Determinants of cerebral oxygenation during cardiac surgery

Well-being and cerebral oxygen saturation during acute heart failure in humans

Cerebral symptoms and near-infrared spectrophotometry-determined cerebral oxygen saturation (ScO2) were followed in patients treated for normotensive acute congestive heart failure. The reproducibility and normal range for ScO2 were established from 39 resting subjects without cardio-respiratory disease: the ScO2 ranged from 55 to 78% with a coefficient of variation for triple determination of 6%. Patients rated cerebral symptoms on a scale with end-points of 0 (best) and 10 (worst). In eight patients with acute heart failure, arterial oxygen tension increased during decongestive treatment, from 9.1 (4.9-10) to 10.4 kPa (7.3-17); median with range, as did arterial oxygen saturation, from 94 (48-97) to 97% (87-99) (P<0.02), whereas the mean arterial pressure, heart rate and arterial carbon dioxide tension remained unchanged. The cerebral symptom score improved from 8 (3-10) to 1 (1-9) and the ScO2 increased from 34 (20-58) to 50% (19-91) (P<0.02). A ninth patient presented with a silent but massive myocardial infarction: she was cerebrally obtunded with a ScO2 of 18% and soon died. In patients with normotensive acute heart failure and cerebral symptoms, cerebral oxygen saturation is low, and during successful treatment ScO2 increases with the well-being of the patient.

Utility of near-infrared spectroscopic measurements during deep hypothermic circulatory arrest

BACKGROUND

Near-infrared spectroscopy (NIRS) is used to monitor cerebral oxygenation during cardiac surgery. However, interpretation of the signals is controversial. The aim of the study was to determine which NIRS variable best correlated with brain damage as assessed by animal behavior and neurohistologic score and to compare the accuracy of NIRS and magnetic resonance spectroscopy (MRS) in predicting brain injury.

METHODS

Forty 5-week-old piglets underwent 60 minutes of deep hypothermic circulatory arrest (DHCA) at 15 degrees C. Changes in brain adenosine triphosphate (ATP), phosphocreatine (PCr), and intracellular pH (pHi) were determined by MRS and correlated to changes in oxygenated hemoglobin (HbO2), deoxygenated hemoglobin (Hb), and oxidized cytochrome a,a3 (CytOx) NIRS signals. Brains were fixed on day 4 and examined using a neurohistologic score.

RESULTS

Reductions in CytOx and HbO2 values were correlated closely with decreases in ATP, PCr, and pHi. The changes in CytOx and PCr showed the strongest correlation (r = 0.623). Maximal CytOx reduction during DHCA of more than -25 microM * differential pathlength factor (DPF) predicted brain damage with a sensitivity of 100% and a specificity of 75%. The histologic score was also correlated with a decrease in ATP (r = -0.52 for CytOx; r = -0.32 for ATP); HbO2, PCr, and pHi showed no correlations.

CONCLUSIONS

Reduction in CytOx correlates with decreased brain energy state and predicts histologic brain injury after DHCA with a high sensitivity. These data suggest that the level of CytOx could be a very important predictor of brain damage during DHCA.

Neurophysiologic monitoring and outcomes in cardiovascular surgery

The first step to make in improving neurologic outcome is to recognize and accept neurologic injury occurs in all patient groups undergoing CPB. Fortunately, that stage has now been passed. Accurate detection and documentation of the incidence of brain injury is the next progression. At the same time, the cause of the injury needs to be established. Since the introduction of CPB, numerous improvements and refinements have been achieved, making it the acceptable, everyday clinical tool that has enabled the development of cardiac surgery. Despite these improvements, CPB-related morbidity persists. The advent of new technologic advances drives the quest for new techniques. New protective strategies for many end organs, including the heart, kidney, and brain, are evolving. No organ system should be viewed in isolation; otherwise, organ-specific protective strategies may arise in conflict. A strategy that confers absolute myocardial protection would be ideal, but at what cost to the protection of the kidneys, intestines, and brain? A neuroprotective strategy would ideally eliminate brain injury and be beneficial for all organs. The only way to continue to make progress is by the scientific evaluation of new techniques. The use of appropriate monitoring and outcome measures is fundamental to this process.

Oxygenation strategy and neurologic damage after deep hypothermic circulatory arrest. II. hypoxic versus free radical injury

OBJECTIVES

Laboratory studies suggest that myocardial reperfusion injury is exacerbated by free radicals when pure oxygen is used during cardiopulmonary bypass. In phase I of this study we demonstrated that normoxic perfusion during cardiopulmonary bypass does not increase the risk of microembolic brain injury so long as a membrane oxygenator with an arterial filter is used. In phase II of this study we studied the hypothesis that normoxic perfusion increases the risk of hypoxic brain injury after deep hypothermia with circulatory arrest.

METHODS

With membrane oxygenators with arterial filters, 10 piglets (8-10 kg) underwent 120 minutes of deep hypothermia and circulatory arrest at 15 degrees C, were rewarmed to 37 degrees C, and were weaned from bypass. In 5 piglets normoxia (PaO2 64-181 mm Hg) was used during cardiopulmonary bypass and in 5 hyperoxia (PaO2 400-900 mm Hg) was used. After 6 hours of reperfusion the brain was fixed for histologic evaluation. Near-infrared spectroscopy was used to monitor cerebral oxyhemoglobin and oxidized cytochrome a,a3 concentrations.

RESULTS

Histologic examination revealed a significant increase in brain damage in the normoxia group (score 12.4 versus 8.6, P =.01), especially in the neocortex and hippocampal regions. Cytochrome a,a 3 and oxyhemoglobin concentrations tended to be lower during deep hypothermia and circulatory arrest in the normoxia group (P =.16).

CONCLUSIONS

In the setting of prolonged deep hypothermia and circulatory arrest with membrane oxygenators, normoxic cardiopulmonary bypass significantly increases histologically graded brain damage with respect to hyperoxic cardiopulmonary bypass. Near-infrared spectroscopy suggests that the mechanism is hypoxic injury, which presumably overwhelms any injury caused by increased oxygen free radicals.

[Cerebral hemodynamics during implantation of cardioverter-defibrillator systems]

OBJECTIVE

During ICD-implantation it is necessary to prove the function and to determine the optimal threshold by means of induced ventricular fibrillation (VF). Provoked cardiac arrests cause a circulator stop of the cerebral perfusion. Our aim was to examine the changes of cerebral blood flow velocity (CBFV(MCA)) after induced VF depending on the duration of fibrillation and prior values of CBFV(MCA).

PATIENTS AND METHODS

Sixty induced episodes of VF in 9 patients (mean age +/- SD 53.5 +/- 8 years) were examined during ICD-implantation. Beside the standardized anaesthesiological monitoring, transcranial Doppler sonography was used to record the cerebral blood flow velocity in the middle cerebri artery CBFV(MCA). The duration of the fibrillation-period and the range and duration of the CBFV increase during the post defibrillation-period were correlated. Additionally, we examined whether systematic differences existed between the episodes of each patient (time-trend) by means of 5 following episodes of a patient.

RESULTS

During all episodes of VF and hyperperfusion was present, that means a time interval showing increased values of CBFV(MCA), compared to the values present before VF. The duration of hyperperfusion depended significantly on the fibrillation time (r = 0.57; p < 0.001). The equation of regression is: hyperperfusion time = 11.1 + 1.22 x fibrillation time. The amount of hyperperfusion, that means the maximal CBFV after defibrillation, increase significantly with CBFV(MCA) before VF (correlation = 0.88; p < 0.001). The equations of regression is hyperperfusion height = 6.11 + 1.22 x CBFV(MCA) before VF. The duration of hyperperfusion is not influenced by the maximal CBFV(MCA) after defibrillation (r = 0.08; p = 0.52). In the examined patients no significant differences in the hyperperfusion time maximal CBFV(MCA) after defibrillation between the episodes were found.

CONCLUSION

After induced VF you always have to expect a reactive cerebral hyperperfusion. The amount of increase of CBFV after defibrillation depends on the prior values of CBFV before fibrillation and shows a nearly proportional relation to these. The duration of hyperperfusion shows a linear dependency on VF-times. This may show that we had VF-times, in which the cerebral autoregulation and other cerebral physiological reactions compensate the drop of the CBFV(MCA) during VF in the postfibrillation time. In further studies will be examined if there are similar changes in the cerebral metabolism as in CBFV(MCA).

Cerebral hemodynamics and metabolism during infant cardiac surgery. Mechanisms of injury and strategies for protection

There is an established link between congenital heart disease and acquired brain injury, which relates to the dependence of the nervous system on a consistent and responsive supply of oxygen and glucose. The advances in the field of infant cardiac surgery have presented new and different challenges to the arena of child neurology. This review provides an overview of the mechanisms of neurologic injury and cerebral hemodynamics and metabolism during cardiac surgery. This review discusses current and future strategies for the management of children with congenital heart disease.

Near-infrared spectroscopy: theory and applications

In conclusion, NIRS appears to offer both a new monitoring modality and new information about cerebral oxygenation. Technical problems in the application of this technology persist, most notably determination of pathlength and the volume of tissue interrogated. Those familiar with the history of pulse oximetry will recall that although Millikan developed an ear oximeter in 1947, it was not until Aoyagi combined recognition of the pulse signal with spectroscopy in the 1970s that oximetry was transformed into a clinically applicable monitor. In much the same way, NIRS may find the same tremendous usefulness as a noninvasive monitor of cerebral oxygen utilization, pending resolution of the remaining technical problems.