Effects of a bradycardic agent on postischemic cardiac recovery in rabbits

Decreasing heart rate might be beneficial for improvement of myocardial energetics and could reduce the severity of myocardial ischemia. We examined the contribution of heart rate reduction by cilobradine (DK-AH 269), a direct sinus node inhibitor, on left ventricular function and peripheral vasomotion in anesthetized rabbits with experimental myocardial infarction. The rabbits were randomized to receive either placebo (n=10) or cilobradine (n=7). Cilobradine decreased significantly heart rate from 163 +/- 33 to 131 +/- 13 bpm, p< 0.05, without any inotopic or vascular effects. After 60 min coronary occlusion and 30 min reperfusion, both systolic and diastolic ventricular function were more reduced in the cilobradine group; i.e. maximal left ventricular pressure significantly decreased to 62 +/- 11 mmHg, p < 0.05 (placebo: 77 +/- 9 mmHg); dP/dt(min) significantly decreased to -904 +/- 247 mmHg, p < 0.05 (placebo: -1106 +/- 242 mmHg). However, infarct size in the cilobradine group was significantly smaller compared with the placebo group. In conclusion, cilobradine reduced heart rate without any negative inotropic effect and reduced infarct size. On that account, this bradycardic agent might open a promising therapeutical avenue to treat postischemic dysfunction.

Management of the persistent ductus arteriosus in infants of very low birth weight: early and long-term results*1

The hemodynamically relevant persistent ductus arteriosus (PDA) impairs pulmonary and cardiac function. Frequently, PDA can be closed only via surgery. In this retrospective study, early and long-term results in very low birth weight newborns are evaluated. Eighty-seven of 634 very low weight newborns presented with PDA All patients (pts; age: +/-14 days; weight: +/-1064 g) were ventilator-dependent. Surgical closure (after 29+/-5 days) was indicated if echocardiography and prolonged ventilation (>20+/-2 days) evidenced a hemodynamically relevant PDA. Sixteen pts, in which indomethacin therapy failed preoperatively are included in the 36 surgically treated pts; no pt died intra- or early postoperatively (<3 day). Overall mortality 30 days after delivery was n=9. Early plus late mortality was n=19. Long-term follow-up (3-12 years) in 46 (68%) pts: 15 were solely physically, 11 were mentally and neurologically, and 4 were physically, mentally and neurologically retarded. From these 30 pts, 15 were severely (e.g. tetraspasm; severe cerebral paresis) and 15 were slightly (e.g. psychosomatic and language development prolongation) retarded. Sixteen pts exhibited no disability; no long-term complications owing to surgery. The relatively large number of neurological injuries was not owing to chromosomal syndromes or pre-existing abnormalities but can be explained by severe and frequent prematurity, hypoxia, and intracerebral bleeding. Indomethacin was successful only in a few patients. Early surgery (after frustran early indomethacin therapy) of a hemodynamically relevant PDA is recommended. In the long-term, severe disabilities develop.

[Protective effects of a delta-opioid-receptor agonist and an oxygen radical scavenger on postischemic hearts]

BACKGROUND AND PURPOSE

The cardioprotective effects of delta-opioid receptor agonists is mediated-at least in part-via oxygen radicals. Mannitol that is used in cardiac surgery because of its osmotic properties exerts its beneficial effects on stunned myocardium via scavenging hydroxyl radicals. The effects of a delta-opioid receptor agonist (D-Ala2-D-Leu5-Enkephalin [DADLE]), the radical scavenger mannitol and their combination on postischemic dysfunction in rabbit hearts were investigated.

METHODS

Isolated, blood-perfused rabbit hearts were subjected to a 20-min global, normothermic, no-flow ischemia that was followed by a 60-min reperfusion. Systolic and diastolic ventricular function as well as coronary blood flow (CBF) were assessed. The hearts were assigned to one of four groups: 1. placebo (n = 6); 2. DADLE (n = 8; 430 nM); 3. mannitol (n = 7; 8.6 mM); 4. DADLE + mannitol (n = 7).

RESULTS

Ischemic contracture in the DADLE and the mannitol group was significantly smaller compared with the placebo group. Contracture was smallest in the DADLE + mannitol group. The postischemic function in the placebo group was drastically reduced (p < 0.05), while it was best preserved in the DADLE + mannitol group. CBF and MVO(2) were changed similarly in all groups (n. s.). The external efficiency was significantly higher in the groups with DADLE and/or mannitol than in the placebo group. Both DADLE and mannitol exhibit cardioprotective properties. Combination of both substances exerts an additive, positive effect on the ischemic contracture. Noteworthy, the protective effects of DADLE during reperfusion were not antagonized by the oxygen radical scavenger mannitol. On the other hand, DADLE + mannitol did not augment the protective effects of the single substances during reperfusion, except for the isovolumic LVP(max).

CONCLUSION

Both substances improve the postischemic systolic and diastolic function and the relation between cardiac work and oxygen needed for this work. Thus, both substances offer promising properties in the clinic.

Electrophoretic separation of ventricular myosin isoenzymes using a native polyacrylamide minigel system

A method is presented to separate rabbit cardiac ventricular myosin isoenzymes (V(1), V(2), V(3)), which are large and important contractile proteins. This polyacrylamide gel electrophoresis--using a slab minigel format--does not involve preparation of an acrylamide gradient or denaturing conditions. The isoenzyme migration order was confirmed through identification with an anti beta-myosin heavy chain in cardiac ventricles (i.e., V(3)) antibody. Extracts from atrial and soleus muscle were used as positive control for V(1) and V(3), respectively. The relative quantification was obtained densitometrically and analyzed via TINA/Software. The reproducibility of method was additionally tested. The procedure employs Coomassie blue staining and is rapid and reproducible. Thus, the method permits easy and economic analysis of myosin isoenzymes under native conditions.

Static filling pressure in patients during induced ventricular fibrillation

The static pressure resulting after the cessation of flow is thought to reflect the filling of the cardiovascular system. In the past, static filling pressures or mean circulatory filling pressures have only been reported in experimental animals and in human corpses, respectively. We investigated arterial and central venous pressures in supine, anesthetized humans with longer fibrillation/defibrillation sequences (FDSs) during cardioverter/defibrillator implantation. In 82 patients, the average number of FDSs was 4 +/- 2 (mean +/- SD), and their duration was 13 +/- 2 s. In a total of 323 FDSs, arterial blood pressure decreased with a time constant of 2.9 +/- 1.0 s from 77.5 +/- 34.4 to 24.2 +/- 5.3 mmHg. Central venous pressure increased with a time constant of 3.6 +/- 1.3 s from 7.5 +/- 5.2 to 11.0 +/- 5.4 mmHg (36 points, 141 FDS). The average arteriocentral venous blood pressure difference remained at 13.2 +/- 6.2 mmHg. Although it slowly decreased, the pressure difference persisted even with FDSs lasting 20 s. Lack of true equilibrium pressure could possibly be due to a waterfall mechanism. However, waterfalls were identified neither between the left ventricle and large arteries nor at the level of the diaphragm in supine patients. We therefore suggest that static filling pressures/mean circulatory pressures can only be directly assessed if the time after termination of cardiac pumping is adequate, i.e., >20 s. For humans, such times are beyond ethical options.

Minimally invasive coronary artery bypass grafting in high-risk patients. Late follow-up with assessment of left internal mammary artery graft patency and flow by exercise transthoracic Doppler echocardiography

Patients with significant risk factors are at increased risk of higher mortality and morbidity (9-16%) after CABG-procedures with cardiopulmonary bypass (CPB). When catheter interventions are not applicable and conventional CABG with CPB are considered to have an unacceptable perioperative risk, these patients (n=35) were scheduled for minimally invasive coronary artery bypass grafting (MIDCAB).

PATIENTS AND METHODS

The risks leading to exclusion of conventional CABG procedures were: extremely impaired LV-function (EF<20%), severe pulmonary diseases, malignant carcinoma, compromised coagulation system, age >80 years with impaired physical constitution, redo-procedures after complicated initial operation, symptomatic descending thoracic aortic aneurysm, ongoing long-term intensive care treatment with unclear prognosis. All patients received the LIMA as a single graft to the LAD. One year follow-up was performed using transthoracic Doppler echocardiography at rest and during exercise.

RESULTS

In 20 patients incomplete revascularization was accepted. There was no mortality, while signs for myocardial infarction were seen in two patients. Twenty-nine patients (82%) showed clear improvement of clinical symptoms, one patient needed further conventional CABG. Nine to thirteen months postoperatively (mean 10.8+/-1.6 months), there were two deaths due to noncardiac reasons. Three of the survivors (n=33) had symptoms of angina pectoris. Exercise tests revealed an improved stress tolerance (NYHA class improved from preop. III-IV to postop. I-II). The IMA graft flow increased significantly with exercise in all patients. Flow patterns in both flow velocity and volume changed to diastolic-dominant, and the ratio of diastolic to systolic time-velocity integral of >1.5 excluded a graft stenosis.

CONCLUSIONS

In high-risk patients, with an increased likelihood of perioperative morbidity and mortality, the MIDCAB procedure can be performed accurately and safely. Even after incomplete revascularization of some high-risk patients, exercise tolerance was improved. Transthoracic Doppler echocardiography proved to be a clinically useful noninvasive method of assessing IMA graft function at rest and during exercise. Despite the small patient population, our late follow-up results suggest the potential benefit of MIDCAB for patients with otherwise inoperable heart disease.

[The isolated rabbit heart: comparison between five different modifications]

BACKGROUND

The isolated heart as an experimental model has been firmly established for more than 100 years.

MATERIAL AND METHODS

In this study, five modifications are compared: 1. modified Langendorff apparatus (LA) with modified Krebs-Henseleit (KH) solution a) not containing bovine serum albumin (BSA; n = 13) and b) containing BSA (n = 16), 2. LA with KH solution containing BSH and bovine erythrocytes (n = 14), 3. LA with support rabbit (n = 6), and 4. "working heart" preparation with KH solution, BSA and bovine erythrocytes (n = 16). In the latter modification, no balloon was inserted into the left ventricular cavity, i. e., systemic and coronary circuits were not separated from each other. After completion of the preparation and 20-min stabilization, hemodynamic and metabolic data were assessed while the hearts were contracting in the ejecting mode. Thereafter, protocols for different studies were performed that are not presented here. However, the stability of the modifications within their individual protocols is reported.

RESULTS

The results suggest that hearts perfused with KH solution are well suited for short protocols. In spite of the additional costs and time, blood perfusion is required for long-lasting protocols or if changes in coronary flow are to be investigated.

CONCLUSIONS

The working heart exhibits both the best function and stability at a relatively low experimental expenditure. Yet, it is not suited for studies where perfusion pressure needs to be changed independent of arterial pressure.

Preconditioning: myocardial function and energetics during coronary hypoperfusion and reperfusion

BACKGROUND

Ischemic preconditioning (IP) is gaining more acceptance as a protective method in beating heart surgery. Yet it remains controversial whether preconditioning can attenuate myocardial dysfunction during reperfusion after severe coronary hypoperfusion. We examined this issue and also the issue of whether this protection is mediated by adenosine A1 receptors.

METHODS

In isolated, blood-perfused rabbit hearts, the effects of IP (3 minutes of no flow ischemia and 8 minutes of reperfusion) during 30 minutes of coronary hypoperfusion and 60 minutes of reperfusion were investigated. In two groups (n = 8 each) with and without (control group) preconditioning, ventricular function was assessed by load-insensitive measures: slope of the end-systolic pressure-volume relation (Emax), slope of the stroke work/end-diastolic volume relation (Mw), and end-diastolic pressure-volume relation. External efficiency was calculated, and contractile efficiency was assessed using the reciprocal of the myocardial oxygen consumption-pressure-volume area relationship. To investigate the possible role of adenosine, the adenosine A1 receptor antagonist DPCPX (2.5 micromol/L) was administered before preconditioning in a third group (n = 7).

RESULTS

The effects of hypoperfusion on systolic function, diastolic function (dP/dtmin, end-diastolic pressure-volume relation), external efficiency, and contractile efficiency were similar in both the IP and control groups. Lactate efflux was significantly reduced after preconditioning (p = 0.02). During reperfusion, recovery of systolic function and coronary flow were significantly improved in the IP group compared with controls: aortic flow, 85% versus 63% (p = 0.01); dP/dtmax, 91% versus 67% (p = 0.001); pressure-volume area, 97% versus 68% (p = 0.01); Emax, 74% versus 62% (p = 0.03); and Mw, 94% versus 84% (p = 0.04). Release of creatine kinase was reduced in the IP group, 9.6 +/- 1.3 U x 5 min(-1) x 100 g(-1) wet weight, versus controls, 12.7 +/- 2.7 U x 5 min(-1) x 100 g(-1) wet weight (p = 0.04). During reperfusion, contractile efficiency (p = 0.03) and external efficiency (p = 0.02) recovered better in preconditioned than in untreated hearts. Recovery was less pronounced in the DPCPX group compared with the IP group (p, not significant).

CONCLUSIONS

The results, derived from load-insensitive measures, confirm that IP provides protection after episodes of severe hypoperfusion by attenuating systolic dysfunction without improving diastolic dysfunction and reduces the severity of anaerobic metabolism as well as ischemic injury. Contractile efficiency and external efficiency both indicate improved energetics after IP (oxygen utilization by the contractile apparatus). The protective effect, at least in part, is mediated by adenosine A1 receptors.

Mismatch of local blood flow and oxidative metabolism in stunned myocardium

Myocardial blood flow is spatially heterogeneous, reflecting nonuniform oxygen supply. Also, myocardial oxidative metabolism is spatially heterogeneous. The effects of acute ischemia and reperfusion on the relationship between local myocardial blood flow (LMF) and oxidative metabolism are still unknown. LMF was measured in isolated, blood-perfused rabbit hearts using colored microspheres and oxidation water labeled with 18O2 (H2(18)O). Three protocols were performed: 18O2-perfusion during normoxia (N; n=7), during early reperfusion (ER; 10 min, n=6), and late reperfusion (LR; 40 min, n=6) following 20 min no-flow ischemia. LMF and local H2(18)O residues were determined within defined myocardial samples (105+/-15 mg). For interindividual comparison, values were normalized to the mean of the individual experiment and expressed as percentages. LMF ranged from 18 to 193% (N), 12 to 250% (ER), and 11 to 180% (LR). The H2(18)O tissue residue ranged from 63 to 132% (N), 73 to 142% (ER) and 32 to 148% (LR). The correlation between LMF and local oxidative metabolism during N (r=0.77; n=56) was lost in the postischemic heart during ER and LR. LMF during N and ER were only weakly correlated (r=0.24; n=48), whereas LMF during N and LR correlated well (r=0.87; n=48). It is concluded that the heterogeneous LMF pattern at baseline is maintained in the stunned myocardium whereas that of local oxidative metabolism is not. Apart from the established mechanisms underlying myocardial stunning, a mismatch between local flow and oxidative metabolism might also contribute.

Gene expression profiles in the acutely dissected human aorta

OBJECTIVES

heritable connective tissue abnormalities and arterial hypertension may predispose to aortic dissection. This study evaluates gene expression profiles in the acutely dissected human aorta.

DESIGN, MATERIALS AND METHODS

Atlas Human Broad Arrays I, II, and III (Clontech) were used to compare gene expression in acutely dissected (6 patients) and normal ascending aortas (6 multiorgan donors). The tissues were also compared macroscopically.

RESULTS

of 3537 genes analysed, 1250 (35%) were expressed in aortic tissue. For statistical analysis we focused on 627 genes, which had an intensity>0.95 of the mean patients or controls. Dissected and adjacent macroscopically intact aorta displayed similar gene expression patterns. On the contrary, 66 genes were expressed significantly different in dissected aorta, compared with undiseased control aorta of multiorgan donors. Genes, predominantly upregulated in dissection, are involved in inflammation, in extracellular matrix proteolysis, in proliferation, translation and transcription. Predominantly downregulated genes code for extracellular matrix proteins, adhesion proteins and cytoskeleton proteins.

CONCLUSION

our results demonstrate for the first time the complexity of the dissecting process on a molecular level. The ultimate dissection seems to be the dramatic endpoint of a long-lasting process of degradation and insufficient remodelling of the aortic wall. Altered patterns of gene expression suggest a pre-existing structural failure of the aortic wall, resulting in dissection.

Changes in coronary vessel resistance during postischemic reperfusion and effectiveness of nitroglycerin

OBJECTIVE

Microvascular incompetence after ischemia and reperfusion may compromise the normal postischemic coronary perfusion and additionally jeopardize the recovery of the myocytes. We investigated whether such a form of acute endothelial dysfunction occurs in the routine operative setting despite the use of protective measures. For this purpose, we measured pressure-flow relations in the coronary vasculature during heart operations before and after ischemia and after reperfusion and their reaction to the nitric oxide donor nitroglycerin.

METHODS

Forty-eight patients with a low risk profile scheduled for routine coronary artery bypass surgery were included. During normothermic extracorporeal circulation, the fibrillating heart was completely excluded from bypass by clamping of the ascending aorta and snaring of the caval veins. It was relieved of blood by opening the right atrium and venting the left atrium and ventricle to avoid distention. The coronary vessels were perfused under controlled flow, and the perfusion pressures were monitored. This protocol was performed in 24 patients before and immediately after ischemia and after a reperfusion period.

RESULTS

Compared with the preischemic control, vascular resistance was decreased by 17% (P <.003) immediately after ischemia but increased again by 46% (P <.0001) during an average of 25 minutes of reperfusion and, even more important, by 23% (P <.028) in comparison with the preischemic values. In two groups of 12 patients, nitroglycerin was added to the perfusate either in a dosage of 3 microg. kg. min(-1) or as a bolus injection of 2 mg. Low-dose nitroglycerin did not reduce the elevated postreperfusion resistances significantly, but bolus injection did (P <.0002). Coronary vessel resistance increased during reperfusion in particular in patients with a history of hypertension.

CONCLUSION

Coronary vasoconstriction during postischemic reperfusion is regularly present in the routine operative setting in cardiac surgery, despite myocardial protection measures. The amount of vasoconstriction varies considerably and is particularly increased in patients with hypertension. The nitric oxide donor nitroglycerin can normalize the elevated resistances, but only in high dosages. This demonstrates a preserved ability of vascular smooth muscle to relax. The phenomenon had no sequelae in our low-risk patients having elective operations. However, it may gain significance in the case of severe left heart hypertrophy and in patients at risk with both a postoperative low-output syndrome and reduced mean arterial pressures during reperfusion.

Correlation between heterogeneous myocardial flow and oxidative metabolism in normoxic and stunned myocardium

Myocardial blood flow exhibits considerable heterogeneity. Consequently, oxygen supply to the myocardium is also heterogeneous, as is myocardial metabolism. Many lines of evidence show a close correlation between local flow and local metabolism in the normoxic myocardium. So far, myocardial metabolism has predominantly been assessed indirectly by using labeled substrates. We used the (18)O isotope, permitting analytical separation of H2(18)O from the (18)O isotope, as well as quantification of regional oxidative metabolism by measuring the tissue residue of oxidation water in the rabbit myocardium. Correlation of local flow with oxidative metabolism was significant in the normoxic myocardium. This correlation was lost in the postischemic/reperfused myocardium. Apart from the established mechanisms underlying myocardial stunning, a mismatch between local flow and oxidative metabolism might thus also contribute to the postischemic dysfunction. In the normoxic myocardium, function should correlate with metabolism and blood flow. For technical reasons, function has not been assessed on a very local scale. Nevertheless, some considerations are presented on the heterogeneity of function as well as on the scale on which heterogeneity should be investigated to convey physiologically meaningful information on regulatory cardiac mechanisms.

Left ventricular dysfunction and disturbed O(2)-utilization in stunned myocardium: influence of ischemic preconditioning

OBJECTIVE

Myocardial dysfunction during postischemic reperfusion is frequently reported only in terms of left ventricular (LV) systolic properties. We additionally assessed diastolic properties, the cardiovascular tone and in particular, the relation between ventricular function and myocardial oxygen consumption. Moreover, these measures are investigated after cardioprotection via ischemic preconditioning (IP). However, this phenomenon is not fully understood, and therefore cardioprotective methods like ischemic preconditioning might provide only insufficient protection.

METHODS

In a total of 17 isolated rabbit hearts, perfused with an erythrocyte suspension (Hct 30%), we investigated the effect of 20 min low-flow ischemia also on diastolic properties, coronary resistance and cardiac energetics (n=9). During control and 30 min after the onset of reperfusion, LV systolic function was assessed in terms of aortic flow, dP/dt(max) and the end-systolic pressure-volume relation (ESPVR). Early relaxation was evaluated via dP/dt(min) and diastolic properties were assessed via the end-diastolic pressure-volume relation (EDPVR), i.e. using the equation LVP(ed)=c.exp(m.LVV(ed)), where c equals the LVP(ed)-axis intercept and m equals LV stiffness. In addition, coronary resistance (R(cor)) and the pressure-volume area (PVA) were calculated. Total oxygen consumption (MVO(2)) was calculated as well as the contractile efficiency (E = inverse slope of the MVO(2)-PVA relation). In a second series (n=8) the effect of ischemic preconditioning (3 min no-flow and 8 min reperfusion before the 20 min low-flow ischemia) was tested.

RESULTS

In the first series, systolic function was impaired during reperfusion: aortic flow to 32% of control, dP/dt(max) to 74% and the slope of ESPVR to 73%. Early relaxation in terms of dP/dt(min) decreased to 76%. The slope of the EDPVR was steeper in stunned myocardium with an increase of the ventricular stiffness (m increased from 3.2 to 4.1) and with an upward shift of the EDPVR (c from 0.6 to 2.4 mmHg). Coronary resistance was increased (from 0.9 to 1.4 mmHg/ml per min) and PVA was significantly decreased to 68%, whereas MVO(2) was not, indicating also a decrease in contractile efficiency E from 28 to 14%. In the second series, recovery of systolic function was significantly improved by IP compared with the first series (aortic flow 56% of preischemic control, dP/dt(max) to 91% and ESPVR to 78%). LV stiffness m was also slightly increased from 3.1 to 3.9 and again, c was elevated, indicating no beneficial effect for diastolic properties including dP/dt(min) (77%). But IP improved R(cor) significantly (from 0.9 to only 1.0 mmHg/ml per min) and efficiency E to 21% (from 27% during control).

CONCLUSION

Brief episodes of ischemia not only induce systolic but also diastolic and vascular stunning at almost maintained MVO(2). The decreased contractile efficiency clearly indicates an impaired O(2)-utilization of the contractile apparatus. Ischemic preconditioning did not improve diastolic function during reperfusion, but it provided protection with respect to vascular stunning and myocardial energetics.

Effect of immersion, submersion, and scuba diving on heart rate variability

BACKGROUND

Heart rate variability (HRV) describes the cyclic variations in heart rate and offers a non-invasive tool for investigating the modulatory effects of neural mechanisms elicited by the autonomic nervous system on intrinsic heart rate.

OBJECTIVE

To introduce the HRV concept to healthy volunteers under control conditions and during scuba diving. In contrast with more established manoeuvres, diving probably activates both the sympathetic and parasympathetic nervous system through various stimuli-for example, through cardiac stretch receptors, respiration pattern, psychological stress, and diving reflex. A further aim of the study was to introduce a measure for determining a candidate's ability to scuba dive by providing (a) standard values for HRV measures (three from the time domain and three from the frequency domain) and (b) physiological responses to a strenuous manoeuvre such as scuba diving.

METHODS

Twenty five trained scuba divers were investigated while diving under pool conditions (27 degrees C) after the effects of head out immersion and submersion on HRV had been studied.

RESULTS AND CONCLUSIONS

(a) Immersion under pool conditions is a powerful stimulus for both the sympathetic and parasympathetic nervous system. (b) As neither the heart rate nor the HRV changed on going from immersion to submersion, the parasympathetic activation was probably due to haemodynamic alterations. (c) All HRV measures showed an increase in the parasympathetic activity. (d) If a physiological HRV is a mechanism for providing adaptability and flexibility, diving should not provoke circulatory problems in healthy subjects. (e) Either a lower than normal HRV under control conditions or a reduction in HRV induced by diving would be unphysiological, and a scuba diving candidate showing such characteristics should be further investigated.

Another view of myocardial hibernation

This manuscript brings together three newer concepts: myocardial hibernation, heterogeneity in myocardial blood flow and oxidative metabolism, and effects of hibernating animal serum on non-hibernators. Myocardial hibernation is viewed as a protective mechanism that helps to maintain myocardial integrity and viability by down-regulating contractile function as an adaptation to reduced blood flow. Myocardial flow is considerably heterogeneous. Consequently, oxygen supply to the myocardium is also heterogeneous. Many lines of evidence show a close correlation between regional flow and regional metabolism. In low-flow/low-metabolism areas, myocardial function must be reduced, since the myocardium would otherwise undergo necrosis. Because no regional histological differences exist, the pattern of heterogeneity seems to shift over time. Hence, we hypothesize that such very regional hibernation presents an evolutionary, protective mechanism, permitting subsequent myocardial areas to rest within the ceaselessly working heart. We also hypothesize that a similar mechanism ensures the down-regulation of function following myocardial ischemia in order to induce myocardial hibernation on a broader level. Surprisingly, a substance (opioid in nature) contained in hibernator serum both induced hibernation-like state in non-hibernators and suppressed myocardial oxygen consumption. Thus, we lastly hypothesize that myocardial hibernation is a remnant of the early stages of evolution and is closer to physiological hibernation than traditionally viewed.

[Gregg phenomenon and garden hose effect]

Under physiologic circumstances, cardiac function determines myocardial oxygen consumption and consequently coronary perfusion. Surprisingly, in a reverse direction, improved coronary perfusion also increased myocardial oxygen consumption and contractile function. This experimental finding, now 40 years old, is termed the Gregg phenomenon. Some 10 years later, in experiments by Arnold and co-workers, an isolated increase in perfusion pressure improved ventricular function. In this context, the term 'gardenhose effect' was coined, implying a hydraulic explanation of the Gregg phenomenon. In the following, we attempt to distinguish the Gregg phenomenon from the gardenhose effect and to critically evaluate them.

Optimal method to achieve consistently low defibrillation energy requirements

Reduction of the defibrillation energy requirement offers the opportunity to decrease implantable cardioverter defibrillator (ICD) size and to increase device longevity. Therefore, the purpose of this prospective study was to obtain confirmed defibrillation thresholds (DFTs) of < or = 15 J in each patient with an endocardial dual-coil lead system incorporating an active pectoral pulse generator (TRIAD lead system: RV- --> SVC+ + CAN+). According to our previous clinical and experimental studies, we tried to lower DFTs that were > 15 J by repositioning the distal coil of the endocardial lead system in the right ventricle. A total of 190 consecutive patients requiring ICDs for ventricular fibrillation and/or recurrent ventricular tachycardia were investigated at the time of ICD implantation (42 women, 148 men; mean age 61.9 +/- 12.0 years; mean left ventricular ejection fraction 42.7 +/- 16.6%). Coronary artery disease was present in 139 patients; nonischemic dilated cardiomyopathy in 34 patients; and other etiologies in 17 patients; 47 patients had undergone previous cardiac surgery. Regardless of optimal pacing and sensing parameters, for patients having DFTs > 15, we repositioned the distal coil of the endocardial lead system toward the intraventricular septum to include this part of both ventricles within the electrical defibrillating field. In 177 of 190 patients, induced ventricular fibrillation was successfully terminated with < or = 15 J (group I) using the initial lead position. Repositioning of the endocardial lead was necessary in 13 patients whose DFT(plus) (DFT(plus) = second additional success at lowest energy level) were > 15 J (group II). In all patients, repositioning was successful within a 15 J energy level (100% success). The mean DFT(plus) was 7.3 +/- 3.5 J (group I) and 11.0 +/- 4.5 J (group II; p<0.005). The mean DFT(plus) of all patients enrolled in the study was 7.6 +/- 3.7 J (range: 2 to 15 J). In 87% of all patients, DFT(plus) of < or = 10 J was achieved. Repositioning of the endocardial lead in the right ventricle is a simple and effective method to reduce intraoperative high DFTs. As a result of this procedure, ICDs with a 20 J output should be sufficient for the vast majority (87%) of our patients. Furthermore, we were able to avoid additional subcutaneous or epicardial electrodes in all patients.

Heterogeneity of local myocardial flow and oxidative metabolism

In mammalian hearts, local myocardial flow (LMF) varies between 20 and 200% of the mean. It is not clear whether oxidative metabolism has a similar degree of heterogeneity. Therefore, we investigated the relation between LMF and local oxidative metabolism in isolated rabbit hearts. Buffer oxygenation with (18)O(2) resulted in labeled myocardial oxidation water (H(2)(18)O). In four hearts, myocardial oxygen consumption (MVO(2)) was calculated from the H(2)(18)O production and compared with that calculated according to Fick. In eight additional hearts, LMF was measured using microspheres. Coronary venous H(2)(18)O kinetics and local H(2)(18)O residues were determined and analyzed by mathematical modeling. MVO(2) recovery from H(2)(18)O was >93% compared with that according to Fick. LMF ranged from 1.91 to 11.24 ml. min(-1). g(-1), and local H(2)(18)O residue ranged from 0.41 to 1.04 micromol/g. Both variables correlated (r = 0.62, n = 64, P < 0.001). Measurements in nine hearts were fitted by modeling using capillary permeability-surface area products (PS(c)) from 2 to 10 ml. min(-1). g(-1). With flow-proportional PS(c), a 3.33-fold difference in LMF was associated with a 6.45-fold difference in local MVO(2). Both LMF and local oxidative metabolism are spatially heterogeneous, and they correlate to one another.

Potential role of endothelin-1 and endothelin antagonists in cardiovascular diseases

The endothelins comprise a family of three isopeptides ET-1, ET-2 and ET-3, whereby ET-1 appears to be the most relevant in humans. They act in a paracrine manner on ETA and ETB receptors. ET-1 plays an important role in the cardiovascular system. In addition, it modulates vasomotion and growth processes, and it participates in thrombogenesis and neutrophil adhesion. This review summarizes some of the current literature pertaining to the physiological and pathophysiological significance of ET-1, focusing the assets and drawbacks of elevated ET-1 levels. In this regard, modulation of the endothelin system by either receptor blockade or by inhibition of endothelin converting enzyme is expected to provide novel therapeutic drug strategies.

Myocardial efficiency in stunned myocardium. Comparison of Ca(2+)-sensitization and PDE III-inhibition on energy consumption

OBJECTIVE

In stunned myocardium oxygen consumption is relatively high compared with the reduced ventricular function. On the other hand, inotropic stimulation is frequently required to improve postischemic ventricular dysfunction. However, inotropic agents which act via intracellular increased calcium result in a higher oxygen demand. Therefore Ca(2+)-sensitization might be a favorable alternative.

METHODS

The effects of a novel Ca(2+)-sensitizer (EMD 60263, 10 microM, group 1) were compared with a phosphodiesterase (PDE) III-inhibitor (enoximon, 20 microM, group 2) on 14 isolated, blood-perfused rabbit hearts during reperfusion after a global ischemia of 20 min. Ventricular function, the pressure-volume area (PVA, a measure of total mechanical work), and total myocardial oxygen consumption (MVO(2)) were assessed. Contractile efficiency (EF(cont)), derived from the reciprocal of the slope of the MVO(2)-PVA relation, and external efficiency (EF(ex), stroke work/MVO(2)), were calculated.

RESULTS

At matched heart rate (group 1: 141+/-10 min(-1) group 2: 151+/-28 min(-1)) and end-diastolic volume (1.3+/-0.2 ml) systolic variables were significantly decreased in stunned myocardium: LVP(max) to 57+/-13% of control value in group 1 and to 76+/-7% in group 2, aortic flow to 20+/-4 vs. 25+/-8%. PVA was decreased to 57+/-13 and 67+/-11%, MVO(2) was non-significantly decreased to 73+/-22 and 88+/-14%. After administration of either inotropic agent LVP(max) was significantly improved to 96+/-12 vs. 90+/-8% compared with preischemic levels, aortic flow to 103+/-24 vs. 88+/-9%, and PVA 99+/-11 vs. 89+/-16%, respectively. EMD 60263 increased MVO(2) to control levels (107+/-9%), and enoximon raised MVO(2) even more significantly above control (139+/-13%). Both myocardial efficiency indices were significantly diminished during reperfusion: EF(ex) to 14+/-9 vs. 23+/-7% and EF(cont) to 71+/-7 vs. 65+/-9% compared with preischemic levels. EF(ex) (109+/-21%) was significantly, but EF(cont) only slightly (84+/-11%) increased after administration of EMD 60263, whereas EF(ex) (57+/-13%) and EF(cont) (71+/-12%) remained depressed after enoximon.

CONCLUSIONS

In stunned myocardium, the decreased efficiency indices show that energy utilization is disturbed. Both agents recruited an inotropic reserve, whereas Ca(2+)-sensitization seemed to be more favorable in terms of myocardial efficiency indices. These results indicate that alteration of myocardial calcium sensitivity contributes a major part to postischemic dysfunction. Therefore, Ca(2+)-sensitization may potentially be a superior method for inotropic support in the postischemic heart.

[Myocardial hibernation: another view]

In the following, three newer concepts are brought together: myocardial hibernation, heterogeneity in myocardial blood flow and oxidative metabolism, and effects of hibernating animal serum on non-hibernators. Myocardial hibernation is viewed as a protective mechanism that helps to maintain myocardial integrity and viability by down-regulating contractile function as an adaptation to reduced blood flow. Myocardial flow is considerably heterogeneous. Consequently, oxygen supply to the myocardium is also heterogeneous. Many lines of evidence show a close correlation between regional flow and regional metabolism. In low-flow/low-metabolism areas, myocardial function must be reduced, since the myocardium would otherwise undergo necrosis. Thus, others and we hypothesize that function must be down-regulated to induce hibernation in low-flow areas. Because no regional histologic differences exist (the mitochondria are uniformly distributed within the myocardium), the pattern of heterogeneity seems to shift over time. Hence, we hypothesize that such very regional hibernation presents an evolutionary, protective mechanism, permitting subsequent myocardial areas to rest within the ceaselessly working heart. We also hypothesize that this mechanism ensures the down-regulation of function following myocardial ischemia in order to induce myocardial hibernation on a broader level. Surprisingly, a substance (opioid in nature) contained in hibernator serum both induced hibernation-like state in non-hibernators and suppressed myocardial oxygen consumption. Thus, we lastly hypothesize that myocardial hibernation is a remnant of the early stages of evolution and is closer to physiologic hibernation than traditionally viewed.

Does ischemic preconditioning require reperfusion before index ischemia?

BACKGROUND

Ischemic preconditioning (IP) is initiated through one or several short bouts of ischemia and reperfusion which precede a prolonged ischemia. To test whether a reperfusion must precede the prolonged index ischemia, a series without reperfusion (intraischemic preconditioning: IIP) and a series with gradual onset of ischemia, i.e. ramp ischemia (RI), which is possibly related to the development of hibernation, was compared to conventional IP (CIP).

METHOD

Experiments were performed an 27 blood-perfused rabbit hearts (Langendorff apparatus) that were randomized into one of four series: (1) control (n = 7): 60 min normal flow - 60 min low flow (10%) ischemia - 60 min reperfusion. (2) CIP (n = 7): 4 times 5 min zero flow with 10 min reperfusion each - 60 min low flow (10%) - ischemia 60 min reperfusion. (3) IIP (n = 7): 50 min normal flow - 10 min no flow - 60min low flow (10%) ischemia -4 60min reperfusion. (4) RI (n=6): gradual reduction to 10% flow during 60min - 60min low flow (10%) ischemia - 60min reperfusion. At the end of each protocol, the infarcted area was assessed.

RESULTS

The infarct area in control hearts was 6.7+/-1.4% (means+/-SEM) of LV total area, in CIP hearts 2.6+/-0.8%, in IIP hearts 3.1+/-0.5%, and in RI hearts 3.0+/-0.3% (all p<0.05 vs. control). The differences between the three protection protocols were statistically not significant, and no protective protocol reduced post-ischemic myocardial dysfunction.

CONCLUSION

The preconditioning effect (infarct size reduction) appears not to depend on intermittent reperfusion. Thus, the protective mechanism of IP develops during the initial ischemia that precedes the index ischemia. Alternatively, low-flow ischemia is effectively a sort of reperfusion.

Effect of implantable cardioverter/defibrillator lead placement in the right ventricle on defibrillation energy requirements. A combined experimental and clinical study

OBJECTIVES

The effect of implantable cardioverter/defibrillator (ICD) lead placement in the right ventricle (RV) on defibrillation efficacy has not been thoroughly investigated. Therefore, the goal of this combined experimental and clinical study was to evaluate the effect of a septal and a non-septal position of the right ventricular endocardial spring lead on defibrillation energy.

METHODS

In 12 isoflurane-anaesthetized swine and subsequently in 8 patients who underwent ICD implantation, two different positions of the distal spring lead in the RV were investigated in randomized order: non-septal position (free wall of the RV) and septal position (interventricular septum). For each position, separate 50% probability determinations of energy (E50), peak voltage (V50) and peak current (A50) were calculated using the three reversal up/down defibrillation procedure. The E50, V50, A50 and impedance (I) were averaged and compared using the two-sided t-test for paired samples.

RESULTS

Both the experimental study and the clinical study demonstrated that placing the distal defibrillation lead near to the septum rather than near to the ventricular free wall resulted both in the swine and in the patients in significantly lower E50-31.6%/ - 37.1%, V50-16.1%/-20.9% and A50 -10.0%/ - 24.2%, respectively. Defibrillation impedances were significantly reduced only in the experimental study.

CONCLUSIONS

Defibrillation efficacy depends on the position of the distal spring electrode in the RV. A septal position significantly reduces the energy requirements compared to a non-septal position. The decrease in energy requirements might be explained by an increase in current flow through the septum and the posterolateral wall of the left ventricle. reserved