Cerebral vasoreactivity to carbon dioxide during cardiopulmonary perfusion at normothermia and hypothermia

Non-pulsatile blood flow is associated with enhanced cerebrovascular carbon dioxide reactivity and an attenuated relationship between cerebral blood flow and regional brain oxygenation

BACKGROUND

Systemic blood flow in patients on extracorporeal assist devices is frequently not or only minimally pulsatile. Loss of pulsatile brain perfusion, however, has been implicated in neurological complications. Furthermore, the adverse effects of absent pulsatility on the cerebral microcirculation are modulated similarly as CO₂ vasoreactivity in resistance vessels. During support with an extracorporeal assist device swings in arterial carbon dioxide partial pressures (PaCO₂) that determine cerebral oxygen delivery are not uncommon-especially when CO₂ is eliminated by the respirator as well as via the gas exchanger of an extracorporeal membrane oxygenation machine. We, therefore, investigated whether non-pulsatile flow affects cerebrovascular CO₂ reactivity (CVR) and regional brain oxygenation (rSO₂).

METHODS

In this prospective, single-centre case-control trial, we studied 32 patients undergoing elective cardiac surgery. Blood flow velocity in the middle cerebral artery (MCAv) as well as rSO₂ was determined during step changes of PaCO₂ between 30, 40, and 50 mmHg. Measurements were conducted on cardiopulmonary bypass during non-pulsatile and postoperatively under pulsatile blood flow at comparable test conditions. Corresponding changes of CVR and concomitant rSO₂ alterations were determined for each flow mode. Each patient served as her own control.

RESULTS

MCAv was generally lower during hypocapnia than during normocapnia and hypercapnia (p < 0.0001). However, the MCAv/PaCO₂ slope during non-pulsatile flow was 14.4 cm/s/mmHg [CI 11.8-16.9] and 10.4 cm/s/mmHg [CI 7.9-13.0] after return of pulsatility (p = 0.03). During hypocapnia, non-pulsatile CVR (4.3 ± 1.7%/mmHg) was higher than pulsatile CVR (3.1 ± 1.3%/mmHg, p = 0.01). Independent of the flow mode, we observed a decline in rSO2 during hypocapnia and a corresponding rise during hypercapnia (p < 0.0001). However, the relationship between ΔrSO₂ and ΔMCAv was less pronounced during non-pulsatile flow.

CONCLUSIONS

Non-pulsatile perfusion is associated with enhanced cerebrovascular CVR resulting in greater relative decreases of cerebral blood flow during hypocapnia. Heterogenic microvascular perfusion may account for the attenuated ΔrSO₂/ΔMCAv slope. Potential hazards related to this altered regulation of cerebral perfusion still need to be assessed.

TRIAL REGISTRATION

The study was retrospectively registered on October 30, 2018, with Clinical Trial.gov (NCT03732651).

Enhanced cerebral CO2 reactivity during strenuous exercise in man

Light and moderate exercise elevates the regional cerebral blood flow by approximately 20% as determined by ultrasound Doppler sonography (middle cerebral artery mean flow velocity; MCA V(mean)). However, strenuous exercise, especially in the heat, appears to reduce MCA V(mean) more than can be accounted for by the reduction in the arterial CO(2) tension (P(a)CO(2)). This study evaluated whether the apparently large reduction in MCA V(mean) at the end of exhaustive exercise relates to an enhanced cerebrovascular CO(2) reactivity. The CO(2) reactivity was evaluated in six young healthy male subjects by the administration of CO(2) as well as by voluntary hypo- and hyperventilation at rest and during exercise with and without hyperthermia. At rest, P(a)CO(2) was 5.1 +/- 0.2 kPa (mean +/- SEM) and MCA V(mean) 50.7 +/- 3.8 cm s(-1) and the relationship between MCA V(mean) and P(a)CO(2 )was linear (double-log slope 1.1 +/- 0.1). However, the relationship became curvilinear during exercise (slope 1.8 +/- 0.1; P < 0.01 vs. rest) and during exercise with hyperthermia (slope 2.3 +/- 0.3; P < 0.05 vs. control exercise). Accordingly, the cerebral CO(2) reactivity increased from 30.5 +/- 2.7% kPa(-1) at rest to 61.4 +/- 10.1% kPa(-1) during exercise with hyperthermia (P < 0.05). At exhaustion P(a)CO(2) decreased 1.1+/- 0.2 kPa during exercise with hyperthermia, which, with the determined cerebral CO(2) reactivity, accounted for the 28 +/- 10% decrease in MCA V(mean). The results suggest that during exercise changes in cerebral blood flow are dominated by the arterial carbon dioxide tension.

Effects of milrinone on jugular bulb oxygen saturation and cerebrovascular carbon dioxide reactivity in patients undergoing coronary artery bypass graft surgery

BACKGROUND

Jugular bulb oxygen saturation (Sjv(o(2))) is a surrogate marker for global cerebral oxygenation. The effect of milrinone on Sjv(o(2)) and the cerebrovascular carbon dioxide reactivity (CCO2R) was investigated.

METHODS

Thirty patients scheduled for coronary artery bypass graft surgery (CABG) were studied prospectively. After sternotomy, normoventilation (at T(1); Pa(co(2))=4.7-5.0 kPa) and hyperventilation (at T(2); Pa(co(2))=3.3-3.7 kPa) were induced and the changes in Sjv(o(2)) (DeltaSjv(o(2))) and Pa(co(2)) (DeltaPa(co(2))), and DeltaSjv(o(2))/DeltaPa(co(2)) (CCO(2)R) were measured. After normoventilation was re-established (at T(3)), milrinone 50 microg kg(-1) was given (at T(4)), followed by hyperventilation (at T(5)), and DeltaSjv(o(2)), DeltaPa(co(2)) and CCO(2)R were measured.

RESULTS

After milrinone administration at normoventilation (T(3) and T(4)), cardiac index and mixed venous oxygen saturation increased, while mean arterial pressure and systemic vascular resistance index decreased, without a significant change in Sjv(o(2)). Before milrinone administration (T(1) and T(2)), hyperventilation decreased Pa(co(2)) and Sjv(o(2)), and DeltaSjv(o(2)) showed positive linear correlation with DeltaPa(co(2)). After milrinone administration (T(4) and T(5)), hyperventilation decreased Pa(co(2)) and Sjv(o(2)), and DeltaSjv(o(2)) showed positive linear correlation with DeltaPa(co(2)). There was no significant difference in CCO(2)R before and after milrinone administration (13.3 (5.7)% kPa(-1) and 12.3 (3.9)% kPa(-1), respectively).

CONCLUSIONS

Although milrinone induced significant haemodynamic changes, Sjv(o(2)) and CCO(2)R were unchanged during its administration.

Changes in cerebral oxygenation during cold (28 degrees C) and warm (34 degrees C) cardiopulmonary bypass using different blood gas strategies (alpha-stat and pH-stat) in patients undergoing coronary artery bypass graft surgery

BACKGROUND

Impaired cerebral oxygenation, which is reflected by measuring jugular bulb oxygenation, is thought to play an important role in the development of neurological injury after cardiac operations with cardiopulmonary bypass (CPB). The effects of cardiopulmonary temperature and blood gas strategy on cerebral oxygenation are not fully appreciated.

METHODS

Sixty patients were randomly allocated into four equal groups (cold alpha-stat, cold pH-stat, warm alpha-stat and warm pH-stat) to compare the effect of these perfusion strategies on cerebral oxygenation monitored by jugular bulb oximetry [jugular bulb oxygen saturation (SjO(2)) and arterial-jugular bulb oxygen content difference (AjDO(2))]. Jugular bulb oxygen saturation and AjDO(2) were measured before CPB, after 5, 20, 40 min on CPB, at start and end of rewarming, 5 min before the end of CPB and 10 min after CPB. Two-way analysis of variance was used to model the lowest SjO(2) and highest AjDO(2) during CPB, with CPB temperature and blood gas management as contributing factors.

RESULTS

Significant changes in SjO(2) were only related to the type of blood gas management, with no significant difference between warm and cold CPB patients. In addition, during rewarming, desaturation (SjO(2) </= 50%) occurred in seven patients of 30 in the alpha-stat groups and in one patient of 30 in the pH-stat groups (P = 0.021), and in five patients of 30 in the cold groups vs. three of 30 in the warm groups (P = 0.434). However, no significant changes were found in the highest AjDO(2) between the four groups.

CONCLUSION

Cold CPB failed to offer any further brain protection in terms of better preservation of cerebral oxygenation than warm CPB. Therefore, warm CPB (34 degrees C) with different blood gas strategies appears to be a satisfactory alternative to cold CPB (28 degrees C).

Cerebral blood flow and vascular physiology

The cerebral circulation is tightly regulated to meet the brain's metabolic demands. Although the mechanism is not fully understood, the major physiologic influences on cerebral blood flow have been well documented. In this chapter the basic vascular anatomy, and physiologic control of the cerebral circulation are reviewed. Clinical implications are emphasized.

Can hypocapnia reduce cerebral embolization during cardiopulmonary bypass?

BACKGROUND

Cerebral embolization is a major cause of central nervous dysfunction after cardiopulmonary bypass. Experimental studies demonstrate that reductions in arterial carbon dioxide tension (PaCO2) can reduce cerebral embolization during cardiopulmonary bypass. This study examined the effects of brief PaCO2 manipulations on cerebral embolization in patients undergoing cardiac valve procedures.

METHODS

Patients were prospectively randomized to either hypocapnia (PaCO2 = 30 to 32 mm Hg, n = 30) or normocapnia (PaCO2 = 40 to 42 mm Hg, n = 31) before aortic cross-clamp removal. With removal of the aortic cross-clamp embolic signals were recorded by transcranial Doppler ultrasonography for the next 15 minutes.

RESULTS

Despite significant differences in PaCO2, groups did not differ statistically in total cerebral emboli counts. The mean number of embolic events was 107 +/- 100 (median, 80) in the hypocapnic group and 135 +/- 115 (median, 96) in the normocapnic group, respectively (p = 0.315).

CONCLUSIONS

Due to the high between-patient variability in embolization, reductions in PaCO2 did not result in a statistically significant decrease in cerebral emboli. In contrast to experimental studies, the beneficial effect of hypocapnia on cerebral embolization could not be demonstrated in humans.

Changes in jugular bulb oxygenation in patients undergoing warm coronary artery bypass surgery (34-37 degrees C)

BACKGROUND AND OBJECTIVE

Imbalance between cerebral oxygen supply and demand is thought to play an important role in the development of cerebral injury during cardiac surgery with cardiopulmonary bypass.

METHODS

We studied jugular bulb oxygen saturation, jugular bulb oxygen tension, arterial-jugular bulb oxygen content difference and oxygen extraction ratio in 20 patients undergoing warm coronary artery bypass surgery (34-37 degrees C) with pH-stat blood gas management.

RESULTS

Only two patients showed desaturation (jugular bulb oxygen saturation < 50%) at 5 min on bypass, and none from 20 min onwards. Multiple regression models were performed after using bypass temperature, mean arterial pressure, cerebral perfusion pressure, haemoglobin concentration and arterial carbon dioxide tension as independent variables, and arterial-jugular bulb oxygen content difference, jugular bulb oxygen saturation, oxygen extraction ratio and jugular bulb oxygen tension as individual dependent variables.

CONCLUSIONS

We found that jugular bulb oxygen saturation, jugular bulb oxygen tension and oxygen extraction ratio are mainly dependent on arterial carbon dioxide tension, and arterial-jugular bulb oxygen content difference is dependent on arterial carbon dioxide tension and the bypass temperature. Our results suggest jugular bulb oxygenation is mainly dependent on arterial carbon dioxide tension during warm cardiopulmonary bypass.

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.

Relative changes in cerebral blood flow during cardiac operations using xenon-133 clearance versus transcranial Doppler sonography

BACKGROUND

Changes in cerebral blood flow (CBF) during cardiac operations have implications in terms of postoperative neurologic and neuropsychological dysfunction. Current techniques of CBF measurement are cumbersome and invasive. Transcranial Doppler sonography offers a noninvasive means of assessing changes in CBF. The aim of this study was validation of this technique with existing methods of CBF measurement during cardiac operations.

METHODS

We compared the changes in CBF using xenon-133 clearance with changes in middle cerebral artery velocity by transcranial Doppler sonography (VMCA) using pH-stat and alpha-stat acid-base management during cardiopulmonary bypass. Measurements were taken (1) before bypass, (2) at 28 degrees C on bypass, (3) at 37 degrees C on bypass, and (4) after bypass. Relative changes in CBF and VMCA, calculated as the percent change from the prebypass baseline value normalized to 100%, were used in this analysis.

RESULTS

During the hypothermic phase of cardiopulmonary bypass, CBF and VMCA increased by 45.9% and 51.8%, respectively (p < 0.001), during pH-stat acid-base management but decreased by only 26.4% and 22.4%, respectively (p < 0.0001), during alpha-stat acid-base management. Linear regression analysis of the absolute changes in CBF (mL . 100 g-1 . min-1) and VMCA (cm/s) showed a significant correlation (r = 0.60; r2 = 0.36; p < 0.0001), but a better correlation was obtained when relative changes in CBF and VMCA were compared (r = 0.89; r2 = 0.79; p < 0.0001).

CONCLUSIONS

Measurements of VMCA, expressed as relative changes of a pre-cardiopulmonary bypass level (using the noninvasive transcranial Doppler sonographic technique), can be used to examine CBF changes during cardiopulmonary bypass.

Alpha-stat acid-base regulation during cardiopulmonary bypass improves neuropsychologic outcome in patients undergoing coronary artery bypass grafting

Neuropsychologic impairment in patients undergoing cardiopulmonary bypass may be associated with cerebral blood flow changes arising from different management protocols for carbon dioxide tension during bypass. Seventy patients having coronary artery bypass grafting were randomized to either pH-stat or alpha-stat acid-base management during cardiopulmonary bypass with a membrane oxygenator. In each patient, cerebral blood flow (xenon 133 clearance), middle cerebral artery blood flow velocity (transcranial Doppler sonography), and cerebral oxygen metabolism (cerebral metabolic rate and cerebral extraction ratio) were measured during four phases of the operation: before bypass, during bypass (at hypothermia and at normothermia), and after bypass. A battery, of neuropsychologic tests were also conducted before and 6 weeks after the operation. During hypothermic (28 degrees C) bypass, cerebral blood flow was significantly (p < 0.001) greater in the pH-stat group (41 mlx100 gm(-1)xmin(-1); 95% confidence interval 39 to 43 mlx100 gm(-1)xmin(-1)) than in the alpha-stat group (24 mlx100 gm(-1)xmin(-1); confidence interval 22 to 26 mlx100 gm(-1)xmin(-1)) at constant pressure and How. Arterial carbon dioxide tensions were 41 mm Hg (40 to 41 mm Hg) and 26 mm Hg (25 to 27 mm Hg), respectively; pH was 7.36 (7.34 to 7.38) and 7.53 (7.51 to 7.55), respectively. Middle cerebral artery flow velocity was significantly (p < 0.05) reduced in the alpha-stat group to 87% (77% to 96%) of the prebypass value, whereas it was significantly (p < 0.05) increased (152%; 141% to 162%) in the pH-stat group. Cerebral extraction ratio for oxygen demonstrated relative cerebral hyperemia during hypothermic (28 degrees C) bypass in both the pH-stat and alpha-stat groups (0.12 [0.11 to 0.14] and 0.25 [0.22 to 0.28], respectively); however, hyperemia was significantly more pronounced in the pH-stat group, indicating greater disruption in cerebral autoregulation. Neuropsychologic impairment criteria of deterioration in results of three or more tests revealed that a significantly (Fisher's exact test, p = 0.02) higher proportion of patients in the pH-stat group fared poorly than in the alpha-stat group at 6 weeks (17/35, 48.6% [32% to 65.1%], and 7/35, 20% [6.7% to 33.2.2%], respectively). In conclusion, patients receiving alpha-stat management had less disruption of cerebral autoregulation during cardiopulmonary bypass, accompanied by a reduced incidence of postoperative cerebral dysfunction.

Early cerebral functional outcome after coronary artery bypass surgery using different acid-base management during hypothermic cardiopulmonary bypass

BACKGROUND

Cerebral injury can render meaningless an otherwise successful cardiac operation. As carbon dioxide management during hypothermic cardiopulmonary bypass has a major impact on cerebral blood flow during bypass, it may also influence postoperative cerebral function. The objective of this prospective, controlled, double-blind study was to compare the effect of pH-stat (temperature correction of blood gas analyses) or alpha-stat (no temperature correction) on brain function.

METHODS

Cerebral function was assessed non-invasively using neurological examination, a neuropsychological test battery, quantitative electroencephalogram and the cognitive evoked potential P300 preoperatively and on postoperative day 7 or 8. Forty-five patients undergoing elective aortocoronary bypass grafting in barbiturate/opiate anesthesia were randomised into a pH-stat-(n = 23) and an alpha-stat-group (n = 22). Membrane oxygenators with arterial line filtration and non-pulsatile flow at a perfussion pressure of 50-60 mmHg were used throughout.

RESULTS

One patient in each group developed a new paresis. Among 49 comparisons of cerebral function parameters, only 3 showed significant differences at the 5%-level, i.e. exactly what is expected purely by chance. These differences were too small to be clinically relevant and favored either pH-stat (theta-amplitude) or alpha-stat (subtest 3 of syndrome-short-test, number of mistakes in aiming).

CONCLUSION

Under the bypass conditions of this study there was no difference in early cerebral functional outcome between pH-or alpha-stat carbon dioxide management during hypothermic cardiopulmonary bypass.

Normothermic versus hypothermic cardiopulmonary bypass: central nervous system outcomes

The recent advent of "warm heart" surgery has resulted in reexamination of the neuroprotective effects of hypothermia in the setting of cardiopulmonary bypass (CPB). Hypothermia has been shown to confer significant protection in the setting of transient, but not permanent, ischemia. The mechanism of this neuroprotection is unclear at this time. Reduction in cerebral metabolic rate is believed to be less important compared with the effect of hypothermia on the release of excitatory neurotransmitters, catecholamines, or other mediators of cellular injury. It is for this reason that mild hypothermia (33-35 degrees C) is believed to confer significant neuroprotection. Two large randomized trials of warm versus cold heart surgery have been reported. Neither study found a difference in terms of neuropsychologic dysfunction. However, one study identified a threefold increase in strokes in the "warm" patients. The reasons for this difference are not clear; however, there were various differences in technique and patient population that may have been important. There are other reports of large series of patients undergoing normothermic bypass, with no increase in stroke rate over what is reported elsewhere in the literature. To date, the evidence would suggest that neuropsychologic function is not affected by CPB temperature, suggesting that the transient ischemia is not an important mechanism in this injury. Stroke after CPB is usually the result of permanent ischemia, and hypothermia's effect in this setting is minimal. It would seem unlikely that hypothermia offers anything more than modest benefits in the clinical situation where there is no circulatory arrest.

Determinants of cerebral perfusion during cardiopulmonary bypass

The risk of postoperative neurologic dysfunction in patients undergoing cardiac surgery remains high despite continued improvements in myocardial protective strategies. Part of this neurologic morbidity can be attributed to patients' increased age and underlying pathology, but other factors adversely affecting cerebral blood flow and cerebral metabolism during cardiopulmonary bypass may also contribute. Particulate microembolization during cardiopulmonary bypass appears to be a major cause of postoperative neurologic dysfunction and the pH-stat method of carbon dioxide management during hypothermia may potentiate neurologic damage by allowing a greater embolic load to be delivered to the brain. Echocardiography and transcranial Doppler methods may contribute to reducing the incidence of cerebral embolization by recognizing the timing and number of microemboli. Although hypothermia confers cerebral protection, rewarming may unmask and perhaps potentiate any ischemic damage that occurred with embolization during hypothermia. Both the degree and speed of rewarming may be important factors contributing to the extent of ischemic damage and ultimately neurologic function. In addition, many other factors related to cardiopulmonary bypass can alter cerebral perfusion and metabolism, such as nonpulsatile flow, hemodilution, pressure autoregulation, anesthetic and cerebroprotective drugs, and the neuroimmune response to bypass. In this review, the major factors affecting cerebral blood flow during cardiopulmonary bypass are discussed and their relative importance evaluated with regard to postoperative neurologic function.

Cardiopulmonary bypass: perioperative cerebral blood flow and postoperative cognitive deficit

Increased cerebral blood flow occurring during cardiopulmonary bypass as a result of changes in arterial carbon dioxide tension during acid-base regulation is thought to increase postoperative cognitive dysfunction. We studied 70 patients undergoing coronary artery bypass procedures who were randomized to two different acid-base protocols: pH-stat or alpha-stat regulation. Cerebral blood flow, cerebral blood flow velocity, and cerebral oxygen metabolism were measured before bypass, during bypass (hypothermic [28 degrees C] and normothermic phases), and after bypass. Detailed cognitive tests were conducted before operation and 6 weeks after operation. During 28 degrees C bypass, cerebral blood flow was significantly (p < 0.05) higher in the pH-stat group than in the alpha-stat group (41 +/- 2 versus 24 +/- 2 mL.100 g-1.min-1), and cerebral blood flow velocity was significantly increased in the pH-stat group and significantly decreased in the alpha-stat group (152% +/- 10% versus 78% +/- 7%). Cerebral extraction ratio of oxygen demonstrated a relatively greater disruption of autoregulation in the pH-stat group than in the alpha-stat group with relative hyperemia of 0.12 +/- 0.02 versus 0.26 +/- 0.03, respectively, during 28 degrees C bypass. Using the criterion of deterioration in three or more neuropsychologic tests, a significantly higher proportion of patients in the pH-stat group fared less well than in the alpha-stat group (49% +/- 17% versus 20% +/- 13%). Patients in the alpha-stat group experienced less disruption of cerebral autoregulation during hypothermic cardiopulmonary bypass, and this was accompanied by a reduction in postoperative cognitive dysfunction.

Cerebral blood flow during cardiopulmonary bypass: influence of temperature and pH management strategy

Because disordered autoregulation of cerebral blood flow may underlie neurologic injury associated with cardiopulmonary bypass (CPB), we studied the effects of normothermic (37 degrees C) and hypothermic (18 degrees C) CPB on cerebral vascular reactivity in 6 to 8-week-old piglets. Hypothermic CPB animals were subdivided into alpha-stat and pH-stat groups (n = 6 animals each group) according to acid-base management protocol. Cerebral blood flow (CBF), cerebral oxygen consumption (CMRO2), cerebral vascular resistance (CVR), and CBF response to hypercapnia were examined before, during, and 1 hour after CPB and used to calculate CVR per millimeter of mercury change in arterial partial pressure of CO2: (CVRnormocapnia - CVRhypercapnia)/(PaCO2 hypercapnia - PaCO2 normocapnia). Before CPB, CBF, CMRO2, and vascular reactivity to elevated CO2 were similar in the three groups; these parameters remained unchanged by normothermic CPB. However, during hypothermic CPB, CBF and CMRO2 decreased in both alpha-stat and pH-stat groups; in the alpha-stat group, CBF decreased from 27 +/- 5 mL.min-1.100 g-1 (normothermic CPB) to 5 +/- 1 mL.min-1.100 g-1 (hypothermic CPB) (p < 0.05) and CMRO2 decreased from 1.8 +/- 0.21 to 0.24 +/- 0.04 mL.min-1.100 g-1 (p < 0.05), whereas in the pH-stat group CBF decreased from 28 +/- 2 to 9 +/- 1 mL.min-1. 100 g-1 (p < 0.05) and CMRO2 decreased from 1.63 +/- 0.07 to 0.31 +/- 0.09 mL.min-1.100 g-1 (p < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of carbon dioxide on cerebral arteries

The constancy of cerebral blood flow and volume relies heavily upon the cerebral arteries' intrinsic ability to respond to changes in the partial pressure of arterial CO2. The physiologic mechanisms underlying these responses have not been determined, although changes in extracellular and intracellular pH, mediation by prostanoids and neural activity have been suggested. CO2 reactivity can be influenced by oxygen status and blood pressure and can vary according to age and brain region. In certain pathological conditions or diseases, it can be severely altered. Modern techniques, which measure CBF in cases of cerebral hemodynamic insufficiency, head injury or tumor, rely on the inherent ability of the cerebral circulation to respond to changing levels of CO2.

Radio-immunoassay of atrial natriuretic peptide (ANP) and characterization of ANP immunoreactivity in human plasma and atrial tissue

A sensitive radio-immunoassay (RIA) was developed to determine the occurrence of atrial natriuretic peptide (ANP) in plasma and atrial extracts from patients undergoing open heart surgery. The immunoreactive ANP (irANP) was characterized by high-pressure liquid chromatography coupled with RIA. The plasma irANP response to releasing stimuli during the operation was determined in simultaneously sampled venous and arterial blood, in order to evaluate any differences. The antiserum recognized the intact ring-structure of alpha-humanANP (alpha-hANP) and its propeptide gamma-hANP, as well as beta-hANP, an anti-parallel dimer of alpha-hANP. Less bioactive N-or C-terminal fragments of alpha-hANP, or an N-terminal fragment of the propeptide, gamma-hANP 1-67, did not cross-react with the antiserum. Sep Pak C18-extraction of plasma resulted in an 80% recovery of synthetic alpha-hANP. The assay had a sensitivity of 1.9 pmol l-1, well below the venous plasma concentrations of irANP found in healthy volunteers (7.4 +/- 1.3 pmol l-1, mean +/- SEM, n = 19), and the local standard was identical to an international standard of alpha-hANP. In atrial extracts three major peaks of irANP were identified as alpha-, beta- and gamma-hANP, with gamma-hANP as the most abundant form. In plasma alpha-hANP dominated, but in two cases high plasma levels of beta-hANP were seen, reflecting the high atrial content in these patients. In peripheral arterial blood, irANP was on an average 56% +/- 20% (p less than 0.01, n = 18) higher than in venous blood; this was associated with more distinct arterial irANP responses to releasing stimuli during the operation.

Cardiopulmonary perfusion and cerebral blood flow in bilateral carotid artery disease

The fear of cerebral complications after cardiopulmonary bypass in patients with heart disease and severe carotid artery disease has led many authors to suggest combined approaches in these patients. The pathogenetic mechanism for stroke is based partly on the stenotic narrowing of the carotid artery. A diameter reduction of 75% is frequently considered hemodynamically significant and indicative of an increased risk for neurological morbidity. We studied the cerebral blood flow in 7 patients undergoing coronary artery bypass grafting who also had severe bilateral carotid disease. The results were compared with the results in 17 patients without carotid disease who had bypass grafting. The cerebral blood flow was measured by xenon 133 washout technique before, during, and after cardiopulmonary bypass with moderate hypothermia. Acid-base regulation was according to the alpha-stat theory, and blood pressure was kept greater than 50 mm Hg. The cerebral blood flow levels (mL.100g-1.min-1) before, during, and after cardiopulmonary bypass in the study group (30 +/- 11, 31 +/- 8, 47 +/- 20) (mean +/- standard deviation) were almost identical to those in the control group (30 +/- 11, 28 +/- 8, 47 +/- 12). The cerebral blood flow levels for the left and right hemispheres in the group with carotid disease were comparable and within normal ranges. In 2 patients, slight differences were noted between hemispheres, and this finding may indicate an increased risk for ischemia. These patients, however, did not show any signs of postoperative deficit. The flow limitations of critical carotid stenoses do not seem to imply a risk for cerebral hypoperfusion if cardiopulmonary perfusion is performed in a controlled manner.(ABSTRACT TRUNCATED AT 250 WORDS)