial remodelling and increased contractile reserve that might be recruited during adrenergic stimulation in the trained group.

Intravenous lipid infusion results in myocardial lipid droplet accumulation combined with reduced myocardial performance in heparinized rabbits

The question addressed in this study was whether a relation between myocardial lipid droplet accumulation and depressed myocardial function existed following intralipid infusion for 45 min in open chest, anaesthetized rabbits. One group of rabbits (n = 8) received intralipid infusion whereas a control group (n = 8) received sodium chloride. Local myocardial performance was obtained by sonomicrometry and the fractional volume of myocardial lipid droplets was measured by morphometric methods. The fractional volume of lipid droplets was 0.667 +/- 0.116% in the intralipid group compared with 0.318 +/- 0.080% in the control group (P < 0.03). Cardiac output and stroke volume fell 26% (P < 0.0001) and 34% (P < 0.0001), respectively, as a result of intralipid infusion. However, myocardial blood flow obtained by radiolabelled microspheres remained unchanged. Local myocardial function was reduced for both segments after intralipid infusion; maximal systolic shortening was reduced from 15.63 +/- 1.45 to 12.07 +/- 1.55% (P < 0.002) in the circumferential segment and from 9.46 +/- 1.17 to 7.40 +/- 0.53% (P < 0.05) in the longitudinal segment. The end-diastolic length of the circumferential segment was reduced by 3% (P < 0.05) after intralipid infusion. The reduced end-diastolic length of circumferential segments together with unchanged left ventricular end-diastolic pressure might indicate reduced left ventricular end-diastolic compliance. We conclude that acute intralipid infusion in rabbits results in myocardial lipid droplet accumulation and depressed local myocardial function.

Is postsystolic shortening area always a marker of myocardial ischaemia?

Postsystolic shortening area has been shown to be a sensitive marker of myocardial ischaemia in a one-vessel model. We tested whether postsystolic shortening provoked by interaction between ischaemic and nonischaemic regions is reduced in a two-vessel model, one vessel occluded and one subjected to coronary artery stenosis. Regional function in the left ventricular anterior wall was studied by orthogonal sonomicrometry during left coronary underperfusion in 14 pentobarbital-anaesthetized cats with an acute circumflex coronary artery occlusion. Left coronary underperfusion in two discrete steps decreased subendocardial blood flow in the left ventricular anterior wall to on average 60% (P < 0.001) and 20% (P < 0.001) of control value, while subepicardial flow did not change. End-diastolic lengths of longitudinal segments increased markedly even during mild subendocardial underperfusion, whereas end-diastolic lengths of circumferential segments only increased during severe subendocardial underperfusion with concomitant increase of left ventricular end-diastolic pressure. Systolic shortening, pressure-length loop area and shortening velocity of circumferential segments did not change. In contrast, systolic shortening and total pressure-length loop area of longitudinal segments decreased progressively approaching zero values during severe coronary underperfusion. Shortening velocity of longitudinal segments also decreased progressively during experimental protocol whereas postsystolic shortening area did not change.

CONCLUSIONS

postsystolic shortening area is not a marker of subendocardial ischaemia in a two-vessel model which indicates that postsystolic shortening is primarily a phenomenon related to model of ischaemia. End-diastolic lengthening is predominant in the longitudinal axis of the heart during subendocardial ischaemia.

The influence of afterload on uniformity of segment shortening in feline left ventricles. Importance of cross-fibre contraction

Previous studies have shown that the combined increase of preload and afterload leads to a more uniform wall contraction. As our previous information with a stable afterloaded situation showed considerable increase of left ventricular end-diastolic pressure, we have as yet no information of how afterload per se effects uniformity of segment shortening. We therefore analysed maximal systolic shortening of cross-oriented segments for three consecutive beats during abrupt elevation of afterload in 12 open-chest cats. Peak left ventricular systolic pressure increased during the three beats from 154 +/- 5 mmHg (mean +/- SEM) to 167 +/- 5 mmHg and 186 +/- 5 mmHg. Left ventricular end-diastolic pressure remained unchanged. Maximal systolic shortening was reduced for both segments with increasing afterload, in longitudinal segment (LONG) from 7.6 +/- 1.1 to 6.1 +/- 1.0% (P < 0.005), and more pronounced for circumferential segments (CIRC) from 12.2 +/- 0.7 to 8.3 +/- 0.9% (P < 0.0005). Uniformity of maximal systolic shortening, LONG/CIRC, increased from 0.63 +/- 0.08 in the first beat to 0.73 +/- 0.10 and 0.82 +/- 0.12 in the following beats (P < 0.02). We conclude that uniformity of contraction for cross-oriented segments in the anterior left ventricular wall is increased during increased afterload. We propose that this can be explained by reduction of the effect of cross-fibre contraction with increasing afterload.

Non-uniformity of two-dimensional myocardial deformation in response to chronotropic and inotropic stimulation in cats

The degree of uniformity of myocardial deformation for cross-oriented segments in the anterior wall of the left ventricle is influenced both by loading conditions and by infusion of isoprenaline. The aim of this study was to clarify the chronotropic influence (atrial pacing) compared to the combined inotropic and chronotropic effects of isoprenaline on uniformity of contraction. In eight open-chest pentobarbitone-anaesthetized cats segment performance was measured by orthogonal sonomicrometry. Heart rate (HR) increased from control state, 199 +/- 5 (mean +/- SEM) beats min-1, to 224 +/- 6 and 227 +/- 7 beats min-1 during atrial pacing and isoprenaline infusion, respectively (P < 0.0005). Circumferential segment shortening remained unchanged during pacing but increased with isoprenaline (P < 0.0005). Longitudinal segments showed reduced shortening during pacing (P < 0.05), whereas shortening during isoprenaline infusion did not differ from control shortening. The ratio between shortening of longitudinal and circumferential segments, long/circ ratio, changed from 0.52 +/- 0.13 in the control state to 0.36 +/- 0.10 during pacing and 0.39 +/- 0.11 with isoprenaline (P < 0.05). End-systolic pressure-length relations were unchanged by atrial pacing but showed leftward shifts during isoprenaline infusion. Myocardial tissue blood flow (microspheres) was unchanged during interventions. We conclude that both atrial pacing and isoprenaline infusion lead to a more non-uniform deformation in the anterior wall. However, non-uniformity during atrial pacing was primarily related to reduced longitudinal shortening whereas the non-uniformity during isoprenaline infusion was due to increased circumferential shortening.

Beta-adrenergic blockade improves uniformity of local contraction in the anterior wall of feline left ventricles

Uniformity of myocardial contraction has been put forward as an important regulatory mechanism of myocardial contraction, and we have previously demonstrated reduced uniformity of local myocardial contraction in the midwall of left ventricles during preload reduction combined with increased inotropy. The aim of the present study was to explore the isolated inotropic influence on uniformity of contraction, keeping loading conditions constant. Segment performance was measured by implanted piezo-electric crystals in ten open-chest, anaesthetized cats. One crystal pair, circumferential segment, aligned with midwall and epicardial fibres, whereas another perpendicular segment, longitudinal, ran close to endocardial fibre direction. Maximal systolic shortening of longitudinal segments remained unchanged following inotropic interventions, isoprenaline and timolol, whereas circumferential segments revealed reduced performance with timolol (8.4 +/- 1.0% compared with 14.0 +/- 1.3% during control; P < 0.001). In the control state the longitudinal-to-circumferential ratio (LONG/CIRC) a quantitative measure of uniformity, was 0.32 +/- 0.08. No change in uniformity occurred during isoprenaline infusion (0.32 +/- 0.11) but during beta-adrenergic blockade with timolol a clearly higher LONG/CIRC ratio was observed (0.56 +/- 0.06, P < 0.005). Myocardial tissue blood flow measurement by radioactive microspheres showed a shift in transmural distribution with interventions so that subepicardial blood flow was markedly reduced during beta-adrenergic blockade. The direction of subepicardial fibres approximates our circumferential segments. Thus, there may well exist a relationship between segmental shortening and perfusion. In conclusion, circumferential segments were most influenced by changes in inotropy in contrast to previous observations where pronounced changes in longitudinal segments with loading occur. Furthermore, this study demonstrates that reduced inotropy with timolol helped uniform local contraction as estimated by the increased LONG/CIRC ratio, a transition that could improve contraction efficacy.

Dyssynchrony of segment shortening in the anterior wall of the feline left ventricle

Non-uniformity of regional contraction may be both spatial and temporal. This study was undertaken to deal with the temporal aspects of shortening and to quantify non-uniformity with regard to timing. Nine cats were anaesthetized and artificially ventilated. Two pairs of ultrasonic crystals were situated in the anterior midwall of the left ventricle to measure regional shortening. One pair, longitudinal segment, was oriented to align with midwall fibres. The other pair, transverse segment, was placed perpendicular to the first one. Registrations in control state, during caval occlusion, and during aortic constriction were carried out with and without isoprenaline infusion. Cyclic events were analysed in terms of phase angle, 0-2 pi representing one heart cycle. Transverse segments showed marked shift of duration of shortening, from 1.19 pi +/- 0.06 pi (mean +/- SEM) in the control state to 0.40 pi +/- 0.14 pi during caval occlusion with isoprenaline infusion. Duration of shortening of longitudinal segments showed less prominent shift with mean values between 1.38 pi and 1.11 pi. Regional uniformity of timing, expressed as synchronization index, varied markedly with interventions (P less than 0.0005). Dyssynchrony was most prominent during caval occlusion with mean values less than 0.6. A simple model of force generation for the two segments visualizes that segment shortening of the transverse segment is of shorter duration than the longitudinal segment and a common mechanism for temporal and spatial non-uniformity within a region could be elaborated. This study quantifies both the time course of shortening and temporal non-uniformity of two cross-oriented segments within the same myocardial region.