Evidence of functional myocardial ischemia associated with myocardial dysfunction in brain-dead pigs.

BACKGROUND: Cardiac dysfunction after brain death has been documented, but its mechanisms remain unclear. Myocardial ischemia has been suggested as a possible cause. The aim of the present study was to investigate the existence of an imbalance between myocardial oxygen delivery and demand as a possible cause of myocardial dysfunction in brain-dead pigs. METHODS AND RESULTS: Interstitial myocardial lactate and adenosine concentrations were assessed with cardiac microdialysis in 2 groups of animals: brain-dead pigs (n=7) and brain-dead pigs treated with labetalol (10+/-3 mg/kg) (n=7). Heart rate (HR), left ventricular (LV) dP/dt(max), rate-pressure product (RPP), cardiac output (CO), and left anterior descending coronary artery blood flow (QLAD) were continuously monitored. Brain-dead pigs exhibited a transient significant increase in HR, LV dP/dt(max), RPP, and CO and a limited increase in QLAD. This resulted in functional myocardial ischemia attested to by the significantly increased adenosine and lactate microdialysate concentrations. In brain-dead pigs treated with labetalol, there was a moderate increase in HR, QLAD, and adenosine microdialysate concentrations; LV dP/dt(max), RPP, CO, and myocardial lactate concentrations remained stable, confirming the preservation of aerobic metabolism. CONCLUSIONS: Brain death was associated with an increase in myocardial interstitial adenosine and lactate concentrations, as well as with myocardial dysfunction; all were attenuated by labetalol, suggesting an imbalance between oxygen consumption and oxygen delivery as a possible cause of myocardial dysfunction after brain death.

High-performance liquid chromatographic analysis of muscular interstitial arginine and norepinephrine kinetics. A microdialysis study in rats.

Complex interactions between the L-arginine/nitric oxide synthase (NOS) pathway and the sympathetic nervous system have been reported. Methods capable of measuring L-arginine and norepinephrine (NE) have mainly been reported for plasma. We report the use of the microdialysis technique combined with high-performance liquid chromatography (HPLC) for measurement of both L-arginine and NE within the same tissue microdialysis sample. The microdialysis probe consisted of linear flexible probes (membrane length: 10 mm, outside diameter: 290 microm, molecular weight cut-off 50 kDa). The method used for L-arginine measurement was HPLC with fluorescence detection, giving a within-run and a between-day coefficient of variation of 2.9 and 12.8%, respectively. The detection limit was 0.5 pM/20 microl injected for L-/D-arginine. The method used for NE measurement was HPLC with electrochemical detection. The coefficients of variation were 4% for within-assay precision and 7.5% for between-assay precision. The detection limit for NE was 1 fmol/20 microl injected. The microdialysis technique coupled with HPLC system was validated in vivo to measure muscular interstitial concentrations of both arginine and NE under baseline conditions and after intravenous infusion of 500 mg/kg of L-arginine or D-arginine. In conclusion, the microdialysis technique coupled to HPLC allows the simultaneous measurements of both L-arginine and NE within the same tissue microenvironment and will enable the study of the complex interactions between the L-arginine/NO pathway and sympathetic nervous system within the interstitial space of different organs.

Consequences of inspired oxygen fraction manipulation on myocardial oxygen pressure, adenosine and lactate concentrations: a combined myocardial microdialysis and sensitive oxygen electrode study in pigs.

Adenosine is a potent vasodilator whose concentration has been shown to increase in cardiac tissue in response to hypoxia. However, the time-dependent relationship between the levels of myocardial interstitial adenosine and tissue oxygenation has not yet been completely established. Therefore, the purpose of this study was to investigate the complex relationship between tissue myocardial oxygen tension (PtiO(2)) and interstitial myocardial adenosine and lactate concentrations by developing a new technique which combines a cardiac microdialysis probe and a Clark-type P O(2)electrode. The combined and the single microdialysis probes were implanted in the left ventricular myocardium of anesthetized pigs. The consequences of the combined use of microdialysis and P O(2)probes on myocardial PtiO(2)and microdialysis performances against glucose were evaluated. A moderate but significant reduction in the relative recovery against glucose of the combined probe was observed when compared to that of the single microdialysis probe (42+/-2 v 32+/-1%, mean+/-S.E. M.n=5 P<0.05), at 2microl/min microdialysis probe perfusion flow. Similarly, myocardial oxygen enrichment, measured by the P O(2)electrode, was negligible when microdialysis probe perfusion flow was 2microl/min. Systemic hypoxia (FiO(2)=0.08) resulted in a significant decrease in PtiO(2)from 30+/-4 to 11+/-2 mmHg, limited increase in coronary blood flow (CBF), and a significant increase in myocardial adenosine and lactate concentrations from 0.34+/-0.05 to 0.98+/-0.06micromol/l and from 0.45+/-0.05 to 0.97+/-0.06 mmol/l respectively (P<0.05). Increasing the FiO(2)to 0.3 restored the PtiO(2)and hemodynamic parameters to baseline values with no changes in interstitial adenosine and lactate concentrations. Nevertheless, myocardial interstitial adenosine remained significantly higher than baseline values. In conclusion, this study demonstrates the ability of a combined probe to measure simultaneously regional myocardial PtiO(2)and metabolite concentration during hypoxia. The hypoxia-induced increase in myocardial adenosine persists after correction of hypoxia. The physiological significance of this observation requires further studies.

Consequences of labetalol administration on myocardial beta adrenergic receptors in the brain dead pig.

OBJECTIVES: Cardiac dysfunction following brain death is associated with highly increased myocardial norepinephrine, lactate and adenosine concentrations. Administration of labetalol, a mixed alpha-, beta-adrenergic receptor antagonist, attenuates metabolic disturbances and improves myocardial function. The purpose of this study was to investigate beta-adrenergic receptor (beta AR) density and affinity in the presence or absence of labetalol administration, as a possible mechanism of the protective effects of this drug. METHODS: Experimental animals were divided into three groups: sham-operated, brain-dead pigs, and brain-dead pigs treated with labetalol (10 +/- 3 mg/kg). The maximum number of binding sites (Bmax) and the dissociation constant (Kd) of beta AR were determined with (-)-[125I]cyanopindolol on myocardial samples harvested 3 hours after brain death. RESULTS: Left ventricular beta AR density and affinity were identical in brain-dead and sham-operated animals. Labetalol-treated pigs exhibited a significant decrease of Bmax and an increase of Kd as compared with brain-dead pigs. Bmax decrease was due to the persistence of labetalol in the membrane preparations. Increased Kd was too low to be biologically significant. Therefore, beta AR number and affinity can be considered as unchanged after adrenergic blockade with labetalol. CONCLUSIONS: The protective mechanism of labetalol on brain death-induced myocardial dysfunction cannot be explained by changes in beta AR density and affinity but is probably related to a preservation of the oxygen consumption/oxygen delivery balance during the autonomic storm.

Consequences of static and pulsatile pressure on transmembrane exchanges during in vitro microdialysis: implication for studies in cardiac physiology.

Microdialysis is an established technique for measuring the kinetics of various neurotransmitters within the extracellular space in the field of neurochemistry. Recently, its use has been extended to sampling in other tissues, including liver, kidney and the heart. A persistent problem in cardiac microdialysis concerns two parameters related to myocardial function: pressure and frequency (heart rate). The aim of the study is to evaluate the consequences of pressure and frequency on transmembrane exchanges. Linear flexible microdialysis probes (membrane length: 12 mm, outside diameter: 390 microns, MWCO 50,000 Daltons) were designed in our laboratory. The probes, perfused at 2 microL/min with sterile water, were placed in a system filled with a glucose solution (2 g/L) and able to generate either static: 0 to 400 mmHg (0 to 53.31 kPa) or pulsatile pressure: 0-100; 0-200; 0-300 mmHg (0-13.32; 0-26.65; 0-39.98 kPa) at different frequencies: 1, 2 and 3 Hz. At 2 mu litre min-1 perfusion rate, the pressure inside the probe is estimated to be 80 mmHg (10.66 kPa). Under static pressure conditions, the glucose recovery rate can be expressed as an exponential function, and the outflow rate can be expressed as a linear function of the external pressure level. Under dynamic conditions, the external mean pressure must be accounted for. When external mean pressure exceeds 80 mmHg (10.66 kPa) (pressure generated by the flow rate of perfusion inside the probe), the recovery rate increases with frequency. Conversely, if the outer mean pressure is lower than 80 mmHg (10.66 kPa), the recovery rate decreases with frequency. Theoretical and experimental modelling results in a nomogram that can be used to estimate in vivo recovery. In conclusion, mass transfer across a microdialysis membrane is dependent on the direction of the transmembrane pressure gradient and increases with heart rate. These findings must be taken into account when in vivo recovery rates during cardiac microdialysis are determined.

Increase in myocardial interstitial adenosine and net lactate production in brain-dead pigs: an in vivo microdialysis study.

BACKGROUND: Brain death-related cardiovascular dysfunction has been documented; however, its mechanisms remain poorly understood. We investigated changes in myocardial function and metabolism in brain-dead and control pigs. METHODS: Heart rate, systolic (SAP) and mean (MAP) arterial pressure, left ventricular (LV) dP/dtmax, rate-pressure product, cardiac output (CO), left anterior descending coronary artery blood flow, lactate metabolism, and interstitial myocardial purine metabolite concentrations, monitored by cardiac microdialysis, were studied. A volume expansion protocol was performed at the end of the study. RESULTS: After brain death, a transient increase in heart rate (from 90 [67-120] to 158 [120-200] beats/min) (median, with range in brackets), MAP (82 [74-103] to 117 [85-142] mmHg), LV dP/dtmax (1750 [1100-2100] to 5150 [4000-62,000] mmHg x sec(-1), rate-pressure product (9100 [7700-9700] beats mmHg/min to 22,750 [20,000-26,000] beats mmHg/min), CO (2.2 [2.0-4.0] to 3.3 [3.0-6.0] L/min), and a limited increase in left anterior descending coronary artery blood flow (40 [30-60] to 72 [50-85] ml/min) were observed. Net myocardial lactate production occurred (27 [4-40] to -22 [-28, -11] mg/L, P<0.05) and persisted for 2 hr. A 6-7-fold increase in adenosine dialysate concentration was observed after brain death induction (2.9 [1.0-5.8] to 15.8 [7.0-50.7] micromol/L), followed by a slow decline. Volume expansion significantly increased MAP, CO, and LV dP/dtmax in control animals, but decreased LV dP/dtmax and slightly increased CO in brain-dead animals. A significant increase in adenosine concentration was observed in both groups, with higher levels (P<0.05) in brain-dead animals. CONCLUSIONS: Brain death increased oxygen demand in the presence of a limited increase in coronary blood flow, resulting in net myocardial lactate production and increased interstitial adenosine concentration consistent with an imbalance between myocardial oxygen demand and supply. This may have contributed to the early impairment of cardiac function in brain-dead animals revealed by rapid volume infusion.

[Protective effect of labetalol on cardiovascular consequences of brain death in the swine]. / Effet protecteur du labétalol contre les conséquences cardiovasculaires de la mort cérébrale chez le porc.

OBJECTIVE: Assessment of the preventive effect on cardiovascular changes following experimental brain death (BD) in the pig by pretreatment with labetalol, an alpha and beta adrenoreceptor blocking agent. STUDY DESIGN: Experimental study. ANIMALS: Ten 25-35 kg domestic pigs allocated either in the control group (n = 5) or the labetalol group (n = 5). METHODS: BD was achieved in anaesthetized animals by the rapid inflation of a Foley catheter inserted into the sub-dural space. In the labetalol group, the agent (total: 10 +/- 3 mg.kg-1) was administered immediately before BD and thereafter over a 20-min period, in order to maintain haemodynamic parameters at control values. The following haemodynamic data were recorded over a 3 hour period after BD: heart rate (HR), dP/dtmax, mean arterial pressure (MAP), pulmonary capillary wedge pressure (PCWP), cardiac output (CO) and left anterior descending coronary artery blood flow (CBF). Afterwards, a dynamic loading test with 500 mL of dextran over 20 min was performed. RESULTS: In the control group, BD elicited a significant increase in HR (from de 96 +/- 9 to 176 +/- 11 b.min-1), dP/dtmax (from 1,960 +/- 123 to 4,904 +/- 930 mmHg.s-1), MAP (from 88 +/- 5 to 119 +/- 11 mmHg), CO (from 2.4 +/- 0.2 to 3.6 +/- 0.7 L.min-1) and CBF (from 45 +/- 6 to 73 +/- 7 mL.min-1) respectively. Apart from a slight increase in HR and a significant increase in CBF (from 34 +/- 4 to 55 +/- 6 mL.min-1), no other modifications occurred in the labetalol group. Following volume expansion, the labetalol group animals experienced a significant increase in CO (from 2.3 +/- 0.3 to 3.7 +/- 0.2 L.min-1), dP/dtmax (from 1,400 +/- 91 to 2,100 +/- 212 mmHg.s-1) and MAP (from 55 +/- 5 to 70 +/- 5 mmHg). In the opposite, a significant decrease in dP/dtmax (from 1,645 +/- 450 to 628 +/- 152 mmHg.s-1) occurred in the control group. CONCLUSION: The protective effect of labetalol confirms the role played by the activation of the cardiac sympathetic nervous system in the cardiocirculatory changes following BD.