H. Sonntag

Bedside Assessment of Intravascular Volume Status in Patients Undergoing Coronary Bypass Surgery

BackgroundManagement of intravascular volume is crucial in patients after cardiopulmonary bypass as myocardial dysfunction is common. The purpose of this study was to validate a novel bedside technique for real-time assessment of Intravascular volumes. MethodsEleven patients undergoing cardiopulmonary bypass were studied. In addition to standard monitors, a fiberoptic thermistor catheter was placed in the descending aorta and central venous Injections of 10 ml ice-cold indocyanine green dye were performed. Total blood volume was measured by a standard in vitro technique. Circulating and central blood volume were calculated by using cardiac output, mean transit times, and a newly developed recursive convolution algorithm that models recirculation. Measurements were performed after Induction of anesthesia and at 1, 6, and 24 h after surgery. ResultsA two-compartment model of the circulation was required for adequate fit of the data. We found a significant correlation between total and circulating blood volumes (r = 0.87). One hour after surgery, central blood volume was decreased by 10% (P < 0.05). At 6 and 24 h after surgery, circulating blood volumes were significantly increased by 29% and 20%, respectively (P < 0.01), although central blood volume was similar to control values. Before surgery stroke volume index correlated with circulating blood volume (r = 0.87) but not with pulmonary capillary wedge and central venous pressures. ConclusionsThis study shows that bedside determinations of intravascular blood volumes are feasible and that these measurements are more Indicative of Intravascular volume status than are either pulmonary capillary wedge or central venous pressures in the post-cardiopulmonary bypass period. Our data also demonstrate that despite a normal central blood volume both circulating and total blood volume are significantly Increased in the immediate post-cardiopulmonary bypass period.

Comparison of cardiac output assessed by pulse-contour analysis and thermodilution in patients undergoing minimally invasive direct coronary artery bypass grafting

OBJECTIVE To investigate the precision and accuracy of continuous pulse contour cardiac output (PCCO) compared with intermittent transcardiopulmonary (TCPCO) and pulmonary artery thermodilution (TDCO) measurements in patients undergoing minimally invasive coronary bypass surgery (MIDCAB). DESIGN Prospective, controlled, clinical study. SETTING University hospital. PARTICIPANTS Twelve patients undergoing MIDCAB. INTERVENTIONS Thirty-six measurements of PCCO and thermodilution cardiac output (CO) were simultaneously performed after the start of surgery, during bypass grafting, and at the end of surgery. TCPCO and TDCO were simultaneously assessed by three injections of ice-cold saline randomly spread over the respiratory cycle. The pulse contour device was initially calibrated with an additional set of aortic thermodilution measurements. MEASUREMENTS AND MAIN RESULTS Absolute values of CO ranged between 1.6 and 9.2 L/min. A close agreement among the three techniques was observed at all measurements. Mean bias between PCCO and TDCO and TCPCO was 0.003 L/min (2 SD of differences between methods = 1.26 L/min) and 0.27 L/min (2 SD of differences between methods = 1.16 L/min), respectively. The correlation coefficients were r2 = 0.90 for TCPCO versus PCCO and r2 = 0.88 for TDCO versus PCCO. CONCLUSION The results of the present study show that compared with thermodilution CO, pulse contour analysis enables accurate measurement of continuous CO in patients undergoing MIDCAB.

Cerebrovascular tone rather than intracranial pressure determines the effective downstream pressure of the cerebral circulation in the absence of intracranial hypertension.

Cerebral perfusion pressure is commonly calculated from the difference between mean arterial pressure and intracranial pressure because intracranial pressure is known to represent the effective downstream pressure of the cerebral circulation. Studies of other organs, however, have shown that effective downstream pressure is determined by a critical closing pressure located at the arteriolar level. This study was designed to investigate the effects of PCO2-induced variations in cerebrovascular tone on the effective downstream pressure of the cerebral circulation. Sixteen patients recovering from head injury were studied. Intracranial pressure was assessed by epidural pressure transducers. Blood flow velocity in the middle cerebral artery was monitored by transcranial Doppler sonography. Effective downstream pressure was derived from the zero flow pressure as extrapolated by regression analysis of instantaneous arterial pressure/middle cerebral artery flow velocity relationships. PaCO2 was varied between 30 and 47 mm Hg in randomized sequence. Intracranial pressure decreased from 18.5 ± 5.2 mm Hg during hypercapnia to 9.9 ± 3.1 mm Hg during hypocapnia. In contrast, effective downstream pressure increased from 13.7 ± 9.6 mm Hg to 23.4 ± 8.6 mm Hg and exceeded intracranial pressure at hypocapnic PaCO2 levels. Our results demonstrate that, in the absence of intracranial hypertension, intracranial pressure does not necessarily represent the effective downstream pressure of the cerebral circulation. Instead, the tone of cerebral resistance vessels seems to determine effective downstream pressure. This suggests a modified model of the cerebral circulation based on the existence of two Starling resistors in a series connection.

Flow velocity measurements as an index of cerebral blood flow. Validity of transcranial Doppler sonographic monitoring during cardiac surgery.

Background:Transcranial Doppler sonography is increasingly used to monitor changes in cerebral perfusion intraoperatively. However, little information is available about the validity of velocity measurements as an index of cerebral blood flow (CBF). The purpose of this study was to compare invasive and Doppler-derived measurements of cerebral hemodynamic variables during coronary artery bypass graft surgery. Methods:In 15 male patients, measurements of CBF and middle cerebral artery flow velocity (VMCA) were performed before and after induction of fentanyl-midazolam anesthesia, during hypothermic cardiopulmonary bypass (CPB), and at the end of the surgical procedure. Transcranial Doppler sonography recordings of systolic, diastolic, and mean VMCA, and derived parameters such as pulsatility (PI) and resistance (RI) indexes were recorded from the proximal segment of the right middle cerebral artery. CBF was measured by the Kety-Schmidt inert gas saturation method with argon as a tracer. To facilitate comparisons of CBF and VMCA measurements, changes between consecutive measurements were expressed as percentage values. Calculations of cerebral perfusion pressure and cerebral vascular resistance (CVR) were based on jugular bulb pressure. The cerebral metabolic rate for oxygen was calculated from CBF and the arterial — cerebral venous oxygen content difference. Results:Changes In mean VMCA paralleled changes in mean CBF except for hemodynamic changes associated with hypothermic CPB. At this stage of surgery, mean VMCA increased while actual CBF decreased. Separate analysis of the periods before and after CPB revealed a poor association between percentage changes In CBF and VMCA (r=0.26, P=0.36; r=0.51, P=0.06, respectively). Mean values of CVR, PI, and RI showed consistent changes after induction of anesthesia. After termination of CPB, mean CVR significantly decreased, whereas mean PI and RI remained virtually unchanged. Neither before nor after CPB was a clinically useful correlation found between percentage changes in PI, RI, and CVR (PI r=0.28, P=0.34; r=-0.47, P=0.09, respectively; RI r=0.16, P=0.59; r=-0.53, P=0.06, respectively). Conclusions:Hypothermic CPB seems to alter the relation between global CBF and flow velocity in basal cerebral arteries. Inconsistency in directional changes in CBF and VMCA at this stage of surgery might be attributable to changes in middle cerebral artery diameter, red blood cell velocity spectra, and regional flow distribution. Although changes in mean VMCA before and after CPB appear to parallel changes In mean CBF, individual responses of VMCA cannot reliably predict percentage changes in CBF. Furthermore, Doppler sonographic PI and RI cannot provide an approximation of changes in CVR during cardiac surgery.

Effects of one minimum alveolar anesthetic concentration sevoflurane on cerebral metabolism, blood flow, and CO2 reactivity in cardiac patients.

UNLABELLED We investigated the cerebral hemodynamic effects of 1 minimum alveolar anesthetic concentration (MAC) sevoflurane anesthesia in nine male patients scheduled for elective coronary bypass grafting. For measurement of cerebral blood flow (CBF), a modified Kety-Schmidt saturation technique was used with argon as an inert tracer gas. Measurements of CBF were performed before the induction of anesthesia and 30 min after induction under normocapnic, hypocapnic, and hypercapnic conditions. Compared with the awake state under normocapnic conditions, sevoflurane reduced the mean cerebral metabolic rate of oxygen by 47% and the mean cerebral metabolic rate of glucose by 39%. Concomitantly, CBF was reduced by 38%, although mean arterial pressure was kept constant. Significant changes in jugular venous oxygen saturation were absent. Hypocapnia and hypercapnia caused a 51% decrease and a 58% increase in CBF, respectively. These changes in CBF caused by variation of Paco2 indicate that cerebrovascular CO2 reactivity persists during 1 MAC sevoflurane anesthesia. IMPLICATIONS We used a modified Kety-Schmidt saturation technique to investigate the effects of 1 minimum alveolar anesthetic concentration (MAC) sevoflurane on cerebral blood flow, metabolism, and CO2 reactivity in cardiac patients. We found that the global cerebral blood flow and global cerebral metabolic rate of oxygen remained coupled and that cerebrovascular CO2 reactivity is not impaired by the administration of 1 MAC sevoflurane.

Effects of respiratory alkalosis and acidosis on myocardial blood flow and metabolism in patients with coronary artery disease

Background Variation of the arterial carbon dioxide partial pressure (PaCO2) is not uncommon in anesthetic practice. However, little is known about the myocardial consequences of respiratory alkalosis and acidosis, particularly in patients with coronary artery disease. The aim of the current study was to investigate the effects of variation in PaCO (2) on myocardial blood flow (MBF), metabolism, and systemic hemodynamics in patients before elective coronary artery bypass graft surgery. Methods In 10 male anesthetized patients, measurements of MBF, myocardial contractility, metabolism, and systemic hemodynamics were made in a randomized sequence at PaCO2 levels of 30, 40, and 50 mmHg, respectively. The MBF was measured using the Kety‐Schmidt technique with argon as a tracer. End‐diastolic left ventricular pressure and the maximal increase of left ventricular pressure were assessed using a manometer‐tipped catheter. Results The cardiac index significantly changed with varying PaCO (2) levels (hypocapnia, ‐ 9%; hypercapnia, 13%). This reaction was associated with inverse changes in systemic vascular resistance index levels. The MBF significantly increased by 15% during hypercapnia, whereas no change was found during hypocapnia. Myocardial oxygen and glucose uptake and the maximal increase of left ventricular pressure were not affected by varying Pa (CO)2 levels. Conclusions In anesthetized patients with coronary artery disease, short‐term variations in PaCO2 have significant effects on MBF but do not influence global myocardial oxygen and glucose uptake. Changes in systemic hemodynamics associated with respiratory alkalosis and acidosis are caused by changes in systemic vascular resistance rather than by alterations in myocardial contractility.

Cerebral effects of anaesthesia and hypothermia.

Cerebral blood flow, cerebral oxygen and glucose consumption, and cerebral lactate and pyruvate release were measured; spectral analysis of the EEG was recorded in 10 male patients who had coronary artery bypass surgery. The measurements were taken to evaluate the effects of fentanyl–midazolam anaesthesia during normothermia and during hypothermic nonpulsatile cardiopulmonary bypass at 26°C venous blood temperature, when a temperature‐corrected Paco2‐value of 5.3 kPa was maintained. Anaesthesia with fentanyl 7 μg/kg and midazolam 200 μg/kg as induction doses, followed by infusions of fentanyl 0.15 μg/kg/minute and midazolam 3 μg/kg/minute, was characterised by a decrease in fast‐wave activity and an increase in high‐amplitude, slow‐wave activity in the EEG. There was also a decrease in cerebral blood flow (38%), oxygen consumption (22%) and glucose consumption (25%), while lactate and pyruvate production remained unchanged. Hypothermia of 26°C venous blood temperature suppressed EEG almost completely and decreased oxygen and glucose consumption by a further 61% and 54%, respectively, with no changes in lactate and pyruvate production while cerebral blood flow increased by 145%. These results show that the effects of fentanyl–midazolam anaesthesia on cerebral metabolism are enhanced during hypothermic cardiopulmonary bypass while the influence of anaesthesia on cerebral blood flow is overshadowed by the practice of a temperature‐corrected acid‐base management.

Application of a transpulmonary double indicator dilution method for postoperative assessment of cardiac index, pulmonary vascular resistance index, and extravascular lung water in children undergoing total cavo-pulmonary anastomosis: preliminary results in six patients

Total cavo-pulmonary anastomosis (TCPA) is used for the functional correction of an increasing spectrum of congenital heart diseases. The passive pulmonary perfusion after surgical exclusion of the right ventricle has significant implications for the postoperative hemodynamic management of these patients. Because conventional pulmonary artery thermodilution catheters present methodologic problems in patients after TCPA, important cardiovascular variables such as cardiac index (CI) and pulmonary and systemic vascular resistance indices (PVRI, SVRI) usually cannot be assessed directly. In a preliminary series of six patients undergoing TCPA (age 6-22 years), the applicability of a transpulmonary double indicator dilution technique for postoperative determinations of CI, PVRI, SVRI, and extravascular lung water (EVLW) was investigated. After central venous injection of ice-cold indocyanine green (5 mg), thermal and dye dilution curves were recorded in the abdominal aorta using a combined 4F fiberoptic thermistor catheter. Qualitative assessment of the tracer curves did not show major differences in measurements in patients with pulsatile perfusion of the lungs. CI, SVRI, and EVLW could be determined by use of standard algorithms. Pulmonary perfusion pressure for the calculation of PVRI was based on the gradient between central venous and left atrial pressure. The quality of indicator dilution curves allowed determination of flow-related variables in 33 of a total of 34 sets of measurements. No catheter-related problems occurred during or after the period of investigation. Postoperative EVLW was within the range that is commonly accepted as normal for adults. Mean PVRI initially decreased during the postoperative course but showed a significant increase after extubation.(ABSTRACT TRUNCATED AT 250 WORDS)

The influence of anesthesia on myocardial oxygen utilization efficiency in patients undergoing coronary bypass surgery

A maximum efficiency of myocardial oxygen utilization is desirable, especially in patients with limited coronary blood supply. Little is known about the effects of anesthesia on the efficiency of myocardial oxygen utilization. The aim of this retrospective study was to investigate the effects of different types of anesthesia on myocardial efficiency in patients undergoing coronary bypass surgery. Myocardial blood flow (Argon wash in technique), myocardial oxygen uptake, and standard hemodynamics were measured awake, after induction of anesthesia, and during sternal spread. Myocardial oxygen utilization efficiency was calculated from the ratio of external work divided by myocardial oxygen consumption. Sixty-five patients in eight groups with different anesthetic techniques were studied: 1) halothane/nitrous oxide, 2) enflurane/nitrous oxide, 3) high-dose morphine, 4) high-dose fentanyl, 5) fentanyl/midazolam, 6) high-dose sufentanil, 7) sufentanil/nitrous oxide, and 8) propofol. In all groups induction of anesthesia was associated with a decrease of cardiac work (from 86 +/- 17 to 55 +/- 16 J/min in pooled data) which resulted from a decrease of both stroke volume index and blood pressure. However, myocardial oxygen consumption did not decrease proportionally (from 11.2 +/- 3.0 to 8.5 +/- 2.3 mL.min-1 x 100 g-1 in pooled data) and myocardial oxygen utilization efficiency was therefore decreased in all groups (from 29.2% +/- 2.5% at awake state to 23.9% +/- 5.8% after induction of anesthesia in pooled data). Surgical stimulation by sternotomy and sternal spread was associated with different patterns of hemodynamic response between groups. Blood pressures and external work tended to be higher in the high-dose narcotic groups while it remained less affected in the other groups. However, myocardial efficiency remained depressed in all groups (22.2% +/- 8.2% in pooled data) and with respect to myocardial efficiency no differences between anesthetic techniques were found. We conclude that the specific anesthetic technique does not influence impairment of myocardial oxygen utilization efficiency by anesthesia.

[The effect of nitroglycerin on cerebrovascular circulation, cerebrovascular CO2-reactivity and blood flow rate in basal cerebral arteries].

ZusammenfassungZiel dieser prospektiven kontrollierten Studie war es, 1.) den Einfluß von Nitroglyzerin (NTG) auf den globalen zerebralen Blutfluß (CBF) und die zerebrovaskuläre CO2-Reaktivität zu untersuchen, und 2.) den Einfluß von NTG auf den Zusammenhang zwischen CBF und der Flußgeschwindigkeit in der A. cerebri media (VMCA) zu bestimmen. Bei 10 kardiochirurgischen Patienten wurde zunächst während einer Kontrollphase in randomisierter Reihenfolge eine Hypo- und Hyperkapnie (paCO2≈30 bzw. 50 mmHg) induziert. Anschließend wurden alle Messungen unter einer Infusion von 1,5·μg·kg−1·min−1 NTG bei identischen paCO2-Niveaus wiederholt. Die Bestimmung des CBF erfolgte mittels der Kety-Schmidt-Technik. Simultan wurde jeweils die VMCA mittels eines 2-MHz-Dopplersystems aufgezeichnet. Unter NTG-Infusion nahm der zerebrale Perfusionsdruck um 15–17% ab, dennoch zeigte sich aufgrund einer Reduktion des zerebrovaskulären Widerstands eine erhebliche Zunahme des CBF um 96 bzw. 69%, während die VMCA geringfügig abfiel. Die CO2-Reaktivität des CBF zeigte keine signifikante Änderung. Die Ergebnisse der vorliegenden Untersuchung zeigen zum einen, daß NTG zu einer ausgeprägten Zunahme der globalen Hirndurchblutung führt, sofern kein kritischer Abfall des zerebralen Perfusionsdrucks eintritt. Zum anderen legen die Ergebnisse des durchgeführten Methodenvergleichs nahe, daß auch die proximalen Segmente der A. cerebri media (MCA) unter dem Einfluß von NTG eine Vasodilatation aufweisen, die zu einer methodisch relevanten Diskrepanz zwischen relativen Änderungen der Hirndurchblutung und der MCA-Strömungsgeschwindigkeit führt.AbstractThe cerebral haemodynamic effects of vasodilators are of clinical interest because a decrease in mean arterial pressure (MAP) might alter global cerebral blood flow (CBF). Luxury perfusion of the brain, in contrast, might be unfavourable in patients with reduced intracranial compliance. Despite the widespread use of nitroglycerine (NTG), little is known about the cerebral haemodynamic consequences of NTG infusions in humans. This prospective, controlled study was designed: (1) to investigate the effects of NTG on CBF and cerebrovascular CO2 reactivity and (2) to compare reference measurements o