Myocardial metabolism and efficiency after warm continuous blood cardioplegia

The effect of Riociguat on cardiovascular function and efficiency in healthy, juvenile pigs

INTRODUCTION

Riociguat is a soluble guanylate cyclase stimulator approved for the treatment of pulmonary hypertension. Its effect on cardiometabolic efficiency is unknown. A potential cardiac energy sparing effect of this drug could imply a positive prognostic effect, particularly in patients with right heart failure from pulmonary hypertension.

METHOD

We infused Riociguat in six healthy juvenile pigs and measured the integrated cardiovascular effect and myocardial oxygen consumption. To assess the interplay with NO-blockade on cardiac function and efficiency we also administered the NO-blocker L- NAME to the animals after Riociguat.

RESULTS AND DISCUSSION

Infusion of 100 µg/kg Riociguat gave modest systemic vasodilatation seen as a drop in coronary and systemic vascular resistance of 36% and 26%, respectively. Right and left ventriculoarterial coupling index (Ees/Ea), stroke work efficiency (SWeff), and the relationship between left ventricular myocardial oxygen consumption (MVO₂ ) and total mechanical work (pressure-volume area; PVA) were unaffected by Riociguat. In contrast, systemic and pulmonary vasoconstriction induced by L-NAME (15 mg/kg) shifted the Ees/Ea ratio toward reduced SWeff in both systemic and pulmonary circulation. However, there was no surplus oxygen consumption, that was measured by the MVO₂ /PVA relationship after L-NAME in Riociguat-treated pigs. This suggests that Riociguat can reduce the NO-related cardiometabolic inefficiency previously observed by blocking the NO pathway.

An anaesthetic protocol in the young domestic pig allowing neuromuscular blockade for studies of cardiac function following cardioplegic arrest and cardiopulmonary bypass

BACKGROUND

Neuromuscular blockade should, for ethical reasons, not be allowed in animal experiments unless the use is strongly motivated. Beforehand, the anaesthetic protocol must be documented without muscle relaxation in the species studied. Documentation is difficult to obtain from the scientific literature. When focusing on cardiac function over time, in particular, the ideal anaesthetic protocol should cause no or minor alterations in cardiac variables.

METHODS

We intended to document an anaesthetic protocol involving ventilation with N(2)O combined with loading doses and continuous infusions of pentobarbital, fentanyl and midazolam in seven pigs by applying potentially painful stimuli every 15 min for 7 h. Subsequently, left ventricular global and regional function was studied with conductance catheter and strain rate imaging by echocardiography in eight pigs with pancuronium included.

RESULTS

Pigs without pancuronium were completely immobilized and unresponsive to potentially painful stimuli and sternotomy, with no accumulation or degradation of anaesthetic agents. With pancuronium included, left ventricular preload gradually decreased together with reduction of cardiac index from 3.52 +/- 0.14 at 2 h to 2.84 +/- 0.11 L min(-1). m(-2) (+/-SEM) after 7 h of observation. Preload recruitable stroke work decreased after 7 h, whereas peak systolic strain in the anterior left ventricular wall and load-independent indices of diastolic function were not significantly altered.

CONCLUSION

In specific experimental protocols, the anaesthetic protocol described could allow the use of muscular paralysis in young domestic pigs, for instance when involving hypothermic cardiopulmonary bypass, cardioplegic arrest and reperfusion.

Continuous cold blood cardioplegia improves myocardial protection: a prospective randomized study

BACKGROUND

To assess the influence on myocardial protection of the rate of infusion (continuous vs intermittent) of cold blood cardioplegia administered retrogradely during prolonged aortic cross-clamping. The end-points were ventricular performance and biochemical markers of ischemia.

METHODS

Seventy patients undergoing myocardial revascularization for three-vessel disease were prospectively randomized to receive intermittent or continuous retrograde cold blood cardioplegia. Hemodynamic measurements were obtained using a rapid-response thermodilution catheter and included right ventricular ejection fraction, cardiac output, left and right ventricular stroke work index, and systemic and pulmonary vascular resistance. Blood samples were obtained from the coronary sinus before cross-clamp application and immediately after cross-clamp removal for determinations of lactate and hypoxanthine.

RESULTS

The left ventricular stroke work index trend was significantly superior (p = 0.038) by repeated-measures analysis in continuous cardioplegia. Other hemodynamic measurements revealed a similar trend. The need for postoperative inotropic drugs support was reduced in continuous cardioplegia. The release of lactate in the coronary sinus after unclamping was 2.30 +/- 0.12 mmol/L after intermittent cardioplegia and 1.97 +/- 0.09 mmol/L after continuous cardioplegia (p = 0.036). The release of hypoxanthine was 20.47 +/- 2.74 micromol/L in intermittent cardioplegia and 11.77 +/- 0.69 micromol/L in continuous cardioplegia (p = 0.002).

CONCLUSIONS

Continuous cold blood cardioplegia results in improved ventricular performance and reduced myocardial ischemia in comparison with intermittent administration.

Both glucose-insulin-potassium and glutamine in warm blood cardioplegia increase the rates of myocardial glucose and free fatty acid oxidation

OBJECTIVE

We wanted to assess the effect of glucose-insulin-potassium (GIK) and glutamine on the oxidative metabolism during and after prolonged warm continuous blood cardioplegia (WB).

DESIGN

WB was given to 21 pigs divided into three equally sized groups: WB (control), WB and GIK, or WB and glutamine. Oxidation rates of radiolabeled glucose (14C) and free fatty acid (FFA) (3H) were assessed before, during, and at 30 and 60 min after 3 h of cardiac arrest with WB.

RESULTS

During standstill the substrate oxidation dropped markedly (<60%), glucose oxidation was highest in the WB + GIK group (p < 0.05) and FFA oxidation highest in the WB + glutamine group (NS). During recovery the GIK group had an elevated glucose oxidation (47 and 40% vs WB at 30 and 60 min recovery, respectively -p < 0.05). Following 30 min recovery the addition of GIK suppressed FFA oxidation some 60%. Glutamine increased the oxidation of both glucose (30%) and FFA (150%) following 60 min recovery (p < 0.05). During the whole recovery phase the relative FFA oxidation was significantly lowered in the GIK group. There were no differences between the groups regarding arterial levels or uptake of substrates, except for a higher myocardial oxygen consumption (MVO2) during cardioplegia in the glutamine group. All the hearts performed similarly.

CONCLUSION

Addition of GIK or glutamine to the well-perfused and oxygenated heart during WB led to a postcardioplegic increase in oxidative metabolism and MVO2. GIK resulted in a significant metabolic shift from FFA to glucose.

Cardiac dysfunction and inefficiency after substrate-enriched warm blood cardioplegia

OBJECTIVE

The study assessed the outcome after prolonged warm continuous antegrade blood cardioplegia (WCBC) with substrate enrichment, in terms of mechanical performance and mechanoenergetic efficiency.

METHODS

WCBC was given for 3 h to three groups of pigs on cardiopulmonary bypass; WCBC alone (n=7), WCBC+glucose and insulin (+GIK, n=7) and WCBC+L-glutamine (+GLN, n=7). Cardiac systolic and diastolic function, pressure-volume area (PVA) and myocardial oxygen consumption (MVO(2)) were assessed before, and twice after WCBC using pressure-conductance catheter, coronary flow-probes and O(2)-content difference.

RESULTS

In the WCBC, +GIK and +GLN groups respectively, the following parameters decreased after WCBC compared to baseline: left ventricular developed pressure by 26, 19 and 25% (P<0.001); dP/dt(max) by 36, 37 and 34% (P<0.001); preload recruitable stroke work by 35, 41 and 28% (P<0.001); mechanoenergetic efficiency (PVA/MVO(2)) by 44, 41 and 22% (P<0.001). End-diastolic stiffness increased early after WCBC in the WCBC and +GLN groups, while it was unchanged in the +GIK group (P=0.032).

CONCLUSION

Despite continuous aerobic conditions and additional substrates, post-WCBC cardiac contractile function and mechanoenergetic efficiency was severely depressed. The results demonstrate the hazards of sustained normothermic hyperkalemic perfusion.