Linear and non-linear dynamics of heart rate variability in brain dead organ donors.

Recent studies outlined linear and non-linear dynamics in heart rate variability; however, their physiological origin is still unknown. The present study investigated the impact of cerebral function on linear and non-linear dynamics in heart rate variability. Electrocardiograms from seven brain dead organ donors and seven healthy volunteers were analyzed. Atropine was used in healthy volunteers to adjust their heart rate to that of the donors. As compared to healthy volunteers without atropine, the linear dynamics of heart rate variability, determined by time and frequency domain analyses, were significantly reduced in healthy volunteers with atropine and, to an even greater extent, in donors. Atropine tended to increase the complexity and non-linearity of heart rate variability in healthy volunteers, as determined by the correlation dimension D and the largest Lyapunov exponent L, respectively (D = 9.43 +/- 2.93 vs. 7.65 +/- 0.97 and L = 0.525 +/- 0.099 vs. 0.504 +/- 0.047 bits.beat-1; both NS), while these indices were significantly reduced in donors by 19.5 +/- 12.8% and 15.0 +/- 11.7%, respectively (D = 6.16 +/- 0.98 and L = 0.428 +/- 0.059 bits.beat-1; both p < 0.05 vs. volunteers). Thus, loss of cerebral function reduces both linear and non-linear components of heart rate variability.

Perfusion-contraction mismatch with coronary microvascular obstruction: role of inflammation.

A close relationship exists between regional myocardial blood flow (RMBF) and function during acute coronary inflow restriction (perfusion-contraction matching). However, the relationship of flow and function during coronary microvascular obstruction is unknown. In 12 anesthetized dogs, the left circumflex coronary artery was perfused from an extracorporeal circuit. After control measurements, 3,000 microspheres (42 micrometer diameter) per milliliter per minute inflow were injected to cause a microembolism (ME, n = 6). With unchanged systemic hemodynamics and RMBF, posterior systolic wall thickening (PWT) decreased from 19.8 +/- 1.9% SD at control to 13.3 +/- 4.0, 10.3 +/- 3.8, and 6.9 +/- 4.7% (P < 0.05 vs. control) at 1, 4, and 8 h, respectively. For comparison, inflow was progressively reduced to match PWT to that of the ME group at 1, 4, and 8 h (stenosis, STE, n = 6). RMBF in the STE group was reduced in proportion to PWT. Infarct size was not different among groups (6.5 +/- 4.5 vs. 3.4 +/- 3.2%). However, the number of leukocytes infiltrating the area at risk was significantly greater in the ME group than in the STE group. Coronary microembolization results in perfusion-contraction mismatch and is associated with an inflammatory response.

Less afterload sensitivity in short-term hibernating than in acutely ischemic and stunned myocardium.

Short-term hibernating myocardium is characterized by reduced contractile function during persistent moderate ischemia, the recovery of metabolic parameters, and the absence of necrosis. To study the afterload dependence of regional wall excursion in short-term hibernating myocardium, in 11 enflurane-anesthetized swine the left anterior descending coronary artery was cannulated and hypoperfused for 90 min to reduce anterior systolic wall thickening (WT, sonomicrometry) by 60%. Under control conditions, at 5 and 90 min ischemia the descending thoracic aorta was acutely constricted to increase left ventricular (LV) pressure by 30 mmHg. Under control conditions, increased LV pressure resulted in decreased WT [i.e., a negative slope of the relationship between WT and LV end-systolic pressure: -11.2 +/- 4.2 (SD) microm/mmHg]. This slope was further significantly decreased at 5 min ischemia (-26.5 +/- 8.8 microm/mmHg) but returned toward control values in short-term hibernating myocardium at 90 min ischemia (-17.2 +/- 6.6 microm/mmHg). At 30 min reperfusion, the slope was once more significantly decreased (-27.8 +/- 8.1 microm/mmHg). In conclusion, WT in short-term hibernating myocardium is less afterload dependent than in acutely ischemic and reperfused myocardium.

Contrast echocardiography for assessment of myocardial perfusion.

It has been suggested that the myocardial perfusion can be qualitatively and quantitatively assessed by different ultrasound contrast techniques. It has been reported that the intracoronary or intraaortic administration of the ultrasound contrast agents can be used to visualize perfusion defects or to analyze the coronary flow reserve. The perfusion analysis after intracoronary injection of ultrasound contrast agents seems to be established, but there are a lot of open questions. A topographic (qualitative) perfusion analysis with visualization of perfusion defects and perfusion areas or analysis of collaterals has been demonstrated. A quantitative analysis of myocardial blood flow has been described but the existing studies are inconsistent. It is not known which parameters of the contrast wash-out curves should be used for perfusion analysis and if the Stewart-Hamilton curve analysis can be transferred to all ultrasound contrast agents as a model for quantitative myocardial blood flow assessment. The development of the transpulmonary contrast agents for echocardiographic evaluation of left ventricular cavity has the impact for myocardial perfusion imaging. The increase of myocardial intensity does not mean that a qualitative or quantitative perfusion analysis can be clinically used. In this field we have to differentiate between the possibilities of qualitative discrimination of perfusion defects and quantitative perfusion (myocardial blood flow) analysis. The different scanning conditions, the poor transthoracic ultrasound window and insufficient enhancement of the myocardial intensity make it problematic to quantify the myocardial perfusion. At the moment myocardial intensity will be increased after intravenous injection of transpulmonary contrast agents, but the value for perfusion analysis has not been shown. New ultrasound technologies such as second harmonic imaging, power-mode and raw data analysis have to show the clinical importance of these techniques for perfusion analysis in daily clinical routine. The open questions of the perfusion analysis by contrast echocardiography will be discussed in this review article.

Calcium responsiveness in regional myocardial short-term hibernation and stunning in the in situ porcine heart. Inotropic responses to postextrasystolic potentiation and intracoronary calcium.

BACKGROUND: We tested the hypothesis that decreased calcium responsiveness is responsible for the reduction in contractile function in regional hibernating and stunned myocardium in situ. METHODS AND RESULTS: In 19 anesthetized swine, the left anterior descending coronary artery flow was reduced to decrease anterior myocardial work index (sonomicrometry) by approximately 60%. During 90 minutes of hypoperfusion, creatine phosphate recovered (as determined by biopsy specimens and bioluminescence) and no necrosis developed (as determined by staining with triphenyl tetrazolium chloride). In 10 swine, changes in the intracellular calcium concentration were induced by systematic variation of the postextrasystolic time interval at a constant prematurity. In 9 additional swine, a graded IC calcium infusion was performed. Under control conditions, anterior myocardial work increased with a fully compensated postextrasystolic time interval from 380+/-93 (mean+/-SD) to 523+/-98 mm Hg . mm. IC calcium infusion increased anterior myocardial work under control conditions from 356+/-85 to a maximum of 428+/-93 mm Hg . mm. Although the maximal responses were decreased during postextrasystolic potentiation (222+/-68 versus 523+/-98 mm Hg . mm) and calcium infusion (176+/-32 versus 428+/-93 mm Hg . mm) after 90 minutes of ischemia, the relationships between increases in anterior myocardial work and, respectively, postextrasystolic time interval and IC calcium were not different. The same was true after 30 minutes of reperfusion. CONCLUSIONS: Both regional hibernating myocardium and stunned myocardium in situ are characterized by a decrease in overall myocardial calcium responsiveness; however, there appears to be no significant myocardial desensitization to calcium.

Aspirin does not prevent the attenuation of myocardial stunning by the ACE inhibitor ramiprilat.

The attenuation of myocardial stunning by the ACE inhibitor ramiprilat is prevented by cyclooxygenase inhibition with indomethacin. In the clinical setting of ischemia/reperfusion however, the cyclooxygenase inhibitor aspirin is widely used to prevent platelet aggregation. The present study therefore investigated whether aspirin in dosages sufficient to inhibit platelet aggregation interferes with the attenuation of myocardial stunning by ramiprilat. Fifteen dogs received either 1 mg/(kg.day) (group I, n = 7) or 10 mg/(kg.day) (group II, n = 8) aspirin orally for 1 week. Both dosages of aspirin inhibited ADP-induced platelet aggregation. The dogs were then anesthetized thoracotomized and subjected to 15 min LCx-occlusion and 4 h reperfusion. Before LCx-occlusion, groups I and II received ramiprilat (20 micrograms/kg, i.v.). Systemic hemodynamics, posterior myocardial blood flow (PMBF, colored microspheres) and wall thickening (PWT, sonomicrometry) of these groups were measured and data compared to placebo-controls (group III, n = 11) and dogs receiving only ramiprilat before LCx-occlusion (group IV, n = 11). Four dogs received 10 mg/(kg.day) aspirin without ramiprilat (group V). Mean aortic pressure was kept constant by an intra-aortic balloon, and heart rate did not change. PMBF was not different between the five groups. Under control conditions and during myocardial ischemia PWT was also not different. At 4 h reperfusion PWT was still depressed in group III (-5 +/- 20% of control) and group V (-23 +/- 6%) whereas PWT recovered to the same extent in groups I (46 +/- 23%), II (50 +/- 15%) and IV (58 +/- 18%) (all P < 0.05 v groups III and V). The attenuation of myocardial stunning by the ACE inhibitor ramiprilat is not prevented by aspirin in dosages which are nevertheless sufficient to inhibit platelet aggregation.

Low-frequency spectral power of heart rate variability is not a specific marker of cardiac sympathetic modulation.

BACKGROUND: Heart rate variability in the frequency domain has been proposed to reflect cardiac autonomic control. Therefore, measurement of heart rate variability may be useful to assess the effect of epidural anesthesia on cardiac autonomic tone. Accordingly, the effects of preganglionic cardiac sympathetic blockade by segmental epidural anesthesia were evaluated in humans on spectral power of heart rate variability. Specifically, the hypothesis that cardiac sympathetic blockade attenuates low-frequency spectral power, assumed to reflect cardiac sympathetic modulation, was tested. METHODS: Ten subjects were studied while supine and during a 15-min 40 degrees head-up tilt both before and after cardiac sympathetic blockade by segmental thoracic epidural anesthesia (sensory block: C6-T6). ECG, arterial pressure, and respiratory excursion (Whitney gauge) were recorded, and a fast-Fourier-transformation was applied to 512-s data segments of heart rate derived from the digitized ECG at the end of each intervention. RESULTS: With cardiac sympathetic blockade alone and the subjects supine, both low-frequency (LF, 0.06-0.15 Hz) and high-frequency (HF, 0.15-0.80 Hz) spectral power remained unchanged. During tilt, epidural anesthesia attenuated the evoked increase in heart rate (+11.min-1 +/- 7 SD vs. +6 +/- 7, P = 0.024). However, while during tilt cardiac sympathetic blockade significantly decreased the LF/HF ratio (3.68 +/- 2.52 vs. 2.83 +/- 2.15, P = 0.041 vs. tilt before sympathetic blockade), a presumed marker of sympathovagal interaction, absolute and fractional LF and HF power did not change. CONCLUSIONS: Although preganglionic cardiac sympathetic blockade reduced the LF/HF ratio during tilt, it did not alter spectral power in the LF band during rest or tilt. Accordingly, low-frequency spectral power is unlikely to specifically reflect cardiac sympathetic modulation in humans.

Bradycardic agent UL-FS 49 attenuates ischemic regional myocardial dysfunction and reduces infarct size in swine: comparison with the beta-blocker atenolol.

Heart rate (HR) is a major factor determining the severity of myocardial ischemia, and HR reduction is an effective therapy for myocardial ischemia. We tested the effects of HR reduction induced by either UL-FS 49 or atenolol on regional myocardial blood flow, function, and infarct size (IS) in a porcine model of 90-min low-flow ischemia and 2-h reperfusion. In 24 Göttinger miniswine, the left anterior descending coronary artery (LAD) was cannulated and hypoperfused at constant inflow to reduce anterior systolic wall thickening (AWT, sonomicrometry) by approximately 85%. Eight swine served as a placebo group, and 8 other swine received UL-FS 49 (0.60 mg/kg intravenously, i.v.) after 10-min ischemia. In the remaining 8 swine, atenolol was infused after 10-min ischemia at a dosage [mean 1.75 +/- 1.20 (SD) mg/kg i.v.] to mimic the HR reduction observed with UL-FS 49. Systemic hemodynamics, subendocardial blood flow (ENDO, microspheres) and AWT were measured under control conditions, at 10 and 90 min of ischemia. In the swine receiving UL-FS 49 or atenolol, additional measurements were made 5 min after administration of the respective drug. After 2-h reperfusion, IS (percentage of area at risk) was determined with TTC-staining. Five minutes after administration of UL-FS 49, HR was decreased from 113 +/- 9 to 83 +/- 13 beats/min (p < 0.05) and remained unchanged when ischemia was prolonged to 90 min. In the swine receiving atenolol, HR was reduced from 117 +/- 14 to 93 +/- 7 beats/min (p < 0.05) 5 min after drug administration and decreased further to 87 +/- 10 beats/min when ischemia was prolonged to 90 min. After 10 min of ischemia, AWT in the placebo, UL-FS 49, and atenolol group was decreased to 7.0 +/- 5.5, 6.4 +/- 3.5, and 6.2 +/- 3.3% (all p < 0.05 vs. control), respectively. The reduction in ENDO was also comparable among the three groups. In the placebo group, AWT remained unchanged when ischemia was prolonged to 90 min (4.4 +/- 2.6%). In swine receiving atenolol, AWT tended to increase (13.6 +/- 10.5%), whereas in swine receiving UL-FS 49, AWT was significantly increased to 21.4 +/- 7.1% (p < 0.05 vs. 10-min ischemia and vs. the placebo and atenolol groups). IS was significantly reduced in swine receiving atenolol (3.9 +/- 3.5%) or UL-FS 49 (5.8 +/- 4.6%) as compared with the placebo-group (10.4 +/- 8.9%).(ABSTRACT TRUNCATED AT 400 WORDS)

Characterization of the inotropic and arrhythmogenic action of the sodium channel activator BDF 9148: a comparison to its S-enantiomer BDF 9196, to its congener DPI 201-106, to norepinephrine, and to ouabain.

Positive inotropic substances which enhance the myocardial cAMP level or inhibit the Na+/K(+)-ATPase are known for their proarrhythmic side-effects. This study was performed to investigate the inotropic and arrhythmogenic action of the Na(+)-channel activator BDF 9148 (racemate) in comparison to its S-enantiomer BDF 9196, its congener DPI 201-106 (racemate), to norepinephrine, and to ouabain. In 30 open-chest dogs, the effects of these substances on the first derivative of left ventricular pressure (dP/dt, Millar-tip catheter) and anterior systolic wall thickening (AWT, sonomicrometry) were studied. Concomitantly, myocardial excitability, conduction times, and refractory period were assessed with a transmural, three-dimensional, 16-electrode array in the anterior wall. For the study of the Na(+)-channel activators, alpha- and beta-adrenergic and muscarinic receptors were blocked. A first set of measurements was performed during normoperfusion with administration of BDF 9148 (1 mg/kg, n = 8), BDF 9196 (0.5 mg/kg, n = 8), and DPI 201-106 (1 mg/kg, n = 8), respectively. A second set of measurements was performed with administration of the threefold dosage of either substance. With a severe stenosis on the left anterior descending coronary artery, a final set of measurements was performed, again using the higher dosage of either substance. For the study of norepinephrine (0.5 micrograms/kg/min i.v., n = 6) and ouabain (40 micrograms/kg i.v., n = 4), measurements were performed during normoperfusion in additional animals. Under normal conditions, either Na(+)-channel activator induced increases in dP/dtmax (lower dosage: 45-84%, higher dosage: by 93-117%) and AWT (lower dosage: by 24-37%, higher dosage: by 19-56%). Under ischemic conditions, either drug increased dP/dtmax by 60-98% and AWT by 45-102%. Excitability, conduction times, and refractory period did not change significantly in response to the Na(+)-channel activators, neither under normal nor under ischemic conditions. There was no significant difference in the incidence of spontaneous ventricular extrasystoles before and after administration of either Na(+)-channel activator. In contrast, an equi-inotropic dosage of norepinephrine (increases in dP/dtmax by 148% and AWT by 42%) increased excitability, decreased conduction times and refractory period, and increased the incidence of spontaneous ventricular extrasystoles. Ouabain induced only a moderate increase in dP/dtmax by 56% and AWT by 24%, but elicited sustained and complex ventricular arrhythmias. Excitability was markedly increased, whereas conduction times and refractory period changed only little.(ABSTRACT TRUNCATED AT 400 WORDS)

Transcutaneous pCO2-monitoring for the evaluation of the anaerobic threshold. Comparison to lactate and ventilatory thresholds.

The monitoring of transcutaneous pCO2 (pCO2(tc) is an alternative to the invasive determination of the anaerobic threshold by analysis of arterial lactate concentration or to the uncomfortable determination of the ventilatory threshold. We compared the threshold determination by pCO2(tc)-monitoring to the 4 mmol/l lactate threshold and to the ventilatory threshold (point where the ventilatory equivalent of oxygen started to increase continuously) in 15 athletes during cycle exercise. The first distinct deflection point in the pCO2(tc) time course after the start of exercise was chosen to indicate the anaerobic threshold. The mean threshold determined by pCO2(tc) occurred at the same workload as the ventilatory threshold but at a lower workload than the lactate threshold. In spite of the good correspondence in the respective means there was a wide range of individual differences between the pCO2(tc) derived thresholds and both reference thresholds. Thus, looking at an individual, the continuous monitoring of the pCO2(tc) does not provide reliable data on the occurrence of the transition from aerobic to anaerobic metabolism.

CORDAT II: a new program for data acquisition and on-line calculation of hemodynamic and regional myocardial dimension parameters.

The present paper describes a new computer program for data acquisition and on-line evaluation of hemodynamic parameters. The hardware setup is based on a standard i486 personal computer equipped with an analog to digital converter and an additional display controller. During data acquisition the multitasking program automatically detects the beginning and the end of a cardiac cycle. Immediately after the recognition of a cardiac cycle the program calculates and displays user-defined hemodynamic parameters and mean values over a given time or a given number of beats. There is a high correlation between manually determined hemodynamic parameters and the parameters determined automatically by CORDAT II.

Heart rate variability and circulating catecholamine concentrations during steady state exercise in healthy volunteers.

OBJECTIVES: To assess whether exercise induced suppression of heart rate variability in the low frequency domain (0.06-0.15 Hz) is related to the increase in circulating catecholamine concentrations. DESIGN: Randomised crossover trial of three exercise tests characterised by different workloads. Pharmacological simulation of exercise-induced changes in vagal and sympathetic activity. PARTICIPANTS: Six healthy men with a mean age of 31.2 (SD 3.0) years. INTERVENTIONS: Three different workloads of steady state cycling ergometry: control state without cycling, cycling at a target heart rate of 100 beats/min, and cycling at a target heart rate of 150 beats/min. Intravenous infusion of atropine (target heart rate 100 beats/min) followed by the additional infusion of adrenaline and noradrenaline. MAIN OUTCOME MEASURES: Fast Fourier analysis of heart rate variability; blood pressure; and venous plasma concentrations of lactate, adrenaline, and noradrenaline. RESULTS: During the control exercise period there were no changes in the assessed variables compared with the preceding resting period. During exercise at a heart rate of 100 beats/min systolic blood pressure increased and heart rate variability decreased. During exercise at a heart rate of 150 beats/min systolic blood pressure and lactate, adrenaline, and noradrenaline concentrations increased. In addition, low frequency (LF) was lower than during exercise at 100 beats/min, high frequency (HF 0.15-0.80 Hz) resembled that during exercise at 100 beats/min, and diastolic blood pressure was reduced. Infusion of atropine caused no changes in blood pressure or plasma concentrations of lactate, adrenaline, and noradrenaline and decreased heart rate variability. The additional infusion of adrenaline and noradrenaline completely suppressed heart rate variability and increased blood pressure. CONCLUSIONS: The reduction in LF and HF during exercise at a heart rate of 100 beats/min, which is not characterised by increased plasma catecholamine concentrations, and during atropine infusion suggests that heart rate variability in the supine state is largely influenced by vagal activity. The additional reduction in LF during exercise at 150 beats/min and during catecholamine infusion may reflect a negative feedback of circulating catecholamines on the sympathetic control of heart rate.

[Poor reproducibility of parameters of heart rate variations]. / Schlechte Reproduzierbarkeit von Parametern der Herzfrequenzvariabilität.

The analysis of heart rate variability is supposed to be a marker of autonomic cardiac activity and is used for risk stratification of post-infarction patients. Analysis of heart rate variability in the frequency domain may permit a differentiation of vagal and sympathetic control; for such analyses only short time intervals characterized by a steady-state autonomic balance can be used. Yet, it is unclear whether single determinations of heart rate variability indices derived from short time intervals yield reproducible results. Therefore, the reproducibility of heart rate variability indices was studied with weekly measurements in 10 healthy volunteers under the following defined conditions: 13 min supine rest, 10 min standing, 13 min sitting, and 15 min cycle ergometry followed by a 14 min recovery period. Heart rate variability was determined in the frequency domain (fast Fourier transformation) and in the time domain. The reproducibility was estimated by the coefficient of variation (CV). Additionally, the reproducibility of heart rate, blood pressure, and the expiratory-inspiratory ratio of heart rate was determined. The reproducibility of the frequency domain indices (36.6-74.9% CV) and of the time domain indices (19.6-32.8% CV) was considerably worse than that of heart rate (5.2-8.2% CV), blood pressure (5.1-8.2% CV) and the expiratory-inspiratory ratio of heart rate (4.6% CV). The reproducibility of heart rate variability indices was not improved by orthostatic or ergometric challenge. This poor reproducibility does not permit a reliable interpretation of heart rate variability on the basis of single measurements in healthy volunteers. Given the wide range and scatter of the measured parameters, the diagnostic and prognostic value of heart rate variability indices derived from short recording periods appears questionable.

Three-dimensional analysis of regional mechanical function, blood flow and electrophysiological parameters during early myocardial ischemia in dogs.

Ventricular arrhythmias are primarily responsible for sudden cardiac death early after the onset of acute myocardial ischemia. We designed an experimental model to simultaneously characterize regional myocardial function, myocardial blood flow, and electrophysiological parameters, and to determine predisposing factors for the development of early ventricular arrhythmias (EVA). The left circumflex coronary artery was occluded in six anesthetized (n = 2 piritramide/N2O, n = 4 chloralose/urethane) mongrel dogs. Systolic wall thickening (%WT) in a control zone and in the central ischemic zone was measured with sonomicrometry and regional myocardial blood flow (RMBF) with colored microspheres. Excitability and relative refractory period at the stimulus electrode and conduction times to all other electrodes were determined with a three-dimensional transmural multi(16)-electrode assay using a computer algorithm. In three of six dogs spontaneous EVA occurred 4 to 6 min after coronary occlusion, degenerating to ventricular fibrillation in two of these dogs. The three dogs developing EVA were not distinguished from those not developing EVA, neither by the kind of anesthesia nor by ischemic % WT (-6.6 +/- 3.8 [SD] vs -7.8 +/- 1.6, ns). Also, dogs with and without EVA did not differ significantly in excitability and relative refractory period. In contrast, dogs with EVA were characterized by a greater mass of severely ischemic myocardium, i.e., exhibiting a RMBF reduction to less than 0.1 ml/(min.g) (18 +/- 3 g vs 7 +/- 4 g, p less than 0.05), and by an increase in subendocardial conduction times of greater than 100% above the respective pre-ischemic values (120 +/- 18% vs 66 +/- 9%, p less than 0.05). Dogs with and without EVA were not as clearly distinguished by the increases in subepicardial (81 +/- 22% vs 46 +/- 15%, ns) and transmural (98 +/- 31% vs 67 +/- 14%, ns) conduction times. The development of EVA is associated with a greater mass of severely ischemic myocardium and a greater increase in subendocardial conduction times.

[A two-stage regulatory system for pressure-constant perfusion of coronary vessels]. / Zweistufiges Regelsystem zur druckkonstanten Perfusion von Koronargefässen.

This paper describes a double-loop servo-controlled pump system for the constant-pressure perfusion of a coronary artery. Due to the transient nature of changes in coronary vasomotor tone, such a perfusion system must have a fast regulatory response. In the first stage, a servo-controlled pump primes a windkessel having a volume of 35 ml with blood. The pumping rate is electronically controlled to maintain a constant pressure within the windkessel max. 700 mmHg. The maximal flow rate is 300 ml/min. To reduce the high pressure in the windkessel to the desired coronary perfusion pressure, a variable flow resistance, comprising a clamped thin-walled silicone tube, is provided in the output line of the system. A fast servo-motor drives the clamp and is controlled by an electronic regulator, using a second feedback loop from the pressure signal measured at the tip of the perfusion cannula. The system stabilizes the coronary perfusion pressure within 300 ms. An additional modulation of the setpoint signal in synchrony with the cardiac cycle improves the phasic pattern of the blood flow, and thus prevents changes in transmural blood flow distribution. The dead volume of the overall system is about 60 ml. Hemolysis caused by this system during five hours of perfusion in vivo is negligible.