Neurologic and cardiovascular effects of hypotension in the monkey

"Silent" Presentation of Hypoxemia and Cardiorespiratory Compensation in COVID-19

Severe hypoxemia presents variably, and sometimes silently, without subjective complaints of dyspnea. The adequacy of cardiovascular compensation for oxygen delivery to tissues should be a focus in all hypoxemic patients.

Cerebral blood flow distribution and hypoxic-ischemic brain damage in newborn rats

OBJECTIVE

Our purpose was to assess the cerebral blood flow distribution and resulting grade of hypoxicischemic brain damage in newborn rats.

METHODS

Seven-day-old Wistar rats (n = 75) underwent left common carotid artery ligation followed by 2 hours hypoxia (8% oxygen in nitrogen) at 33 degrees C. The control animals were exposed to hypoxia without ligation (n = 8). Colored microspheres of 15 microm in diameter were administered into the left cardiac ventricle percutaneously at the end of hypoxia. They were killed 24 hours after induced injury. Brain sections 2 mm in thickness were removed for microtubule-associated protein 2 (MAP-2) staining, and remaining parts were separated into left and right hemispheres for counting the microspheres. The blood flow distribution to the ligated side was expressed as the difference from the non-ligated control side. Severity of MAP-2 disappearance was ranked as normal, mild or severe.

RESULTS

In the control rats, there was no loss of MAP-2 staining. The blood flow equally distributed into both cerebral hemispheres. The cerebral blood flow distribution on the side of carotid artery ligation decreased by 44.7 +/- 21.9% in the mildly damaged group and 65.8 +/- 16.8% in the severely damaged group.

CONCLUSION

The greater the percentage difference of blood flow distribution from the non-ligated side, the more severe the brain damage.

A piglet survival model of posthypoxic encephalopathy

The aim of this study was to produce a neonatal piglet model which, avoiding vessel ligation, exposed the whole animal to hypoxia and produced dose-dependent clinical encephalopathy and neuropathologic damage similar to that seen after birth asphyxia. Twenty-three piglets were halothane-anesthetized. Hypoxia was induced in 19 piglets by reducing the fractional concentration of inspired oxygen (FiO2) to the maximum concentration at which the EEG amplitude was below 7 microV (low amplitude) for 17-55 min. There were transient increases in Fio2 to correct bradycardia and hypotension. Posthypoxia, the piglets were extubated when breathing was stable. Four were sham-treated controls. We aimed at 72-h survival; seven died prematurely due to posthypoxic complications. EEG and a videotaped itemized neurologic assessment were recorded regularly. We found that 95% of the animals showed neuropathologic damage. The duration of low amplitude EEG during the insult and the arterial pH at the end of the insult correlated with cortical/white matter damage; r = 0.75 and 0.81, respectively. Early postinsult EEG background amplitude (r = 0.86 at 3 h) and neurologic score (r = 0.79 at 8 h) correlated with neuropathology. Epileptic seizures in seven animals were always associated with severe neuropathologic damage. We conclude that EEG-controlled hypoxia and subsequent intensive care enabled the animals to survive with an encephalopathy which correlated with the cerebral hypoxic insult. The encephalopathy was clinically, electrophysiologically, and neuropathologically similar to that in the asphyxiated term infant. This model is suitable for examining mechanisms of damage and evaluation of potential protective therapies after birth asphyxia.

New monkey model of severe-volume controlled hemorrhagic shock

Three series of experiments were conducted to develop a model of volume-controlled severe hemorrhagic shock in the unanesthetized analgesic cynomolgus monkey. This report concerns the insult without resuscitation. In Series I, seven monkeys were sedated with 75% N2O/25% O2, bled 40% of their measured blood volume over 20 min and observed until death. Mean arterial pressure (MAP) decreased to 21 +/- 6 mmHg, spontaneously increased to 46 +/- 5 mmHg, then gradually decreased to pulselessness at 146 +/- 42 min (range 101-213). Hemodynamic variables, lactate, base excess, electroencephalogram and sagittal sinus PO2 followed the same biphasic pattern. In Series II, eight monkeys were bled 27 ml/kg (43% of estimated blood volume) over 20 min under the same N2O analgesia and with similar responses as in Series I. In Series III 26 monkeys were bled 27 ml/kg over 20 min (time zero) as in Series II. Three developed apnea and pulselessness at end of hemorrhage. In 23 the shock period was prolonged for testing resuscitation therapies. Starting at 0 + 30 min, MAP was controlled with minute blood volume adjustments at 30 mmHg until 0 + 2 h. Three died due to inaccurate (preventable) MAP adjustments. At MAP 30 mmHg, all animals lost consciousness, EEG activity decreased, and brain stem reflexes disappeared. The "volume-pressure controlled" hemorrhagic shock model of Series III retains the initial natural response to bleeding, simulates the clinical picture of severe prolonged shock without anesthesia, and represents a more controllable insult than volume controlled hemorrhage alone.

Loss of the somatosensory evoked response as an indicator of reversible cerebral ischemia during hypothermic, low-flow cardiopulmonary bypass

We assessed somatosensory evoked response (SSER) as a monitor of cerebral protection during nonpulsatile, hypothermic cardiopulmonary bypass (CPB). In 13 dogs under CPB, extracorporeal flow rate (EFR) thresholds for loss of SSER were determined by stepwise reduction of the EFR from 2.0 to 0.25 L/min/m2 at perfusion temperatures of 35 degrees C, 30 degrees C, 25 degrees C, and 20 degrees C. Testing began at 35 degrees C in Group 1 (N = 6) and at 20 degrees C in Group 2 (N = 7). Immediately on loss of SSER (denoted as a decrease of 80% or more in the amplitude of the somatosensory evoked potentials), EFR was restored to 2.0 L/min/m. Thresholds for loss of SSER ranged between 0.75 and 0.25 L/min/m2. SSER was always restored on return of EFR to 2.0 L/min/m2; thus loss of SSER was a reversible ischemic change. Both groups had similar threshold values at 35 degrees C, but at lower temperatures, Group 1 thresholds were significantly higher than those in Group 2. Since 35 degrees C was the first test temperature for Group 1 but the last for Group 2, EFR reduction at 35 degrees C apparently caused neurophysiological changes (depletion of cortical energy reserves), which diminished subsequent tolerance to ischemia, but EFR reduction at 20 degrees C did not. Our findings show that loss of SSER warns of reversible cerebral ischemia, and support SSER monitoring as a useful measure of cerebral function during low-flow, hypothermic CPB.

Mechanisms of ischemic cerebral injury

Normal compensatory mechanisms protect the central nervous system (CNS) from moderate hypoxia and ischemia; however, after more severe ischemia progressive brain hypoperfusion ensues and irreversible damage occurs. Ischemic brain injury remains greatly significant clinically and elucidating the determinants of ischemic neuronal injury and death continues to challenge researchers. Although altered perfusion and decreased energy charge may contribute to the production of irreversible damage, the distribution of lesions seen after insult does not correspond with the degree of ischemic blood flow impairment, nor can neuronal energy deprivation explain the cell damage. Other factors, such as derangements in astrocyte function, calcium homeostasis, free radical metabolism, acid-base regulation and excitatory neurotransmitters also probably mediate ischemic neuronal death. Continued investigation to establish the cellular pathophysiology of cerebral ischemia can guide rational research and therapeutic strategies.

The effect of low-flow cardiopulmonary bypass on cerebral function: an experimental and clinical study

Systemic flow rates (Q) during nonpulsatile hypothermic cardiopulmonary bypass (CPB) that are consistent with preservation of cerebral function have not to our knowledge been objectively defined. The effect of a sequential reduction in flow rates on cerebral cortical metabolism and function was evaluated in 6 mongrel dogs during hypothermic (25 degrees C) CPB. Cerebral function was assessed using somatosensory cortical evoked potentials (SSEP); cerebral metabolism was assessed by adenosine triphosphate (ATP) and lactate content of snap-frozen gray matter biopsies taken from the hemisphere contralateral to that monitored for SSEP. A progressive decline in ATP levels was observed during flow reduction with virtually complete depletion of ATP at 0.25 L min-1 m-2(p = .0003). The significant (p = .028) dependence of cortical ATP levels on perfusion pressure was no longer evident after adjusting for the effects of flow rate. Lactate levels increased during flow reduction (p = .028), especially at flow rates less than 0.5 L min-1 m-2. Somatosensory neural transmission remained intact until flow was reduced to 0.25 L min-1 m-2 in 5 animals and until total circulatory arrest in 1, at which time loss of the signal occurred. In addition, 5 patients were subjected to brief periods of low-flow CPB (Q = 1.0 L min-1 m-2) at 21 degrees to 25 degrees C. SSEPs remained intact during flow reduction, and postoperative neurologic evaluation was normal in all patients. We conclude that, in the absence of cerebral vascular disease, the flow rate threshold for incurring functional cerebral injury during hypothermic (25 degrees C) nonpulsatile CPB is less than 1.0 L min-1 m-2.(ABSTRACT TRUNCATED AT 250 WORDS)

Duration versus degree of hypotension as critical conditions of brain infarction in the albino rat

In 41 anesthetized, spontaneously breathing male adult albino rats, cerebral hypotension of precisely defined duration and magnitude was induced by means of controlled arterial hemorrhage. One common carotid artery was occluded throughout the hypotensive period, and the target pressure was monitored in the ipsilateral internal carotid artery. Regional brain infarcts developed in all 16 animals with a target pressure of 14 mm Hg maintained for 90 minutes and in all five animals with a target pressure of 12 mm Hg maintained for 70 minutes. However, the brains of all 10 rats with a target pressure of 17 mm Hg maintained for 80 minutes remained intact. In two further groups of five animals each with target pressures of 15 mm Hg for 80 minutes and 16 mm Hg for 90 minutes the incidence of infarct was about 30%. There were no marked differences between the five groups of rats in body weight, body temperature, heart rate, respiratory rate, PaO2, PaCO2, arterial pH, or hematocrit. The data suggest that, in the rat, the clear-cut threshold for the induction of brain infarcts is a function of the severity and duration of arterial hypotension. Evidence is presented indicating that this function is distinctly species-dependent, due to species differences in the dilatory capacity of the arteries supplying the brain rather than species differences in brain vulnerability.

Common carotid artery stump pressure in the gerbil stroke model

In 106 slightly anaesthetised adult mongolian gerbils one common carotid artery (CCA) was ligated and the blood pressure in the distal and in the proximal stump was monitored for 8 minutes. The mean distal CCA stump pressure of the 39 nonsurvivors was 15 (+/- 6) mm Hg, that of the 25 survivors with retinocerebral infarcts was 25 (+/- 6) mm Hg, and that of the 42 intact survivors was 31 (+/- 7) mm Hg. The corresponding mean arterial blood pressures (MABP), as measured in the proximal CCA stump, were 81 (+/- 12) mm Hg, 84 (+/- 13) mm Hg, and 87 (+/- 11) mm Hg, respectively. There were no differences between the samples concerning sex, body weight, rectal temperature, arterial blood gases, arterial pH, and haematocrit. Measurements in a second series of 10 awake gerbils showed that the mean values of MABP, heart rate, and respiratory rate of the nonsurvivors were less than those of the survivors during 90 minutes after CCA ligation. It is inferred that in the mongolian gerbil the lower threshold of the arterial blood pressure for the development of brain infarcts ranges within 22 and 25 mm Hg, that is, within the values found in monkeys and cats. The longlasting depression of respiration and circulation in the nonsurvivors is considered to be related to the phenomenon of diaschisis .

Cerebral infarction due to carotid occlusion and carbon monoxide exposure III. Influence of neck vein occlusion

Unilateral cerebral infarcts were produced in the rat by ligation of one common carotid artery and a subsequent exposure to carbon monoxide. In animals which had undergone an additional ligation of the external jugular veins leading to a moderate increase of the cephalic venous pressure the outcome of the procedure was ameliorated significantly. Venous pressure elevation was thought to reduce the venous vascular resistance effectively by preventing the leptomeningeal veins from collapsing. Collapse of the leptomeningeal veins probably occurred during the severe carbon monoxide-induced hypotension causing a steep increase of cerebral vascular resistance.

Cerebral infarction due to carotid occlusion and carbon monoxide exposure. I. Pathophysiological and neuropathological investigations

Ligation of one common carotid artery and exposure to carbon monoxide has proved to be a reliable method of producing unilateral cerebral infarcts in the rat, allowing controlled experiments in any given sample size. Pathophysiological measurements in awake and narcotized rats has shown that, in contrast to hypoxic hypoxia, the carbon monoxide-induced functional anaemia (3000 ppm in room air) did not stimulate chemoreceptors, thus causing a severe systemic hypotension owing to peripheral vascular dilatation. This hypotension is likely to represent the main pathogenetic factor in this model. An inhibition of ferro-enzymes by carbon monoxide did not seem to be involved. The only cause of death was shown to be diffuse ipsilateral brain oedema with or without extra-vasation of serum proteins. The EEG, the systemic arterial pressure, rotational behaviour, and carotidal stump pressure proved to be reliable predictors of outcome.

Cerebral and systemic effects of hypotension induced by trimetaphan or nitroprusside in dogs

The effects of hypotension induced by trimetaphan (TMP) or nitroprusside (NTP) together with controlled haemorrhage on cerebral electrical activity, cerebrospinal fluid pressure (CSFP) and systemic circulatory and metabolic variables were measured in 10 mongrel dogs anaesthetized with halothane (end-tidal, 0.88 +/- 0.03%). Induced hypotension was maintained at cerebral perfusion pressure (CPP) of 45 mmHg for 45 min and then at 30 mmHg for 45 min. In five TMP dogs, there were significant decreases in EEG-power and slowing of peak power frequency from the frontal area, but not from the occipital area. The CSFP did not change significantly except for an increase during the induction stage of hypotension. In five NTP dogs, there were no significant changes in EEG-power and peak power frequency throughout the study from either frontal or occipital areas, but CSFP increased significantly. With both drugs, an increase in glucose, lactate and lactate pyruvate ratio (L/P), and a decrease in PaO2 were more pronounced at 30 mmHg than 45 mmHg. With NTP, there were sustained increases in lactate and L/P, even after restoration of arterial pressure. The same magnitude and duration of the decrease in CPP induced by either TMP or NTP produced different effects on cerebral and systemic function.