Multiparametric monitoring of brain oxygen balance under experimental and clinical conditions

In order to evaluate the relationship between brain oxygen supply and demand (O2 balance) in real time, it is necessary to use a multiparametric monitoring approach. Cerebral blood flow (CBF) is a representative parameter of O2 supply. The extracellular level of K+ is a reliable indicator of O2 demand since more than 60% of the energy consumed by the brain is utilized by active transport processes. Mitochondrial NADH redox state can represent the balance between O2 supply and demand. In order to monitor the brain of experimental animals or patients, we constructed the multiparametric assembly (MPA) and the following parameters were monitored simultaneously and in real time: CBF, CBV, NADH redox state, extracellular K+, DC potential, EEG, tissue temperature and ICP. Animals were exposed to hypoxia, ischemia, hypercapnia, hyperoxia and spreading depression (SD) and the relative changes in CBF and NADH were calculated and found to be significant indicators of brain energy state. Monitoring these two parameters increases the possibility of differentiating between various pathophysiological states. Each added parameter increases the power of diagnosis and determination of the functional state of the brain. Preliminary results obtained in patients monitored in the ICU or in the OR show that the responses to hypercapnia, spreading depression or ischemia are similar to those measured in experimental animals.

Effect of hyperbaric oxygen therapy on survival after global cerebral ischemia in rats

BACKGROUND

Hyperbaric oxygenation (HBO) has been considered for many years for the treatment of severe brain ischemia. However, its efficacy has not been proven. The aim of this study was to shed light on this question.

METHODS

Acute global cerebral ischemia was induced in 18 rats using the four-vessel occlusion model. Regional cerebral blood flow (CBF) was determined by laser-Doppler flowmetry using a flexible 1 mm fiberoptic probe. Two stainless steel screws were used to measure the spontaneous electrical activity from the contralateral hemisphere. After ischemia monitored by laser-Doppler flowmetry and ECoG, the animals were divided into two groups: (1) control animals that breathed air at atmospheric pressure and (2) rats exposed to HBO at three atmospheres absolute pressure (ATA) for 1 hour. Survival time and rate were recorded for both groups of animals for 14 days.

RESULTS

The survival rate in the study group was significantly higher (45%) than in the control group (0%). In the animals that did not survive the 14-day period, those exposed to HBO survived longer than the control animals (59.8+/-9.1 hour versus 17.9+/-2.7 hours, p < 0.05).

CONCLUSION

This investigation demonstrates that HBO administered after global cerebral ischemia can increase survival in a rat stroke model.

Thiopental induced cerebral protection during ischemia in gerbils

Temporary interruption or reduction of cerebral blood flow during cerebrovascular surgery may rapidly result in ischemia or cerebral infarction. Thiopental has been shown to have cerebroprotective effects. However, the cerebroprotective dose of thiopental causes burst suppression of the EEG, thus this parameter cannot be used continuously for the detection of metabolic changes in the brain during thiopental anaesthesia. This study was performed in order to examine whether the multiparametric assembly (MPA), which measures energy metabolism CBF and mitochondrial (NADH) as well as extracellular ion concentrations (K+), can shed light on the mechanism of the cerebroprotective effects of thiopental. The MPA was placed on the brain of Mongolian gerbils and burst suppression of the ECoG was induced by thiopental. Cerebral ischemia was induced by occlusion of carotid arteries after burst suppression. Burst suppression of the ECoG was accompanied by a significant decrease in cerebral blood flow. In animals that received thiopental prior to ischemia, NADH increased to a lesser degree and extracellular potassium ion concentration increased to a lesser degree than in the control animals, indicating that thiopental affords protection of the brain under ischemic conditions due to improved energy metabolism. This study also demonstrates that the MPA can monitor changes occurring in the cerebral cortex even after the ECoG can no longer be used. Those findings have a significant value in the development of a new clinical monitoring device.

Responses of rat brain to induced spreading depression following exposure to carbon monoxide

Until recently carbon monoxide (CO) was known only for its noxious effects. Exposure to CO results in an autoregulatory increase in cerebral blood flow (CBF). Little information is available on brain energy metabolism under low CO concentrations and on the effect of CO on the stimulated brain. In this study cortical spreading depression (SD) was induced in order to cause transient brain depolarization and increased energy demand. The multisite assembly (MSA), which contains four bundles of optical fibers for monitoring the intramitochondrial NADH redox state and tissue reflectance as well as four DC electrodes enabling measurement from four consecutive points on the cerebral cortex, was used to measure energy metabolism and the propagation of SD waves during exposure to CO. CBF in the contralateral hemisphere was measured using the laser Doppler technique. Three experimental groups of animals were examined: SD was induced during exposure to 1000 ppm CO, immediately after exposure to CO and 90 min after cessation of exposure to CO. Three control groups were also examined, in which the animals underwent the same procedures but were not exposed to CO. In all animals exposure to CO was followed by a significant increase in CBF. The greatest effect was found when SD was induced immediately after cessation of exposure to CO. SD wave frequency decreased when induced immediately after exposure to CO, whereas it increased when SD was induced 90 min after exposure. The amplitude of the NADH oxidation waves and their integral were smaller during SD induced immediately after exposure to CO. The DC potential did not change, suggesting that CO did not affect the SD initiation mechanism but rather resulted in energy depletion during recovery from SD. This study demonstrates that even at a concentration of 1000 ppm CO interferes with the metabolic activity of the brain during repolarization of the SD-induced negativity.

Can the Indo-1 fluorescence approach measure brain intracellular calcium in vivo? A multiparametric study of cerebrocortical anoxia and ischemia

Indo-1 fluorescence was used to monitor intracellular calcium levels in the cat brain in vivo, using the approach proposed by Uematsu et al. [Uematsu D., Greenberg J. H., Reivich M., Karp A. In vivo measurement of cytosolic free calcium during cerebral ischemia and reperfusion. Ann Neurol 1988; 24: 420-428]. In addition, extracellular calcium and potassium levels, NADH redox state, electrocorticogram (ECoG), DC potential and relative cerebral blood flow were monitored simultaneously. Changes in the Indo-1 fluorescence ratio F400/F506 were monitored during anoxia, reversible ischemia and irreversible ischemia. Although these perturbations resulted in the expected changes in extracellular calcium and potassium levels, NADH redox state, ECoG and other physiological parameters, they did not result in significant increases in the F400/F506 ratio. The apparent insensitivity of the in vivo Indo-1 approach is due to the difficulty in obtaining accurate fluorescence signals from Indo-1 in the brain. Two reasons for this difficulty appear to be problems in loading Indo-1 into the brain, and problems in correcting Indo-1 fluorescence signals for changes in NADH fluorescence and changes in absorption of intrinsic chromophores. Under the conditions of our in vivo cat experiments, Indo-1 fluorescence is not a viable approach for measuring changes in cerebral intracellular calcium levels.

Cortical spreading depression recorded from the human brain using a multiparametric monitoring system

The number of parameters (i.e., EEG or ICP-intracranial pressure) routinely monitored under clinical situations is limited. The brain function analyzer described in this paper enables simultaneous, continuous on-line monitoring of cerebral blood flow (CBF) and volume (CBV), intramitochondrial NADH redox state, extracellular K+ concentrations, DC potential, electrocorticography and ICP from the cerebral cortex. Brain function of 14 patients with severe head injury (GCS < or = 8), who were hospitalized in the neurosurgical or general intensive care unit was monitored using this analyzer. Leao cortical spreading depression (SD) has been reported in many experimental animals but not in the human cerebral cortex. In one of the patients monitored, spreading depression was observed. This is the first time that spontaneous repetitive cortical SD cycles have been recorded from the cerebral cortex of a patient suffering from severe head injury. Typical SD cycles appeared 4-5 h after the beginning of monitoring this patient. During the first 3-4 cycles the responses of this patient were very similar to the responses to SD recorded in normoxic experimental animals. Electrocorticography was depressed whereas extracellular K+ levels increased. The metabolic response to spreading depression was characterized by oxidation of intramitochondrial NADH concomitant to a large increase in CBF. During brain death, an ischemic depolarization, characterized by decrease in CBF and an irreversible increase in extracellular K+, was recorded.

Effects of carbon monoxide on the brain may be mediated by nitric oxide

Carbon monoxide (CO) is known to be a toxic molecule due to the high affinity of hemoglobin for it. However, it has recently been shown that low doses of CO may play a physiological role. The aim of the present study was to examine processes occurring in the brain during exposure to 1,000 parts per million CO that result in an increase in cerebral blood flow (CBF) but are not accompanied by changes in oxidation metabolism. This study was carried out in awake rats with the multiprobe assembly developed in this laboratory for the simultaneous continuous measurement of CBF, intramitochondrial NADH redox levels, direct current potential, and extracellular concentrations of K+, Ca2+, and H+ as well as the electrocorticogram. Exposure to 1,000 parts per million CO in air resulted in an increased CBF without any concomitant changes in any of the other metabolic or ionic parameters measured. This indicates that tissue hypoxia was not the trigger for this vasodilation. Injection of N omega-nitro-L-arginine (L-NNA), a nitric oxide synthase inhibitor, before exposure to CO effectively blocked the increase in CBF that was observed when the animal was exposed to CO without prior injection of L-NNA. Furthermore, electrocorticographic depression was observed after the combined treatment of L-NNA and CO. In conclusion, exposure to relatively low doses of CO apparently does not have a deleterious effect on oxidative metabolism because the increase in CBF after this exposure is sufficient to prevent changes in oxidative metabolism, as indicated by the fact that NADH levels remained constant. This protective autoregulatory effect may be mediated by nitric oxide.

Multiparameter monitoring and analysis of in vivo ischemic and hypoxic heart

We describe a unique in vivo technique which addresses the multifactorial function of the heart, i.e., simultaneous measurement of myocardial ion transport (two mini-electrode systems to measure K+e and Ca2+e), energy metabolism (NADH fluorescence to measure NADH redox state), and coronary flow (laser-Doppler perfusion) using a multiprobe assembly (MPA) which contains transducers for all measurements. The MPA (which is 6 mm in diameter) was applied to the external surface of the heart in an open chest dog model. To test MPA function, myocardial ischemia was produced by application of a balloon occluder to the left anterior descending coronary (LAD) artery, and hypoxia was produced by changing the inspired O2-N2 ratio until the PaO2 was 20-30 torr. The MPA simultaneously monitored changes in ion flux, heart metabolism, and tissue perfusion during pathophysiological intervention.

Continuous multiparametric monitoring of brain activities following fluid-percussion injury in rats: preliminary results

Severe head injury can result in a high mortality rate or irreversible brain damage. One technique used to induce traumatic brain injury (TBI) is exposure of the brain to fluid percussion pressure while monitoring the increase in intracranial pressure (ICP). Since brain injury is a multifactorial, pathological, time-dependent state, the multiparametric monitoring approach was adopted for studying fluid percussion effects on the rat brain. A multiprobe assembly (MPA) connected to the brain in vivo (right hemisphere) enabled the simultaneous monitoring of CBF, NADH redox state, extracellular K+, Ca2+, H+ levels as well as DC potential, ECoG and ICP. The animal was connected to the monitoring system and exposed to TBI after a recuperation period of at least 3 hours after the end of the operation. Two typical responses to TBI were recorded in our preliminary experiments. When severe injury was induced, ischemic depolarization (ID) developed, whereas mild or moderate injury led to repetitive spreading depression (SD) cycles. The relationship between the ID and SD observed under TBI is important to the understanding of the mechanism of brain injury. ICP before injury was between 2-6 mm Hg and increased to 20-22 mm Hg 2-3 minutes after the ID. After severe head injury, ICP remained high and in some cases increased to critical values causing death of these animals. Some animals developed seizures at various stages after the TBI. Hyperbaric oxygenation was used as a therapeutic tool to treat severely injured animals. These preliminary results suggest that it is feasible and practical to use the MPA approach for monitoring the brain after TBI.

Multiparametric monitoring of the awake brain exposed to carbon monoxide

We have applied in vivo real-time techniques to monitor the physiological changes associated with exposure to a pattern of carbon monoxide (CO) known to cause brain oxidative stress. Using a multiparametric monitoring device connected to the brain, we exposed unanesthetized rats to two levels of CO, 0.1 and 0.3% in air. Energy metabolism was evaluated by the optical monitoring of relative cerebral blood flow (CBF) and intramitochondrial redox state. Ionic homeostasis was assessed by measurements of K+,Ca2+, and H+ or Na+ levels in the extracellular space. The electrical parameters monitored were the electrocorticogram and direct current steady potential. Under 1,000 ppm of CO, the CBF was increased significantly without any measurable change in the NADH redox state, suggesting that the cause for the increased CBF was not hypoxia. Exposing the awake rat to 1,000 ppm of CO (40 min) followed by 3,000 ppm of CO (20 min) led to an increase in CBF followed by episodes of spontaneous brain depolarizations characterized by changes in ionic homeostasis and blood flow. These changes were similar to those recorded under cortical spreading depression. In most animals exposed to 3,000 ppm of CO, spontaneous oscillations in CBF and NADH redox state that were negatively correlated were recorded. The results indicate that an inspired CO level of 0.1% had effects largely restricted to blood flow, whereas at a higher CO level an additional impairment in energy supply resulted in a complex pattern of effects similar to that caused by brain ischemia.

Brain vasculature and mitochondrial responses to ischemia in gerbils. II. Strain differences and statistical evaluation

The correlation between the anatomy of brain vasculature and the metabolic responses to ischemia was studied in two strains of the Mongolian gerbil as compared to the albino rat. Gerbils belonging to the Meriones unguiculatus obtained from Tumblebrook Farms and from the NY Institute for Basic Research were compared to two lines of the Meriones tristrami obtained from two different breeders in Israel. In all groups of gerbils and in the albino rats, a significant correlation was found between the anatomical patterns of the Circle of Willis and the metabolic responses to ischemia as evaluated by NADH redox state measurements, namely, that with a complete anterior anatomical Circle of Willis the metabolic response to unilateral occlusion was minimal. In the anterior part of the Circle of Willis the various groups of gerbils exhibited the entire range of ischemia levels, from 0-100%, depending upon the anatomical structure of the arteries. In M. tristrami, the anterior communications between the two hemispheres were very well developed as compared to the various degrees of connection found in the different individuals of M. unguiculatus. This anatomical pattern corresponds well to the metabolic response recorded under unilateral and bilateral occlusion. The best connection in the posterior part, was found in M. tristrami. This was less frequent (10-20% of the gerbils) in M. unguiculatus. A clear correlation was found between the size of the posterior communicating artery and the changes in NADH redox state measured during bilateral carotid artery ligation. The variation between gerbils of the same litter was significantly smaller than that of the general population of the same strain.(ABSTRACT TRUNCATED AT 250 WORDS)

Brain vasculature and mitochondrial responses to ischemia in gerbils. I. Basic anatomical patterns and biochemical correlates

A unique blood supply to the brain, the 'Circle of Willis' (COW), exists in all mammals except for the Mongolian gerbil (Meriones unguiculatus). This system is capable of compensating for a decrease in blood supply in one of the four arteries, which may occur during pathological conditions. The posterior connection between the basilar artery and the carotid artery system have been found to be missing in most gerbils. Furthermore, in some of the animals, the anterior communication was not complete, thus leading to partial ischemia following unilateral carotid artery occlusion. Due to those peculiar characteristics, the Mongolian gerbil today has become a widespread animal model for cerebral ischemia studies. M. unguiculatus has been used in most of the studies while the level of ischemia has been evaluated by the development of neurological symptoms created by the occlusion of the carotid arteries. In the present study we investigated the vasculature structure of the commonly used gerbil, M. unguiculatus (MU-TF) and compared it to the vasculature of the Israeli gerbil, Meriones tristrami as well as to that of the Albino rat. We determined the correlation between the anatomical pattern and the biochemical responses during partial or complete ischemia and anoxia by monitoring the oxidation-reduction state of the intramitochondrial NADH using an in vivo surface fluorescence technique. The corrected fluorescence signal was found to be inversely correlated with oxygen availability and could thus be used as an indicator for the level of ischemia developed after carotid artery occlusion. This is the first time that the brain vasculature of two lines of M. tristrami (MT-HU, MT-BD) has been investigated.(ABSTRACT TRUNCATED AT 250 WORDS)

Multiparametric evaluation of brain functions in the Mongolian gerbil in vivo

We have developed the multiprobe assembly (MPA) by which metabolic, ionic and electrical activities can be monitored from the surface of the brain. In the present study we included optical fibers for the monitoring of intracapillary hemoglobin oxygenation by use of the Erlangen Microlight Guide Spectrophotometer (EMPHO-I) from the surface of the gerbil brain. The newly developed MPA provides simultaneous information about oxygen delivery (oxydeoxy Hb), tissue pO2 level, as well as the intracellular oxygen balance (intramitochondrial redox state). The ionic homeostasis was evaluated by monitoring extracellular K+ and Ca2+ activities reflecting the permeability changes of cation channels as well as the activities of Na+,K(+)-ATPase and other ion linked transport processes. The electrical activities were monitored by a bipolar electrocortical surface probe and DC steady potential. The subjects of the present study were Mongolian gerbils (Meriones unguiculatus) anesthetized and operated according to our routine techniques. After 30 min of recovery from the operation each gerbil was exposed to a short anoxia, graded hypoxia, ischemia as well as spreading depression. The results can be summarized as follows: 1. A clear correlation was recorded between the changes in oxydeoxy Hb spectra, tissue pO2 level and oxidation-reduction state of intramitochondrial NADH under oxygen deficiency situations (hypoxia, ischemia). 2. Blood volume changes under various perturbations monitored by various probes (366 reflectance and EMPHO-I) correlated very well with each other. 3. The degree of inhibition of Na+,K(+)-ATPase induced by oxygen deficiency could be interpreted by changes in extracellular levels of K+ measured by the surface mini-electrode. 4. Brain stimulation induced by spreading depression mechanism led to transient changes in ionic homeostasis and increase in energy requirements. The major HbO2 response was an increase in oxygenation due to the large CBF increase as monitored by the laser Doppler flowmeter. 5. Changes in oxy-deoxy Hb under fast scanning of 500-600 nm during 2-3 seconds of bilateral carotid arterial occlusion provided an indirect index for tissue O2 consumption.

Correlated, simultaneous, multiple-wavelength optical monitoring in vivo of localized cerebrocortical NADH and brain microvessel hemoglobin oxygen saturation

Current forms of brain monitoring, such as electroencephalography (EEG), have had limited clinical utility. The EEG records spontaneous cerebrocortical activity and thus is an indirect indicator of metabolic demand and, to a lesser extent, an indicator of mismatch of supply versus demand. Ischemia modulates EEG activity in ways that can usually be detected, but EEG patterns can be similarly modulated by many other factors, including temperature and pharmacologic manipulation. This in vivo study in physiologically monitored animals evaluated the use of correlated optical spectroscopy, performed with an instrument having a fiberoptic light-guide bundle in contact with the cerebral cortex, for the simultaneous monitoring of cerebrovascular oxygen availability and intracellular oxygen delivery. A highly specific monitor of cerebral intracellular oxygen supply, the cerebrocortical intramitochondrial NADH redox state, was monitored in vivo with a fluorescence technique. Absorption spectroscopy was used concurrently to monitor hemoglobin content (blood volume) and oxygen saturation in the microcirculation. Correlated changes in optical signals from cerebrocortical NADH and hemoglobin were studied in a swine model (n = 7) of nitrogen hypoxia. Measurements were made at four wavelengths with a time-division, multiplexed fluorometer/reflectometer. Because the NADH fluorescence signal at 450 nm is affected by local changes in blood volume, a "corrected" fluorescence signal is usually calculated. In previous studies, where only two wave lengths have been measured, attempts at correction were based on reflectance at the excitation wavelength (366 nm). We compared estimators of changes in microcirculatory blood volume using reflection at two wavelengths: 366 nm and 585 nm, the wavelengths for maximum and isobestic absorption. The results of the studies were as follows: (1) during transient hypoxia, NADH and local hemoglobin saturation signals changed in concert with arterial pulse oximetry, with changes in NADH lagging behind changes in saturation by an average of 5.3 seconds; (2) after hypocapnic ventilation to a mean PaCO2 of 20.2 +/- 0.8 mm Hg, NADH increased by 11.5 +/- 8.7% (as compared with maximal change during anoxia), local hemoglobin saturation decreased by 7.7 +/- 6.4%, and local blood volume decreased by 12.5 +/- 13%, while arterial SpO2 was unchanged; (3) our two measures of local blood volume were closely correlated during carbon dioxide perturbations, but poorly correlated during hypoxic perturbation; and (4) NADH fluorescence provided a more rapid, sensitive indicator of oxygen deprivation than did the EEG. During transient hypoxia, EEG changes occurred 57.4 +/- 10.4 seconds after the onset of decline in local hemoglobin saturation, after NADH had completed 50% of its maximal increase.

Effects of age on the metabolic, ionic and electrical responses to anoxia in the newborn dog brain in vivo

The interrelation between brain energy metabolism, electrical activity and ion homeostasis developing under experimental anoxia in animals of different ages is of significant value in the understanding of brain damage occurring under similar conditions of clinical neuropathology. The purpose of the present study was to compare brain energy states and extracellular ion homeostasis during anoxia in newborn puppies of various ages. We have developed and used a multiparametric monitoring device by which various functions of the brain can be recorded in a real-time mode from a 5 mm diameter area on the surface of the cortex. Intracellular oxygen balance was evaluated in newborn puppies of various ages by monitoring the intramitochondrial NADH redox state using a fluorescence technique. The electrical activity was measured by recording the spontaneous ECoG (electrocorticogram) and DC (direct current) steady potential. Ion homeostasis was evaluated using surface potassium and calcium mini-electrodes. Newborn puppies were anesthetized, the dura mater was removed and the multiprobe assembly was placed on the brain and cemented to the skull. Five groups of puppies (0-1, 2-7, 8-14, 15-21 days and 3-24 weeks) were exposed to 5 minutes of complete O2 deprivation (100% nitrogen exposure) and were monitored during the recovery period until all parameters returned to baseline values. The results may be summarized as follows: 1. Resting baseline levels of extracellular K+ were in the same range as described for other young and adult mammals (2.9 +/- 0.05 mM). 2. Extracellular Ca2+ levels were higher than those published for other mammals (1.6 +/- 0.07 mM). 3. During 5 minutes of anoxia, a significant increase in K+ levels was recorded. This increase was not accompanied by measurable changes in extracellular Ca2+. 4. The effect of age on the length of time to the elevation of the extracellular K+ concentration and on the rate of K+ accumulation from the onset of the anoxic condition was significant, i.e., the younger the animal the longer the time and the lower the rate. 5. The rate of energy depletion was age dependent as indicated by the rate of NADH accumulation during anoxia. However, no significant effect of age on the basal aerobic metabolism was found as measured by the maximum percent increase of NADH during anoxia.(ABSTRACT TRUNCATED AT 400 WORDS)

Cerebral blood flow and oxygen consumption in cortical spreading depression

We determined the effects of spreading depression on local cerebral O2 supply, oxygenation, and O2 consumption in the anesthetized rat. Spreading depression was induced by application of 0.5 M KCl to the frontal cortex. Regional cerebral blood flow was determined with [14C]iodoantipyrine and regional O2 extraction was determined microspectrophotometrically. The passage of the spreading depression wave was determined with a multiprobe assembly that recorded NADH redox state (surface fluorometry), extracellular K+ activity, and DC steady potential (surface minielectrodes). As the wave of spreading depression passed, there was an increase in extracellular K+ and a decrease in NADH. Cerebral blood flow was significantly increased (120 +/- 51 ml/min/100 g, mean +/- SD) during the wave as compared with other regions. In the affected cortex, blood flow was not different from that in the contralateral cortex (69 +/- 28 ml/min/100 g) either before or after the wave of spreading depression passed. Arterial and venous O2 saturation were unaffected by the wave and the histogram of O2 saturations of examined veins followed a similar normal distribution in all regions. Oxygen extraction was not altered by the wave of spreading depression. Oxygen consumption was significantly increased during the wave to 7.4 +/- 3.7 ml O2/min/100 g compared with the contralateral cortex (5.1 +/- 2.6 ml/min/100 g) and other regions. It can be concluded that spreading depression caused an increase in cerebral O2 consumption that was adequately matched by an increase in local blood flow. Oxygen delivery was not limited during spreading depression and its effects were quickly over as evidenced by the lack of alteration in oxygenation after the wave of spreading depression passed.

In vivo correlation of myocardial metabolism, perfusion, and mechanical function during increased cardiac work

STUDY OBJECTIVE

The aim was to study the in vivo interaction and regulation of myocardial perfusion, metabolism, and pump function in an open chest canine model using a combination of potentially non-invasive and clinically useful techniques.

DESIGN

To assess potential regulatory mechanisms and the interaction of myocardial perfusion, metabolism, and contractile function responses during changes in cardiac workload, noradrenaline (1 microgram.kg-1.min-1) was infused and hypoxia was produced by increasing the inspired ratio of nitrogen to oxygen to produce a PaO2 of 2.6-4.0 kPa in separate interventions.

SUBJECTS

Nine mongrel dogs of either sex, age 2-5 years, weight 8.5(SD 2.2) kg, were studied in separate interventions.

MEASUREMENTS AND MAIN RESULTS

Myocardial perfusion was determined using 2H nuclear magnetic resonance (NMR) measured washout of deuterium oxide from the left ventricle interpreted with a one component Kety-Schmidt exponential model. High energy phosphate bioenergetics were determined by 31P NMR measurements of the phosphocreatine/ATP ratio. Redox state was estimated by nicotinamide adenine dinucleotide fluorometry expressed as percent change from the baseline, normalised to maximum response measured at 100% inspired N2. Mechanical function was evaluated using heart rate X systolic blood pressure and oxygen consumption measurements. During both noradrenaline infusion and hypoxia, mechanical function increased significantly from control values: heart rate X systolic blood pressure = 1.9(SD 0.5), 3.6(0.1), and 2.6(0.4), respectively; oxygen consumption = 0.9(2), 1.6(0.1), and 1.2(0.6) ml.min-1.100 g-1. Myocardial perfusion increased to support the increased workloads, from 87(10) to 131(20), and from 60(12) to 182(14) ml.min-1.100 g-1, respectively. ADP, estimated by the phosphocreatine/ATP ratio, did not change during noradrenaline infusion [2.4(0.2) to 2.4(0.7)], but decreased during hypoxia [2.4(0.4) to 1.7(0.5)]. Redox state decreased during noradrenaline infusion, from 100% to 84(0.7)%, and increased during hypoxia, from 100% to 140(10)%.

CONCLUSIONS

Similar changes in workload induced by different physiological stimuli are associated with different biochemical responses even though changes in perfusion are similar. The data suggest that myocardial function is regulated by different biochemical mechanisms under different physiological conditions, ie, there is probably no universal regulator of myocardial function. It is now possible to evaluate potential metabolic regulators of myocardial function in an in vivo animal model.