Critical cerebral perfusion pressure during tepid heart operations in dogs

Renal and Cerebral Hypoxia and Inflammation During Cardiopulmonary Bypass

Cardiac surgery-associated acute kidney injury and brain injury remain common despite ongoing efforts to improve both the equipment and procedures deployed during cardiopulmonary bypass (CPB). The pathophysiology of injury of the kidney and brain during CPB is not completely understood. Nevertheless, renal (particularly in the medulla) and cerebral hypoxia and inflammation likely play critical roles. Multiple practical factors, including depth and mode of anesthesia, hemodilution, pump flow, and arterial pressure can influence oxygenation of the brain and kidney during CPB. Critically, these factors may have differential effects on these two vital organs. Systemic inflammatory pathways are activated during CPB through activation of the complement system, coagulation pathways, leukocytes, and the release of inflammatory cytokines. Local inflammation in the brain and kidney may be aggravated by ischemia (and thus hypoxia) and reperfusion (and thus oxidative stress) and activation of resident and infiltrating inflammatory cells. Various strategies, including manipulating perfusion conditions and administration of pharmacotherapies, could potentially be deployed to avoid or attenuate hypoxia and inflammation during CPB. Regarding manipulating perfusion conditions, based on experimental and clinical data, increasing standard pump flow and arterial pressure during CPB appears to offer the best hope to avoid hypoxia and injury, at least in the kidney. Pharmacological approaches, including use of anti-inflammatory agents such as dexmedetomidine and erythropoietin, have shown promise in preclinical models but have not been adequately tested in human trials. However, evidence for beneficial effects of corticosteroids on renal and neurological outcomes is lacking. © 2021 American Physiological Society. Compr Physiol 11:1-36, 2021.

Evaluation of an oscillometric blood pressure monitor in anesthetized dogs: Agreement with direct measurements and ability to detect hypotension

We evaluated the agreement between oscillometric and direct blood pressure monitors in anesthetized dogs and evaluated the ability of the oscillometric method to detect MAP <60 mmHg. Forty client-owned dogs, three months to 14 years old, 4.6-50.2 kg, under general anesthesia were studied. Systolic (SAP), mean (MAP) and diastolic (DAP) arterial pressures were measured directly via an arterial catheter and with an oscillometric cuff (Cardell Touch monitor). Paired values were compared with mixed effect models. Bias and limits of agreement (LOA) were calculated. The sensitivity and false negative rate to detect hypotension (invasive MAP <60 mmHg) using an oscillometric MAP <60 and < 70 mmHg were calculated. A receiver operating characteristics (ROC) curve was constructed and the area under the curve calculated. SAP and DAP differed between methods (both P < 0.001), but MAP did not (P = 0.5). Bland-Altman plots showed small biases but wide LOA for all variables. The sensitivity to detect hypotension was 40% with a false negative rate of 60% when using an oscillometric MAP <60 mmHg, and 68% and 32%, respectively, with a MAP <70 mmHg. The area under the ROC curve for MAP was 0.82. While the oscillometric MAP did not differ from invasive values, the device failed to recognize hypotensive events when 60 mmHg was used as the threshold to detect hypotension. Higher MAP values increased the ability to correctly recognize hypotension, but at the expense of a higher incidence of false positives.

Optimal Conditions for Cardiopulmonary Bypass

There is no single optimal set of conditions for cardio pulmonary bypass. What is optimal is determined by patient factors, surgical need, and the mechanics of perfusion. Additionally, the best way to manage bypass typically varies over its course.

Two levels of the inspired oxygen fraction in propofol-anesthetized dogs with high intracranial pressure: cardiopulmonary function

In the initial stage of traumatic brain injury, the use of 1.0 inspired oxygen fraction (FiO2) is indicated. However, high FiO2 has been correlated with atelectasis. Thus, the effects of FiO2= 1.0 and FiO2= 0.6 on the cardiopulmonary function in propofol-anesthetized dogs with high intracranial pressure (ICP) were evaluated. Eight dogs were anesthetized on two occasions, receiving, during controlled ventilation, an FiO2= 1 (G100) or an FiO2= 0.6 (G60). Propofol was used for induction (10mg.kg-1) followed by a continuous rate infusion (0.6mg.kg-1.minute-1). An increase in the ICP was induced by temporary obliteration of the right jugular vein (OJv) 50 minutes after induction of anesthesia. The measurement was taken twenty minutes after OJv (T0) and then at 15-minute intervals (T15 to T60). Alveolar oxygen partial pressure in G60 was lower than in G100 during the whole procedure. Alveolar-arterial oxygen gradient in G100 was greater than in G60 at T0 and at T60. No differences were observed for arterial oxygen partial pressure/inspired oxygen fraction ratio, arterial-to-alveolar oxygen pressure ratio, respiratory index, venous admixture, oxygen delivery, oxygen consumption, oxygen extraction, heart rate, mean pulmonary arterial pressure, pulmonary arterial occlusion pressure, cardiac index, stroke index and systemic vascular resistance index. In G100, mean arterial pressure at T0 was higher than at T45. In dogs with high ICP, the cardiopulmonary function was not influenced by the different FiO2 used.

Comparison of invasive versus noninvasive blood pressure measurements before and after hemorrhage in anesthetized greyhounds using the Surgivet V9203

OBJECTIVE

To determine the agreement between blood pressure obtained with a noninvasive technique and direct technique using the Surgivet V9203 at varying blood pressure levels created using a canine hemorrhage model.

DESIGN

Experimental study.

SETTING

University Teaching Hospital.

ANIMALS

Twenty-nine greyhounds with an average weight of 30 kg. Approval from the Murdoch University Animal Ethics Committee was obtained (R228609).

METHODS

Arterial blood pressure (BP) was measured noninvasively (NIBP) via a cuff placed over the dorsal metatarsal artery and via a catheter in the contralateral dorsal metatarsal artery (IBP). Retrospectively the blood pressure measurements recorded were divided into various BP categories. A total of 84 paired readings were obtained. The data were further analyzed with respect to heart rate [<120/min and >120/min]. Comparison of the measurements was performed using the Bland-Altman method.

RESULTS

For all categories, BP measurements were generally lower when recorded using the oscillometric technique [Systolic (Sys) bias 4.20 kPa (31.54 mm Hg), mean bias 0.52 kPa (3.93 mm Hg), and diastolic bias 1.04 kPa (7.83 mm Hg)]. Mean arterial BP had the least bias and greatest precision in the hypotensive group (bias 0.174 kPa (1.3063 mm Hg), precision 0.79 kPa (5.89 mm Hg)), low BP group (bias 0.62 kPa (4.67mm Hg) and precision 0.89 kPa (6.67mm Hg)) and normotensive group [bias 0.76 kPa (5.71 mm Hg) and precision 1.16 kPa (8.73 mmHg)].

CONCLUSIONS

The results demonstrate that the agreement between NIBP measurements and IBP, are within the limits of agreement recommended by the American College of Veterinary Internal Medicine Hypertension Consensus Panel for all pressures except Sys BP. This suggests that mean and diastolic NIBP using the Surgivet V9203 are a clinically acceptable alternative of IBP in hypovolemic hypotensive dogs.

Subarachnoid pressures and cardiorespiratory parameters during cisternal myelography in isoflurane anaesthetized dogs

OBJECTIVE

To measure subarachnoid pressures, systemic circulatory and respiratory effects, and to calculate cerebral perfusion pressure during cisternal myelography.

STUDY DESIGN

Prospective clinical study.

ANIMALS

Forty-three client owned dogs with clinical signs of spinal disease, weighing 6-56 kg.

METHODS

Dogs were premedicated with butorphanol and diazepam intravenously (IV) and anaesthesia was induced with propofol and maintained with isoflurane vaporized in oxygen. Ventilation was spontaneous. Heart and respiratory rates, invasive mean arterial blood pressure (MAP), end tidal carbon dioxide and isoflurane concentration were measured continuously. Initial subarachnoid pressure (SaP0 ) was measured in the cisterna magna with a needle pressure gauge. Iohexol 0.3 mL kg(-1) was injected at a rate of 4.1 mL minute(-1) into the cerebellomedullary cistern. The SaP was recorded during and at 120 seconds after contrast administration. The maximum SaP (SaPmax ) and minimum calculated cerebral perfusion pressure (CPPmin ) were recorded for each case.

RESULTS

Prior to contrast injection, mean ± SD, MAP was 73 ± 20 mmHg and SaP0 was 10 ± 3 mmHg. The cerebral perfusion pressure (CPP) was 64 ± 20 mmHg. The contrast injection increased the SaP0 to 73 ± 33 mmHg (SaPmax ). After injection, MAP increased to 97 ± 25 mmHg and the CPP decreased to 14 ± 34 mmHg. A negative correlation was found between the lowest CPP and body weight (ρ = -0.77, p < 0.0001). Nine dogs had bradycardia, apnoea and hypertension, 21 dogs had at least one of these signs. The number of clinical signs showed significant correlation with body weight (ρ = -0.68, p < 0.0001), SaPmax (ρ = -0.66, p < 0.0001) and CPPmin (ρ = -0.73, p < 0.0001).

CONCLUSIONS AND CLINICAL RELEVANCE

Cerebral perfusion can severely decrease during cisternal myelography using the standard dose of iohexol. Bradycardia, apnoea and systemic hypertension were associated with decreased CPP.

Bispectral index in dogs with high intracranial pressure, anesthetized with propofol and submitted to two levels of FiO2

The effects of inspired oxygen fractions (FiO2) of 1 and 0.6 on bispectral index (BIS) in dogs with high intracranial pressure, submitted to a continuous rate infusion of propofol were evaluated. Eight dogs were anesthetized on two occasions, receiving, during controlled ventilation, an FiO2 = 1 (G100) or an FiO2 = 0.6 (G60). Propofol was used for induction (10mg.kg-1, IV), followed by a continuous rate infusion (0.6mg.kg-1.minute-1). After 20 minutes, a fiber-optic catheter was implanted on the surface of the right cerebral cortex to monitor the intracranial pressure, the baseline measurements of BIS values, signal quality index, suppression ratio number, electromyogram indicator, end-tidal carbon dioxide partial pressure, mean arterial pressure, intracranial pressure and cerebral perfusion pressure were taken. Then, the blood flow from the right jugular vein was interrupted in order to increase intracranial pressure and after 20 minutes additional recordings were performed at 15-minute intervals (T0, T15, T30, T45 and T60). The arterial oxygen partial pressure varied according to the changes in oxygen. For the other parameters, no significant differences were recorded. The BIS monitoring was not influenced by different FiO2.

Intracranial variables in propofol or sevoflurane-anesthestized dogs subjected to subarachnoid administration of iohexol

The effects of subarachnoid administration of iohexol on intracranial hemodynamic in dogs anesthetized with propofol or sevoflurane were evaluated. Thirty adult animals (10.9±2.9kg) were distributed into two groups: PG, where propofol was used for induction (10±0.5mg/kg), followed by a continuous rate infusion at 0.55±0.15mg/kg/hour, and SG, where sevoflurane was administered for induction (2.5 MAC) and for anesthetic maintenance (1.5 MAC). A fiberoptic catheter was implanted on the right superficial cerebral cortex to monitor intracranial pressure (ICP). After 30 minutes, cerebrospinal fluid (CSF) was collected at the cisterna magna and iohexol was injected. The measurements were performed before CSF collection (TA), after the iohexol injection (T0), and at 10-minute intervals (T10 to T60). Intracranial pressure decreased at T0 in SG. Cerebral perfusion pressure at T0 was higher than at TA, T50 and T60 in PG, but in SG, the mean value at T0 was higher than the ones from T20 to T60. Mean arterial pressure at T0 was higher than at TA in PG, while in SG, the values from T20 to T60 were lower than at T0. The heart rate at T60 was lower than at T0 in PG. Cardiac output at TA was lower than at T60 in SG. The cerebrospinal fluid collection and administration of iohexol promoted decrease in intracranial pressure in sevolflurane-anesthetized dogs and increase in cerebral perfusion pressure in propofol-anesthetized dogs.

Lower perfusion pressure during hypothermic cardiopulmonary bypass is associated with decreased cerebral blood flow and impaired memory performance 6 months postoperatively

OBJECTIVES

We undertook to determine the influence of perfusion pressure during hypothermic cardiopulmonary bypass (CPB) on cerebral blood flow (CBF) and cognitive memory outcome at 6 months postoperatively.

METHODS

Nineteen patients who underwent hypothermic nonpulsatile CPB for elective coronary artery bypass (CAB) surgery were evaluated by (133)Xe measurement of the CBF and by the Incidental Memory Assessment for evaluating cognitive memory (IMTscore), both at baseline before the operation (T(1)) and again at 5 to 6 months postoperatively (T(2)).

RESULTS

Overall, the mean CBF fell significantly from 39 +/- 5 mL.(100 g)(-1).min(-1) at T(1) to 33 +/- 3 mL.(100 g)(-1).min(-1) at T(2) (P < .001). The decrease in CBF from T(1) to T(2) (DeltaCBF(2-1)) correlated with a significant reduction in the IMTscore from T1 to T2 (DeltaIMTscore(2-1)) (P < .001) and with a mean arterial pressure during CPB (MAPCPB) of <60 mm Hg (P = .05). Cluster analysis of DeltaCBF(2-1) and DeltaIMTscore(2-1) demonstrated that the patients with the greatest decrease in CBF showed the greatest decrease in IMTscore, whereas cluster analysis of DeltaCBF(2-1) and MAPCPB indicated that patients with a perfusion pressure maintained at a mean of <60 mm Hg during CPB were prone to a greater decrease in later postoperative CBF.

CONCLUSION

This study demonstrated that a MAPCPB of <60 mm Hg during CPB was associated with a significant decrease in CBF 6 months after CAB surgery and with an associated decrease in memory performance.

Mean arterial pressure about 40 mmHg during CPB is associated with cerebral ischemia in piglets

OBJECTIVE

To investigate if a mean arterial pressure below 50 mmHg during CPB may lead to cerebral ischemia.

MATERIAL AND METHODS

Piglets with low mean arterial pressure by nitroprusside (LP-group) (n=6) were compared with piglets given norepinephrine to obtain high pressure (HP-group) (n=6) during normothermic and hypothermic CPB. Intracranial pressure, flow and markers of cerebral energy metabolism (microdialysis) were recorded.

RESULTS

Mean arterial pressure differed significantly between the groups and stabilized about 40-45 mmHg in the LP-group. Cerebral perfusion pressure decreased to 21.3 (7.7) mmHg in the LP-group and increased to 51.8 (11.2) mmHg in the HP-group at 150 min of CPB (P<0.001, between groups). During bypass the intracerebral glucose concentration decreased significantly in the LP-group. In this group the lactate/pyruvate ratio increased from 15.5 (5.3) to 64.5 (87.6) at 90 min and 45.0 (36.5) at 150 min (P<0.05) with no such changes in the HP-group. Similarly the cerebral glycerol concentration increased significantly in the LP-group, whereas glycerol remained stable in the HP-group.

CONCLUSION

Mean arterial pressure about 40 mmHg during CPB is associated with cerebral ischemia.

Optimizing selective cerebral perfusion: deleterious effects of high perfusion pressures

OBJECTIVE

Selective cerebral perfusion is a proven adjunct to hypothermia for neuroprotection in complex aortic surgery. The ideal conditions for the provision of selective cerebral perfusion, however, including optimal perfusion pressure, remain unknown. We investigated the effects of various perfusion pressures during selective cerebral perfusion on cerebral physiology and outcome in a long-term porcine model.

METHODS

Thirty piglets (26.3 +/- 1.4 kg), cooled to 20 degrees C on cardiopulmonary bypass with alpha-stat pH management (mean hematocrit 23.6%), were randomly assigned to 90 minutes of selective cerebral perfusion at a pressure of 50 (group A), 70 (group B), or 90 (group C) mm Hg. With fluorescent microspheres and sagittal sinus sampling, cerebral blood flow and cerebral oxygen metabolism were assessed at baseline, after cooling, at two points during selective cerebral perfusion, and for 2 hours after cardiopulmonary bypass. Visual evoked potentials were monitored during recovery. Neurobehavioral scores were assessed blindly from standardized videotaped sessions for 7 postoperative days.

RESULTS

Cerebral blood flow during selective cerebral perfusion was significantly increased by higher-pressure perfusion (P = .04), although all groups sustained similar levels of cerebral oxygen metabolism during selective cerebral perfusion (P = .88). After the end of cardiopulmonary bypass, the cerebral oxygen metabolism increased to above baseline in all groups, with the highest levels seen in group C (P = .06). Intracranial pressure was significantly higher during selective cerebral perfusion in group C (P = .0002); visual evoked potentials did not differ among groups. Neurobehavioral scores were significantly better in group A (P = .0002).

CONCLUSION

Selective cerebral perfusion at 50 mm Hg provides neuroprotection superior to that at higher pressures. The increased cerebral blood flow with higher-pressure selective cerebral perfusion is associated with cerebral injury, reflected by high post-cardiopulmonary bypass cerebral oxygen metabolism and poorer neurobehavioral recovery.

Low perfusion pressure during CPB may induce cerebral metabolic and ultrastructural changes

BACKGROUND

Recently we reported on cerebral metabolic changes suggesting ischemia in piglets during nitroprusside-induced low-pressure CPB. We here investigated whether a mean arterial pressure (MAP) of 40-45 mmHg could provoke similar changes by a NO-independent intervention.

METHODS

Piglets underwent 60 minutes normothermic followed by 90 minutes hypothermic CPB. The LP-group (n=8) had MAP of 40-45 mmHg by phentolamine while the HP-group (n=8) had MAP of 60-80 mmHg by norepinephrine. Cerebral glucose, lactate, pyruvate and glycerol were determined. In the last two animals of each group, cerebral tissue was examined by electron microscopy.

RESULTS

Cerebral lactate was higher in the LP-group than the HP-group during normothermic CPB. Compared with baseline, cerebral glucose of the LP-group decreased whereas lactate/pyruvate-ratio, lactate and glycerol-concentrations increased during normothermic CPB. In the HP-group these parameters remained unchanged. Electron microscopy showed 31.2% and 8.3% altered mitochondria in the cortical micrographs taken from the LP- and the HP-group, respectively (p<0.001).

CONCLUSION

MAP below 45 mmHg during CPB was associated with cerebral biochemical and morphological changes consistent with anaerobic metabolism and subcellular injury.

Comparing Warfarin With Aspirin After Biological Aortic Valve Replacement

Background— Patients with prosthetic heart valves have a higher risk of developing valve thrombosis and arterial thromboembolism. Antithrombotic therapy in the early postoperative period after biological aortic valve replacement (BAVR) is controversial. The American College of Cardiology/American Heart Association and European Society of Cardiology guidelines recommend the use of warfarin for the first 3 months after BAVR, although the American College Chest Physician guidelines suggest that the recommendations are very weak and that the risk/benefit is unclear. This prospective study investigated the efficacy of postoperative warfarin compared with aspirin in patients after aortic valve replacement.

Methods and Results— Patients undergoing BAVR between 2001 and 2002 received 2 antithrombotic therapies: 141 patients received warfarin for the first 3 months, and 108 patients received only aspirin. The major end points evaluated were the rate of cerebral ischemic events, bleeding, and survival. There were 3 and 5 postoperative cerebral ischemic events between 24 hours and 3 months for patients treated with aspirin and warfarin, respectively. After 3 months, the incidence of cerebral ischemic events did not differ between the 2 groups. The rate of major bleeding events, the stroke-free survival, and the overall survival rates were not statistically significant between the warfarin and aspirin groups.

Conclusions— There seem to be no advantages in performing early anticoagulation therapy compared with a low-antiplatelet regimen with regard to early cerebral ischemic events, bleeding, and survival. Currently there is no evidence to support the fact that warfarin is more effective than aspirin.

The impact of asymptomatic carotid artery disease on the intraoperative course of coronary artery bypass surgery

Asymptomatic carotid artery stenosis (CAS) may result in neurological injury after coronary artery bypass surgery, but routine preoperative carotid screening is not undisputed. We studied whether routinely determined carotid duplex results, beyond detecting high-risk patients, additionally influence intraoperative course. One hundred and eight patients without new signs of impaired cerebral circulation were investigated. Anesthesiology, perfusionist records, and patient files were reviewed for patient characteristics, intraoperative variables and postoperative neurological sequelae. There was a higher incidence of prior cerebrovascular events and peripheral artery disease in CAS patients (p < 0.05). Pulsatile flow was employed more frequently in this group (p < 0.05). Severe hyperventilation, hyperglycemia, hemodilution, hyperthermia, and lactacidosis were avoided in both groups. However, labile hemodynamics of CAS patients required more corrective interventions (p < 0.05). There was also a tendency toward greater mortality. Stroke and transient ischemic attack (TIA) occurred in two patients without CAS. Since CAS was associated with a greater degree of cardiovascular instability requiring frequent measures to control hemodynamics, positive duplex results should heighten vigilance. Although CAS does not appear to be the major source of cerebral ischemia, it involves significant comorbidity.

Importance of blood pressure regulation in maintaining adequate tissue perfusion during cardiopulmonary bypass

Patients undergoing surgery with the aid of cardiopulmonary bypass (CPB) have an incidence of end-organ dysfunction, caused by embolization, regional hypoperfusion, or some combination of the two. In this article, we attempt to define the effect of mean arterial pressure (MAP) during CPB on postoperative end-organ function. Although early studies reported that cerebral perfusion during hypothermic CPB is independent of MAP, recent laboratory and clinical reports have shown a positive slope in the MAP versus cerebral blood flow relationship. In clinical studies, patients who had higher MAPs during CPB had a lower incidence of cardiac and neurologic complications, as well as late neurocognitive abnormalities compared with patients with lower MAPs. Improving collateral flow in the setting of cerebral embolization has been postulated as the main mechanism for the improved neurologic outcomes in the high MAP groups. Higher perfusion pressure during CPB affects regional blood flow to the kidneys and visceral organs. However, the lower autoregulatory limits of perfusion to abdominal organs differ from the limits to the brain. Enhanced visceral perfusion during CPB is best achieved by increasing perfusion pressure via increases in perfusion flow rates rather than by using peripheral vasoconstriction alone. In conclusion, it is clear that maintenance of a high MAP during CPB may have a significant impact in protecting the brain and abdominal organs, particularly in the subset of patients at high risk for embolization and end-organ dysfunction.

Bypass flow, mean arterial pressure, and cerebral perfusion during cardiopulmonary bypass in dogs

OBJECTIVE

To determine if normal cardiopulmonary bypass (CPB) pump flows maintain cerebral perfusion in the context of reduced mean arterial pressure at 33 degrees C.

DESIGN

A prospective investigation.

SETTING

Animal CPB research laboratory.

PARTICIPANTS

Seven dogs that underwent CPB.

INTERVENTIONS

Seven dogs underwent CPB at 33 degrees C using alpha-stat management and a halothane, fentanyl-midazolam anesthetic. Cerebral blood flow was measured using the sagittal sinus outflow technique. After control measurements at 70 mm Hg, cerebral physiologic values were determined under four conditions in random order: (1) mean arterial pressure of 60 mm Hg achieved by a reduction in pump flow, (2) mean arterial pressure of 60 mmHg determined by partial opening of a femoral arterial-to-venous reservoir shunt, (3) mean arterial pressure of 45 mm Hg by reduced pump flow, and (4) mean arterial pressure of 45 mm Hg by shunt. A 9F femoral arterial-to-venous reservoir shunt was controlled by a screw clamp.

MEASUREMENTS AND MAIN RESULTS

Except for the controlled variables of mean arterial pressure and bypass flow, physiologic determinants of cerebral blood flow (temperature, PaCO2 and hematocrit) did not differ under any of the CPB conditions. Pump flow per se was not a determinant of cerebral perfusion. Cerebral blood flow and cerebral oxygen delivery did not differ with changes in pump flow if mean arterial pressure did not differ. Cerebral blood flow depended on mean arterial pressure under all pump flow conditions, however.

CONCLUSIONS

Over the range of flows typical in adult CPB at 33 degrees C, pump flow does not have an effect on cerebral perfusion independent of its effect on mean arterial pressure. A targeted pump flow per se is not sufficient to maintain cerebral perfusion if mean arterial blood pressure is reduced.