Thermodilution Cardiac Output Determinations Research Articles

Thermodilution and Fick Cardiac Outputs Differ: Impact on Pulmonary Hypertension Evaluation

The relationship between thermodilution and indirect Fick cardiac output determination methods has not been well described. To describe the relationship between these two cardiac output determination methods in patients evaluated for pulmonary hypertension and to highlight potential clinical implications. A retrospective review of charts of all adult patients who underwent a right heart catheterization (RHC) between January 1, 2007 and November 10, 2010, and participated in the pulmonary hypertension program of the pulmonary division at an academic institution was conducted. For validation, the charts of all patients who underwent RHC during the same period within the cardiology division were reviewed. A total of 198 patients underwent 213 RHCs, 79 (40%) of whom had pulmonary arterial hypertension, were included. Forty-three per cent of patients had >20% difference between thermodilution and Fick. The average difference (thermodilution - Fick ±SD) was -0.39±2.03 L⁄min (n=213; P=0.006). There was no significant difference in bias or variability between thermodilution and Fick among patients with tricuspid regurgitant jet velocity (TRJ) of <3 m⁄s versus those with TRJ >3 m⁄s (-0.41±2.10 L⁄min versus -0.36±1.93 L⁄min, respectively; P=0.87). In a multivariable analysis, the thermodilution-Fick difference increased with age (P=0.001). The presence of such discrepancy in 36% of patients evaluated for heart failure and⁄or heart transplant validated the results. In total, 37% of the 1315 procedures (213 performed by pulmonologists and 1102 performed by cardiologists) had a difference of >20% between thermodilution and Fick. Significant discrepancy exists between thermodilution and indirect Fick methods. This discrepancy potentially impacts pulmonary arterial hypertension prognostication and diagnosis, and is independent of TRJ.

Comparison of Fick and thermodilution cardiac output determinations in standing horses

The Fick and thermodilution (TD) methods are two currently popular techniques for determination of cardiac output (CO) in adult horses. To our knowledge, a comparison of these two techniques has not been reported. Six healthy, resting, fit, adult horses of either sex and weighing 516.5 ± 33.2 kg (mean ± SD) were instrumented to enable measurement of cardiac output. Resting CO was determined by the Fick method and by thermodilution while the horses stood quietly in the stocks. Fick and thermodilution CO measurements were repeated under conditions of increased cardiac output achieved with the use of a dobutamine infusion (5 μg kg −1 min −1, IV), and again under conditions of decreased CO induced by administration of xylazine (0.5 mg/kg, IV). Fick and thermodilution cardiac outputs were compared using Bland–Altman analysis for repeated measures. The mean of the differences ± 1.96SD (bias and precision) between the two techniques was 1.88 ± 24.17 L/min. Variability between measurements with the two techniques was decreased to 3.41 ± 46.78 mL kg −1 min −1 when CO was normalized for body size by calculation of cardiac index.

Accurate, Noninvasive Continuous Monitoring of Cardiac Output by Whole-Body Electrical Bioimpedance

Cardiac output (CO) is measured but sparingly due to limitations in its measurement technique (ie, right-heart catheterization). Yet, in recent years it has been suggested that CO may be of value in the diagnosis, risk stratification, and treatment titration of cardiac patients, especially those with congestive heart failure (CHF). We examine the use of a new noninvasive, continuous whole-body bioimpedance system (NICaS; NI Medical; Hod-Hasharon, Israel) for measuring CO. The aim of the present study was to test the validity of this noninvasive cardiac output system/monitor (NICO) in a cohort of cardiac patients. Prospective, double-blind comparison of the NICO and thermodilution CO determinations. We enrolled 122 patients in three different groups: during cardiac catheterization (n = 40); before, during, and after coronary bypass surgery (n = 51); and while being treated for acute congestive heart failure (CHF) exacerbation (n = 31). MEASUREMENTS AND INTERVENTION: In all patients, CO measurements were obtained by two independent blinded operators. CO was measured by both techniques three times, and an average was determined for each time point. CO was measured at one time point in patients undergoing coronary catheterization; before, during, and after bypass surgery in patients undergoing coronary bypass surgery; and before and during vasodilator treatment in patients treated for acute heart failure. Overall, 418 paired CO measurements were obtained. The overall correlation between the NICO cardiac index (CI) and the thermodilution CI was r = 0.886, with a small bias (0.0009 +/- 0.684 L) [mean +/- 2 SD], and this finding was consistent within each group of patients. Thermodilution readings were 15% higher than NICO when CI was < 1.5 L/min/m(2), and 5% lower than NICO when CI was > 3 L/min/m(2). The NICO has also accurately detected CI changes during coronary bypass operation and vasodilator administration for acute CHF. The results of the present study indicate that whole-body bioimpedance CO measurements obtained by the NICO are accurate in rapid, noninvasive measurement and the follow-up of CO in a wide range of cardiac clinical situations.

Improved accuracy and precision of thermodilution cardiac output measurement using a dual thermistor catheter system

OBJECTIVESTo assess whether thermodilution cardiac output determination based on measurement of injectate temperature in vivo leads to more accurate and precise estimates and to study the influence of chilled injectate on test performance.BACKGROUNDCardiac output measurement via right heart catheterization is used extensively for hemodynamic evaluation in a variety of diagnostic, perioperative and critical care settings. Maximizing accuracy is essential for optimal patient care.METHODSThis prospective study of 960 thermodilution cardiac output measurements was conducted using conventional and dual thermistor techniques. Specialized dual thermistor right heart catheters were constructed using a second thermistor positioned to measure injectate temperature in vivo just prior to entry into the right atrium. To eliminate interinjection variability, a custom set-up was developed that permitted output measurement using both techniques simultaneously. Both ambient temperature injections and cooled injections were investigated.RESULTSThe dual thermistor technique demonstrated significantly less measurement variability than the conventional technique for both ambient temperature (precision = 0.41 vs. 0.55 L/min, p < 0.001) and cooled (precision = 0.35 vs. 0.43 L/min, p = 0.01) injections. Similarly, the average range of cardiac output values obtained during five sequential injections in each patient was less using the dual thermistor approach (1.05 vs. 1.55 L/min, p < 0.001). The use of cooled injectate reduced the mean error of the dual thermistor technique but actually increased the mean error of the conventional technique. Even with ambient temperature injections, injectate warming during catheter transit varied considerably and unpredictably from injection to injection (2 SD range = −0.22 to 5.74°C). Conventional ambient temperature and cooled measurements significantly overestimated Fick cardiac output measurements by 0.32 and 0.50 L/min, respectively (p < 0.001). In contrast, dual thermistor measurements were statistically similar (−0.08 and −0.08 L/min, p = 0.34) to Fick measurements.CONCLUSIONSThis new dual thermistor approach results in a significant improvement in both precision and accuracy of thermodilution cardiac output measurement.

Iced temperature injectate for thermodilution cardiac output determination causes minimal effects on cardiodynamics.

Controversy exists regarding the ideal injectate temperature for measuring cardiac output. Iced temperature injectate gives a higher signal/noise ratio and less variability in the measured cardiac output. Thus, less volume and fewer measurements are required. Advocates of room temperature injectate have suggested that iced temperature injectate may perturb cardiodynamics. This concern has remained largely untested. To help resolve this controversy, we examined the effects of 5 mL iced injectate (0 degrees to 4 degrees) infusions on cardiodynamics. Prospective, randomized, controlled study. A critical care research laboratory. Five domestic pigs, weighing between 20 to 25 kg. Under barbiturate anesthesia, pigs underwent placement of a) a thermodilution catheter in the right internal jugular vein; b) a right carotid artery catheter for mean arterial pressure; and c) sonomicrometry crystals for dynamic measurements of left ventricular dimensions. Calculations were made of end-systolic and end-diastolic left ventricular volume and ejection fraction. Six cardiac output measurements were performed in each pig. Data were obtained at baseline (just before iced temperature injectate infusion) and every 3 sec for 9 secs. The only significant effect seen with iced temperature injectate infusion was a small, transient decrease in heart rate (-5.9 +/- 1.1 beats/min from a baseline heart rate of 144.8 +/- 20.6 beats/min). Indices of preload, contractile function, and dynamic cardiac geometry were unaffected. Iced temperature injectate used in clinically relevant volumes causes transient negative chronotropic effects, but reservations regarding other perturbations of cardiodynamics are unfounded. Thus, the use of iced temperature injectate for cardiac output determination is still a viable alternative to room temperature injectate use, especially when a larger signal/noise ratio is required.

Effect of Injectate Temperature and Thermistor Position on Reproducibility of Thermodilution Cardiac Output Determinations

The purpose of this study was to evaluate the effect of thermistor position with varying injectate temperatures on the reproducibility of thermodilution cardiac output determination. The key hypothesis to be tested was that the positioning of the proximal thermistor at the right atrial port would improve the reproducibility of thermodilution cardiac output determination, independent of injectate temperature. Prospective randomized trial. The study was performed in the cardiac catheterization laboratory of the West Haven Veterans Affairs Medical Center. Twenty consecutive patients undergoing right and left heart catheterizations were enrolled in the study. Each patient underwent triplicate determination of thermodilution cardiac output measurements under four experimental conditions: (1) ambient or room temperature injectate using an external thermistor in the injectate reservoir; (2) iced injectate using an external thermistor; (3) room temperature injectate using an internal right atrium (RA) thermistor; and (4) iced injectate using an RA thermistor. Reproducibility was assessed by the coefficient of variation (CV) and standard error of the mean percent (SEM%), of the triplicate measurements. Using an internal RA thermistor improved the reproducibility of cardiac output determinations independent of injectate temperature. Using room temperature injectate, the CV was 12.8 percent using an external thermistor and 7.9 percent using an internal RA thermistor (p < 0.05). Using iced injectate, the CV was 10.2 percent using an external thermistor and 5.5 percent using an internal RA thermistor (p < 0.05). Reproducibility of thermodilution cardiac output determinations is improved when injectate temperature is measured internally, at the RA, as opposed to externally in the reservoir. This has clinical implications for determining significant changes in serial cardiac output determinations.

Injectate port selection affects accuracy and reproducibility of cardiac output measurements with multiport thermodilution pulmonary artery catheters

To compare the accuracy and reproducibility of thermodilution cardiac output measurements obtained from the injectate and infusion ports of a multilumen pulmonary artery catheter. The thermodilution results were compared with an independent measure of flow obtained from an electromagnetic flow meter. In an experimental study conducted at an animal research laboratory of a health sciences university, two virgin western breed ewes were surgically instrumented with an inferior vena cava occluder, which reduced venous return and thus lowered cardiac output, and an ascending aortic electromagnetic flow probe, which provided an independent reference measure of cardiac output. On the day of study, a multilumen pulmonary artery catheter was inserted. Cardiac output was manipulated over a range of 2.9 to 12.1 L/min with i.v. isoproterenol or inferior vena cava occlusion. Approximately 30 simultaneous thermodilution and electromagnetic flow meter measurements of cardiac output were obtained from both the infusion and injectate ports in each of the two subjects. Correlation coefficients were lower and standard error of the estimates was higher for the infusion port (r = .83; SEE = 1.19 L/min) vs the injectate port (r = .94; SEE = .74 L/min), indicating reduced reproducibility with infusion port thermodilution cardiac output determinations. Accuracy was also adversely influenced in the infusion port results. The linear regression for the infusion port data intercepted the Y axis at +2.64 L/min, indicating significant overestimation of cardiac output at flows of less than 5 L/min and underestimation of flow when cardiac output exceeded 8 L/min. The use of the infusion port for the measurement of thermodilution cardiac output measurements may result in nonreproducible and inaccurate results.

Hemodynamic changes associated with thermodilution cardiac output determination in canine acute blood loss or endotoxemia

Since the technique of thermodilution (TD) cardiac output measurement, per se, causes hemodynamic alterations, the author examined whether the alterations elicited by iced injectate are augmented in the presence of acute blood loss or endotoxemia, compromised conditions frequently associated with critically ill patients. Acute blood loss (N = 8) and endotoxemia (N = 8) were induced by withdrawing arterial blood approximately 20-30 ml.kg-1 over 30 min and by a slow intravenous infusion of E. coli endotoxin 2.5-3.0 mg.kg-1 over 10 min, respectively, in anesthetized dogs. The magnitudes of decreases in mean arterial and pulmonary artery pressures during slowing of heart rate (HR) following injection of iced injectate 3 ml were slightly less in acute blood loss than in normovolemia, whereas in endotoxemia the degree of mean arterial pressure decrease during slowing of HR following iced injectate 3 ml was slightly less as compared with that before endotoxemia. However, the alterations in other hemodynamic variables following injection of iced injectate 3 ml were similar between dogs with and without acute blood loss or endotoxemia. No profound hemodynamic changes were observed during any TD cardiac output measurements under both conditions. Cardiac output estimated by TD correlated closely with pulmonary blood flow measured by electromagnetic flowmeter in endotoxemia (r > 0.9) but not during acute blood loss. These results indicate that TD cardiac output determination does not cause serious hemodynamic alterations in endotoxemia or acute blood loss, and can estimate right ventricular output accurately in endotoxemia but not in acute blood loss.

Room-temperature thermodilution cardiac output: proximal injectate lumen vs proximal infusion lumen

To assess the accuracy of room-temperature thermodilution cardiac output measurements from the venous infusion port. Central venous port cardiac output measurements were compared with venous infusion port measurements in 48 right-heart catheters. Three 10-mL injections of 5% dextrose in water were made through each port. The order of port injection was random. The cardiac outputs were 5.8 +/- 1.8 L/min from both ports, with no difference between ports (paired t test). Room-temperature thermodilution cardiac output determinations from the venous infusion port can be used in place of central venous port cardiac outputs if the central venous port becomes nonfunctional.

Measurement of cardiac output by automated single-breath technique, and comparison with thermodilution and Fick methods in patients with cardiac disease

Accurate noninvasive methods are needed for determination of cardiac output. Current methods are generally complex or may be unreliable. A previously described method, based on absorption of acetylene gas during a constant exhalation that enables calculation of cardiac output by estimating pulmonary capillary circulation, is incorporated in a new, automated commercial product (SensorMedics 2200). In this study, cardiac output by single-breath acetylene blood flow measured with this device was compared with the standard thermodilution and direct Fick methods in 20 patients undergoing cardiac or pulmonary artery catheterization. Patients inhaled test gas mixture to total lung capacity and exhaled at a constant rate through an adjustable resistor. Lung volumes and noninvasive acetylene blood flow value were calculated automatically. Correlation between the automated single-breath technique and both thermodilution and Pick cardiac output determinations was very high (correlation coefficients were 0.90 and 0.92, respectively), regression slopes were close to identity (0.98 and 0.90), and bias (−0.39 and −0.79 liter/min) and precision (0.94 and 1.02) were good; when shunt correction was applied, bias was reduced to 0.06 and 0.35 liter/min, respectively. Rapid, accurate, noninvasive measurement of cardiac output was easily obtained using the automated device. This technique may have a wide applicability for noninvasive evaluation of patients with cardiac disease and for monitoring effects of therapeutic interventions.

Continuous breathing circuit flow and tracheal tube cuff leak: sources of error during pediatric indirect calorimetry.

To determine whether continuous gas flow in the breathing circuit or an airleak around the tracheal tube cuff will introduce errors into the measurement of oxygen consumption (VO2) with indirect calorimetry. Nonrandomized, controlled trial. Experimental laboratory. Ten healthy, anesthetized mongrel dogs, weighing 8 to 12 kg. Data were recorded at seven levels of flow, from 0 to 12 L/min in excess of minute ventilation, through a continuous breathing circuit. Data were recorded at five levels of tracheal tube cuff leak from 0% to 40% of inspiratory minute volume. VO2 was measured using an indirect calorimeter with constant internal gas flow and calculated from results of blood gas analysis, cooximetry, and thermodilution cardiac output determinations at all levels of continuous breathing circuit flow and cuff leak. BP, heart rate, respiratory rate, arterial and mixed venous blood gases, and body temperature were measured to assess stability of cardiopulmonary function. Continuous breathing circuit flow did not affect the accuracy of indirect calorimetry until the total flow reached a critical value (11.5 L/min) that was slightly below the internal flow constant of the metabolic monitor (12.4 L/min). At higher circuit flows, measured VO2 decreased in a linear fashion, while calculated VO2 remained unchanged. Above the critical flow, the error of indirect calorimetry correlated significantly only with the total circuit flow (r2 = .64), not with the exhaled concentration of CO2 (r2 = .005) or the inspiratory-expiratory oxygen difference (r2 = .004). The continuous flow rate at the critical circuit flow was 66 +/- 15% of the subjects' peak inspiratory flow. Increasing tracheal tube cuff leak produced a progressive decrease in measured VO2 but not in calculated VO2. The difference between measured and calculated VO2 was linearly related to the magnitude of the leak (r2 = .56), and was statistically significantly larger at all levels of cuff leak, when compared with measurements during complete cuff seal. An indirect calorimeter in which measurement of VO2 is based on internal constant flow rather than spirometry can be used to accurately measure VO2 from a continuous-flow breathing circuit, if the total circuit flow is less than the internal flow. This limitation may restrict the use of continuous flow to a level below the subject's peak inspiratory flow. The accuracy of indirect calorimetry cannot be guaranteed for any amount of tracheal tube cuff leak.

Hemodynamic Status Susceptible to Slowing of Heart Rate During Thermodilution Cardiac Output Determination in Anesthetized Patients

Survey of Anesthesiology is an authoritative source of information on advances in the science and practice of anesthesiology as reported throughout the world literature. Every bimonthly issue contains 50 or more condensed articles selected by the journal’s distinguished Editorial Board from more than 100 international periodicals. Each condensed article carries a critique by a reviewing editor to provide an expert perspective on the material presented.

Hemodynamic Status Susceptible to Slowing of Heart Rate During Thermodilution Cardiac Output Determination in Anesthetized Patients

Department of Anesthesiology, Institute of Clinical Medicine, University of Tsukuba, Tsukuba City, Japan

Continuous thermodilution cardiac output measurement in intensive care unit patients

A new continuous thermodilution cardiac output measurement technique and companion flow-directed pulmonary artery catheter were evaluated in intensive care unit (ICU) patients. Continuous cardiac output was monitored for 6 hours in each patient, and, at selected intervals, a series of bolus thermodilution cardiac output determinations was made and averaged for comparison. A total of 222 data pairs was obtained in 54 patients. The cardiac outputs ranged from 2.8 to 10.8 L/min. The linear regression is represented by the following equation: continuous thermodilution = 0.99 bolus thermodilution + 0.02. The correlation coefficient r was 0.94, the S yx was 0.54. The mean relative error was 0.3%, and the standard deviation of the relative error was 11.5%. The absolute measurement bias was 0.02 L, and the 95% confidence limits were 1.07 and −1.03 L. The results demonstrated that the new continuous thermodilution cardiac output measurement technique provided acceptable accuracy and was considerably easier to use in the clinical situations studied in the ICU.

Hemodynamic status susceptible to slowing of heart rate during thermodilution cardiac output determination in anesthetized patients

To identify hemodynamic variables that might indicate susceptibility to slowing of heart rate (HR) and to mean arterial pressure (MAP) decrease caused by cold injectate during thermodilution (TD) cardiac output determination, we measured hemodynamic variables in 32 anesthetized patients undergoing major surgery. A total of 608 cardiac output determinations were evaluated for any relationship between the magnitude of HR decreases and basal hemodynamic variables. The magnitude of HR decrease was pronounced in low cardiac index (CI), low mean pulmonary artery pressure (MPAP), and high systemic vascular resistance index (SVRI). Although there was some decrease in MAP associated with cold injectate, neither changes in SVRI nor those in the other hemodynamic variables correlated with the decreases in MAP. These results suggest that decreased CI and MPAP associated with elevated SVRI may be more susceptible to slowing of HR during cardiac output determination by TD in anesthetized patients.

A comparison of thermodilution and pulsed Doppler cardiac output measurement in critically ill children

To evaluate the pulsed Doppler cardiac output method as a noninvasive means for determining cardiac output in critically ill children, we performed paired pulsed Doppler and thermodilution cardiac output determinations in 17 critically ill children. Commercially available equipment, specifically designed for this purpose, was employed. Forty paired thermodilution and pulsed Doppler determinations were made. There was a significant correlation between the two measurements (pulsed Doppler = 0.84 thermodilution + 0.39; r = 0.79, p less than 0.01). The ranges of thermodilution measurements (1.02 to 6.26 L/min; median 2.77 L/min) and pulsed Doppler measurements (1.13 to 6.35 L/min; median 2.57 L/min) were not different (p = 0.25). However, differences between individual paired thermodilution and pulsed Doppler measurements were large (-3.13 to 2.03 L/min; median 0.12 L/min), and the percentage difference between individual paired thermodilution and pulsed Doppler measurements ranged from 0.41% to 102.5% (median 12.7%). A discrepancy of 15% or more between thermodilution and pulsed Doppler was encountered in 18 (45%) of 40 of paired measurements (95% confidence interval: 29% to 61%), and one fourth of the paired measurements differed by more than 25%. We conclude that, as employed in this study, pulsed Doppler cardiac output determination is not sufficiently representative of the thermodilution output to be employed for hemodynamic monitoring in critically ill children.

A new closed-system using partially frozen injectate for thermodilution cardiac output determinations.

The FI (partially frozen injectate) system, a new closed-system devised by the authors for thermodilution cardiac output determinations, has two major features: 1) it needs no ice-filled receptacle to keep injectate cold because it uses partially frozen injectate, and 2) it can go without monitoring the injectate temperatures during the whole process of cardiac output determinations. The author evaluated the accuracy and reproducibility of cardiac output determinations with the FI system in 10 critically ill patients, as compared with another closed-system (which is commercially available) and the standard open method. The injectate temperatures in the FI system were also measured in vitro. The mean injectate temperature in the FI system was 0.71 +/- 0.26 degrees C and 80% of the injectate temperatures were lower than 1.0 degrees C. Even when no monitoring of injectate temperatures was made, the predicated error in the calculated cardiac output resulted as low as 2% with the FI system. The mean cardiac output values were not statistically different between the FI system and the other two systems.

Calculating cardiac output from transmitral volume flow using Doppler and M-Mode echocardiography

To simplify transmitral volume flow determination by Doppler echocardiography, a formula for calculating mean mitral valve orifice area using M-mode echocardiography without any 2-dimensional measurements was developed and evaluated in this study. The maximal mitral orifice area was assumed to be circular and its diameter was calculated from the maximal M-mode mitral leaflet separation. The maximal area was multiplied by the mean to maximal anterior mitral leaflet excursion ratio to correct for phasic changes in flow orifice area during ventricular filling. This measurement had a high correlation (r = 0.97, standard error of the estimate + 0.26 cm2) with mean mitral valve orifice area calculated from frame-by-frame analysis of short-axis 2-dimensional echoes in a select group of 10 normal volunteers and 10 patients with cardiomyopathy who had very high quality images of the mitral valve leaflet tips. Cardiac output calculated using the new method for orifice area estimation combined with apex view mitral valve Doppler velocities was then validated in 48 consecutive patients undergoing thermodilution cardiac output determinations with a close correlation between Doppler and thermodilution cardiac output (2.3 to 6.1 liter/min, r = 0.93, standard error of the estimate = 362 ml). The correlation improved when 12 patients with mild mitral insufficiency were excluded (r = 0.95). The M-mode echocardiogram-derived mitral valve orifice method combined with Doppler mitral valve velocities is accurate, easy to perform, has a high success rate and should increase the applicability of Doppler echocardiography for estimation of cardiac output.

Thermodilution cardiac output measurements during conventional and high-frequency ventilation

The thermodilution method for cardiac output determinations correlates well with Fick and dye dilution methods. Experimental work with thermodilution techniques has shown that individual measurements of right heart cardiac output during conventional ventilation vary throughout the respiratory cycle. The aims of this study were to compare thermodilution cardiac output determinations made at a fixed point (zero end-expiratory pressure [ZEEP]) with those made randomly throughout the respiratory cycle during conventional controlled positive pressure ventilation (CPPV) and high-frequency jet ventilation (HFJV) with up to 10cm H 2O positive end-expiratory pressure (PEEP). There were no statistically significant differences between the cardiac output determinations made at ZEEP and randomly in the ventilation cycle in any group and all correlations were significant. The clinical implications of these results are discussed, and it is concluded that it is not necessary to time the measurements of thermodilution cardiac output determinations during CPPV or HFJV with up to 10 cm H 2O of PEEP.

Mechanism for slowing of heart rate and associated changes in pulmonary circulation elicited by cold injectate during thermodilution cardiac output determination in dogs.

The authors investigated the mechanism for slowing of heart rate (HR) and associated changes in the pulmonary hemodynamics caused by cold injectate during thermodilution method. To clarify whether the slowing of HR after cold injectate is due to the cooling of the sinoatrial (SA) node or a reflex mediated by the autonomic nerves, we directly measured the myocardial temperature of the SA node region by a thermistor probe, and evaluated the HR responses to iced injectate after autonomic blockade in anesthetized dogs. Additionally, pulmonary blood flow (PBF) was continuously measured by an electromagnetic flowmeter during the delivery of injectate. The direction and magnitude of changes in HR after injectate were significantly dependent upon the injectate temperature (P less than 0.01). Thereby, the changes in HR correlated well with those in myocardial temperature of the SA node region (r = 0.987, P less than 0.01). However, the HR responses to injectate were unaffected by cervical vagotomy or stellate ganglionectomy. A significant decrease in PBF was noted in most cases during the slowing of HR. It was concluded that the slowing of HR after cold injectate during thermodilution in dogs is primarily due to the direct cooling of the SA node.