Thermal-Dilution Technics

Thermodiluted relative tidal volume estimation using a thermal camera in operating room under spinal anesthesia

Background

Estimating relative tidal volume is an important factor when monitoring breathing status. The relationship between temperature and respiration volume has rarely been studied. In this paper, a formula was derived for calculating thermodiluted respiration volume from temperature changes in the nasal cavity. To evaluate the proposed formula, the study compared the relative tidal volume estimated by the proposed formula with that recorded by a respiration volume monitor (Exspiron1Xi, RVM). Thermal data were obtained for 8 cases at a rate of 10 measurements per second. Simultaneous recordings by the RVM are regarded as the reference.

Results

The mean of ICC coefficient is 0.948 ± 0.030, RMSE is 0.1026 ± 0.0284, R-squared value is 0.8962 ± 0.065 and linear regression coefficient \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\mathrm{\alpha }$$\end{document}α is 0.966 ± 0.104, \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\upbeta$$\end{document}β is 0.042 ± 0.057. Bland–Altman plot showed 96.01% of samples that the difference between the measured and estimated values exists within 2 standard deviations.

Conclusions

In this paper, a model that can thermodynamically calculate the relationship between thermal energy and respiration volume is proposed. The thermodiluted model is a feasible method for estimating relative respiration tidal volumes.

Comparison of cardiac output measured by use of computed tomography and thermodilution in dogs

OBJECTIVE To compare cardiac output (CO) measured by use of CT coronary angiography and thermodilution (criterion-referenced standard) at various CO values, record adverse effects, and determine the time needed to measure CO. ANIMALS 5 healthy purpose-bred Beagles (2 males and 3 females). PROCEDURES A prospective nonrandomized crossover study was conducted. Dogs were premedicated with butorphanol tartrate (0.2 mg•kg^-1, IM). Anesthesia was induced by IV administration of etomidate (1 to 2 mg•kg^-1) and midazolam (0.25 mg•kg^-1). Orotracheal intubation was performed, and anesthesia was maintained by administration of isoflurane. The CO was determined by use of thermodilution and by use of CT at 3 CO values. Dobutamine was infused at various rates to obtain the 3 CO values. RESULTS 13 values were obtained and analyzed. The mean ± SD difference between methods was 0.09 ± 0.71 L•min^-1 (95% confidence interval [CI], 0.52 to -0.34 L•min^-1). Only 1 of 13 values was located on the 100% agreement line (ie, 0 line), 7 of 13 values were located within the 95% CI, and 5 of 13 values were outside the 95% CI. CONCLUSIONS AND CLINICAL RELEVANCE For this study, there was poor agreement between the 2 methods. The 95% CI interval was 0.52 to -0.34 L•min^-1, and 5 of 13 values were outside the 95% CI. Therefore, results for the CT method appeared to be inappropriate for use in making clinical decisions.

Assessment of cardiac output with transpulmonary thermodilution during exercise in humans

The accuracy and reproducibility of transpulmonary thermodilution (TPTd) to assess cardiac output (Q̇) in exercising men was determined using indocyanine green (ICG) dilution as a reference method. TPTd has been utilized for the assessment of Q̇ and preload indexes of global end-diastolic volume and intrathoracic blood volume, as well as extravascular lung water (EVLW) in resting humans. It remains unknown if this technique is also accurate and reproducible during exercise. Sixteen healthy men underwent catheterization of the right femoral vein (for iced saline injection), an antecubital vein (ICG injection), and femoral artery (thermistor) to determine their Q̇ by TPTd and ICG concentration during incremental one- and two-legged pedaling on a cycle ergometer and combined arm cranking with leg pedaling to exhaustion. There was a close relationship between TPTd-Q̇ and ICG-Q̇ ( r = 0.95, n = 151, standard error of the estimate: 1.452 l/min, P < 0.001; mean difference of 0.06 l/min; limits of agreement −2.98 to 2.86 l/min), and TPTd-Q̇ and ICG-Q̇ increased linearly with oxygen uptake with similar intercepts and slopes. Both methods had mean coefficients of variation close to 5% for Q̇, global end-diastolic volume, and intrathoracic blood volume. The mean coefficient of variation of EVLW, assessed with both indicators (ICG and thermal) was 17% and was sensitive enough to detect a reduction in EVLW of 107 ml when changing from resting supine to upright exercise. In summary, TPTd with bolus injection into the femoral vein is an accurate and reproducible method to assess Q̇ during exercise in humans.

Effects of extravascular lung water on the measurement of transpulmonary thermodilution cardiac output in acute respiratory distress syndrome patients

OBJECTIVE

Transpulmonary thermodilution cardiac output is used in calculating aortic impedance for calibrating the pulse-contour analysis and is applied to calculate extravascular lung water (EVLW). Whether pulmonary edema affects the accuracy of transpulmonary thermodilution is controversial. This study aimed to investigate the effects of extravascular lung water index (EVLWI) on the transpulmonary thermodilution measurement in acute respiratory distress syndrome (ARDS).

DESIGN

A prospective study.

SETTING

The medical intensive care unit of one medical center.

PARTICIPANTS

Twenty-four ARDS patients.

INTERVENTIONS

The continuous pulmonary artery thermodilution cardiac index (CCIpa) and the bolus transpulmonary thermodilution cardiac index (BCItp) data were recorded at baseline and repeated immediately and at 2, 4, and 6 hours after volume expansion with a 500-mL infusion of 10% pentastarch (hydroxyethyl starch 200/0.5) at a rate of 10 mL/kg/h.

MEASUREMENTS AND MAIN RESULTS

A total of 120 paired CI measurements were analyzed. Linear regression analysis showed a close correlation between BCItp and CCIpa (R = 0.87). The mean BCItp was higher than CCIpa, and the Bland-Altman analysis revealed a bias of 0.51 ± 0.78 L/min/m(2). The limits of agreement (2 standard deviations) was 1.66 L/min/m(2) (+2.07 and -1.05 L/min/m(2)), and the percentage error was 31.5%. Levels of EVLWI negatively correlated with the difference between BCItp and CCIpa (R = -0.19).

CONCLUSION

In ARDS patients, the agreement between transpulmonary thermodilution and pulmonary artery thermodilution for cardiac output measurement is marginally acceptable. The severity of pulmonary edema expressed as EVLWI weakly and negatively correlates with the difference between BCItp and CCIpa derived from the two techniques.

Cardiac output monitoring using indicator-dilution techniques: basics, limits, and perspectives

The ability to monitor cardiac output is one of the important cornerstones of hemodynamic assessment for managing critically ill patients at increased risk for developing cardiac complications, and in particular in patients with preexisting cardiovascular comorbidities. For >30 years, single-bolus thermodilution measurement through a pulmonary artery catheter for assessment of cardiac output has been widely accepted as the "clinical standard" for advanced hemodynamic monitoring. In this article, we review this clinical standard, along with current alternatives also based on the indicator-dilution technique, such as the transcardiopulmonary thermodilution and lithium dilution techniques. In this review, not only the underlying technical principles and the unique features but also the limitations of each application of indicator dilution are outlined.

Precision of bolus thermodilution cardiac output measurements in patients with atrial fibrillation

BACKGROUND

The precision of bolus thermodilution cardiac output measurements in patients with atrial fibrillation (AF) has not previously been determined. A priori we suspected that the precision would be lower in patients with AF than in patients with sinus rhythm (SR). Consequently, we also determined if the precision could be improved by injecting the thermal indicator into the right ventricle instead of the right atrium.

METHODS

Cardiac output was determined as the average result of four injections of 10 ml of iced saline. Replicate measurements were performed with thermal indicator injections into the right atrium and ventricle. The coefficients of variation and the precisions were calculated.

RESULTS

In the 25 patients with AF, mean cardiac output was 3.96 l min(-1) (range 2.4-7.4), the coefficient of variation 0.073 (95% CI +/- 0.011), and the precision 0.38 l min(-1) (95% CI +/- 0.14) with injection into the right atrium. In the 25 patients with SR, mean cardiac output was 4.73 l min(-1) (range 2.4-7.3), the coefficient of variation 0.047(95% CI +/- 0.006), and the precision 0.38 l min(-1) (95% CI +/- 0.14). In both groups, an agreement analysis demonstrated that the injection of indicator into the right ventricle resulted in a significantly higher cardiac output [AF+0.25 (95% CI +/- 0.15) l min(-1), SR+0.29 ( +/- 0.20) l min(-1)].

CONCLUSION

The coefficient of variation for cardiac output determinations is 55% higher in patients with AF. Two measurements, separated by time or intervention, must differ by 15% in AF patients and 9% in SR patients before one can be 95% confident that a real change has taken place.

Arterial thermodilution: an alternative to pulmonary artery catheter for cardiac output assessment in burn patients

STUDY OBJECTIVE

To study the agreement between cardiac output measurements with the pulmonary artery catheter and with the transpulmonary thermodilution technique in patients with burns.

DESIGN

Prospective, clinical study.

PATIENTS

23 patients with serious burns and an abbreviated burn severity index score (ABSI)>6.

SETTING

intensive care unit for severely burned in a burn center in Germany.

RESULTS

A total number of 218 cardiac output measurements obtained during the first 72 h postburn were analysed. In the pulmonary artery group, mean cardiac index was 3.93 l/min/m2 and ranged from 0.96 to 9.58. In the transpulmonary group the cardiac index measurements ranged from 0.96 to 9.61 with a mean of 4.0 l/min/m2. During the entire observation period cardiac index was consistently higher in the transpulmonary group than in the pulmonary artery group with a bias of 0.32 l/min/m2 and a standard deviation (S.D.) of 0.29 l/min/m2. Linear regression analysis revealed CI(arterial)=0.98xCI(pulm)+0.22l/min/m2 (r=0.9678, P<0.038). Bias and precision to each time point according to Bland and Altman demonstrated a good agreement between both techniques.

CONCLUSION

The transpulmonary thermodilution offers an attractive, less invasive alternative to the pulmonary artery catheter in patients with burns. Arterial thermodilution for CO measurements is as precise as PA thermal dilution, and CO(pulm) can be replaced by CI(arterial) when basic methodological principles are respected.

Errors in the measurement of cardiac output by thermodilution

Cardiac output (CO) determination by thermodilution, which was introduced by Fegler in 1954, has gained wide acceptance in clinical medicine and animal experiments because it has several advantages over other methods with respect to simplicity, accuracy, reproducibility, repeated measurements at short intervals, and because there is no need for blood withdrawal. However, errors in determination of CO by thermodilution may be introduced by technical factors and the patients' pathological conditions. The current review summarizes these issues and provides our recommendations, based on the medical literature published between 1954-1992. To obtain more reproducible and accurate CO values by thermodilution, one should make several determinations (1) by using 10 ml injectate at room temperature for adults and 0.15 ml.kg-1 injectate for infants and children; (2) at evenly spaced intervals of the ventilation cycle; (3) when rapid intravenous fluid administration is discontinued; (4) by observing thermodilution curves so that baseline pulmonary artery temperature drift or the existence of intra- and extracardiac shunts are noticed. Finally, CO determination by thermodilution may be unreliable or impossible in patients with low CO states and tricuspid or pulmonary regurgitation. Since non-invasive CO monitoring has not replaced CO determination by thermodilution, intimate knowledge of this method is crucial for anaesthetists to prevent errors in the management of patients.

The effects of valvular regurgitation on thermodilution ejection fraction measurements

Through the use of thermodilution principles and rapid response thermistors, it is now possible to measure right ventricular ejection fractions serially in patients. However, to our knowledge, the extent to which tricuspid regurgitation affects the accuracy of thermodilution ejection fraction measurements has not been quantified. The purpose of this study was to compare actual and thermodilution ejection fraction measurements in an in vitro model of tricuspid regurgitation over a wide range of ejection fractions. Stepwise perforation of the inlet valve resulted in regurgitant fractions ranging from 4 to 40 percent. At each increment of inlet valve regurgitation, triplicate sets of thermodilution (EFthermo) ejection fraction measurements were obtained and compared with actual ejection fractions (EFactual). The mean difference between EFactual and EFthermo significantly increased with 8 percent regurgitation and significantly increased with greater increments of inlet valve regurgitation. EFthermo consistently underestimated EFactual over the entire range of regurgitant values. Linear regression analysis revealed a significant correlation between EFactual and EFthermo for all degrees of regurgitation; however, the correlation coefficient significantly declined from control valves with 13 percent regurgitation and declined further with 33 percent regurgitation. Qualitative classification of the inlet valve regurgitation into mild, moderate, and severe regurgitation was performed using pulsed Doppler echocardiography. Mild inlet valve regurgitation resulted in a significantly increased difference between EFactual and EFthermo from control values. A significant increase in the difference between EFactual and EFthermo was observed with both moderate and severe regurgitation. In summary, thermodilution underestimated actual ejection fraction in a direct linear relationship to the degree of inlet valve regurgitation. Thus, in the presence of tricuspid regurgitation, this method may still be useful in serially measuring changes in right ventricular ejection fraction.

Validation of the thermodilution technique for the estimation of the cardiac output in the unsedated calf

The purpose of this study was to establish a standardized protocol in order to obtain accurate and reproducible cardiac output (CO) measurements by the thermodilution technique in the unsedated calf. In 5 healthy calves, the effect of various temperatures, volumes, rates, means and sites of injection of the thermal indicator were tested for their effect on accuracy and reproducibility of CO measurement by the thermodilution technique, the Fick method being used as the standard technique. Five milliliters of an iced 5% dextrose solution per 100 kg body weight constituted the correct amount of thermal indicator. Manual injection of the indicator bolus and/or injection of this bolus through the proximal lumen of a thermodilution Swan-Ganz catheter gave poor results of CO measurements by the thermodilution technique. On the other hand, results of CO measurements were highly reliable when the bolus injection was performed by an ECG-synchronized injection pump and/or through a right atrial catheter with a larger diameter and a shorter length than the classical Swan-Ganz catheter. It was concluded that an iced 5% solution of dextrose (5 ml/100 kg body weight) injected through a right atrial catheter by an ECG-synchronized pump gives the most accurate and reproducible CO measurements by the thermodilution technique in unsedated calves.

Cardiovascular interactions between vasopressin, angiotensin and noradrenaline in the Brattleboro rat

1. The cardiovascular effects of bolus intravenous injections of vasopressin, angiotensin II and noradrenaline were studied in 6-hydroxydopamine pretreated, anaesthetized Brattleboro rats with hereditary diabetes insipidus and normal rats of the parent Long Evans strain. 2. Pretreatment with 6-hydroxydopamine did not significantly affect control values for mean arterial blood pressure, cardiac output or total peripheral resistance in either Brattleboro or Long Evans rats but the pressor response to haemorrhage was reduced in both strains compared to the control animals. 3. The pressor responses of the untreated Brattleboro rats to 250 mu kg-1 vasopressin were significantly greater and more prolonged than in control rats of the Long Evans strain. 4. Pretreatment with 6-hydroxydopamine significantly enhanced the peak pressor response to vasopressin, but not to angiotensin II (1 microgram kg-1), in Brattleboro and Long Evans rats. 5. Pretreatment with 6-hydroxydopamine resulted in an enhanced pressor response to 1 microgram kg-1 noradrenaline in both Brattleboro and Long Evans rats, but the effect was significantly greater in the vasopressin-deficient animals. 6. These results indicate differences in the pressor responsiveness of Brattleboro rats to vasopressin and noradrenaline, but not to angiotensin II, compared with control Long Evans rats and provide evidence for important interactions between the sympathetic nervous system and these pressor hormones.

Haemodynamic responses to stimulation of the splanchnic and cardiac sympathetic nerves in the anaesthetized cat

The changes in cardiac output and mean right atrial pressure (R.A.P.) evoked at different circulating blood volumes by stimulation of the splanchnic sympathetic nerves were investigated in adrenalectomized cats under chloralose anaesthesia, with unopened chests and spontaneous respiration and with active vascular reflexes. The cardiac autonomic nerves were cut or blocked pharmacologically. Stimulation of the distal ends of the splanchnic nerves at 4 Hz caused aortic pressure and R.A.P. to rise to maximum values at 2 min before declining slowly. Cardiac output rose more slowly to a steady state at 3 min; at higher circulating volumes it fell initially. Although the output increments were slower in development they were better sustained than those in total peripheral resistance. The proportionate output increments were largest and the R.A.P. increments least at low circulating volumes whereas at high volumes the R.A.P. increments were large but the output changes were small or negative; the pattern of changes resembled that resulting from infusion of blood. Stimulation of the cardiac sympathetic nerves evoked a rise in output and a fall in R.A.P. related in magnitude to the initial value of R.A.P. On simultaneous stimulation of the splanchnic and cardiac sympathetic nerves the changes in output combined whereas the R.A.P. changes cancelled, to give output increments of 25-50% with little change in R.A.P. at all circulating volumes. At high circulating volumes infusion of blood did not usually alter output or aortic pressure, but splanchnic nerve stimulation increased peripheral resistance and aortic pressure and commonly evoked a rise in left ventricular stroke work which could not be accounted for by known adrenergic mechanisms or by elevation of left ventricular end-diastolic pressure. Portal venous pressure was consistently elevated by splanchnic nerve stimulation; it rose more slowly than did aortic pressure or R.A.P. and was independent of a changing central venous pressure provided this did not exceed +5 mmHg. The cardiac output increments were not related to changes in the ratio between the input and output resistances of the portal vein and it is concluded that displacement blood from the peripheral to the central vasculature was induced by contraction capacitance vessels.

The cardiovascular effects of vasopressin after haemorrhage in anaesthetized rats

The cardiovascular effects of an acute haemorrhage (2% of the body weight) were studied over a 60 min period in three groups of rats: (a) Brattleboro rats with hereditary hypothalamic diabetes insipidus (b.d.i.) lacking circulating vasopressin, (b) control rats of the parent Long Evans (l.e.) strain, and (c) l.e. rats treated with an antagonist of the vascular action of vasopressin. Prior to the haemorrhage there were no significant differences between the three groups of rats with respect to mean arterial blood pressure, cardiac output, stroke volume or total peripheral resistance. Following the haemorrhage cardiac output and stroke volume were severely reduced in all three groups of rats. Total peripheral resistance was relatively unaffected in antagonist-treated l.e. rats and b.d.i. rats, but rose substantially in response to the loss of blood in the control l.e. group. Both total peripheral resistance and mean arterial blood pressure were markedly greater in the untreated l.e. control rats than in the other two groups of animals during the first 20 min after haemorrhage. The mean heart rate measured in Brattleboro rats was elevated compared with that of control l.e. rats throughout the experiment and, in addition, significantly greater than that of antagonist-treated l.e. rats during the first 40 min after the haemorrhage. Survival rate for the b.d.i. rats following the 2% haemorrhage was lower than that for l.e. control rats and antagonist-treated l.e. rats. The results indicate that the recovery of the blood pressure following an acute arterial haemorrhage is significantly influenced by vasopressin, particularly during the first 20 min, and that the predominant effect of the hormone is to increase the total peripheral resistance. The higher mortality associated with volume depletion in the b.d.i. rats is unlikely to be directly related to the absence of the vascular action of vasopressin, since administration of the vasopressin antagonist to normal l.e. rats does not reduce their survival rate.

Cardiac output measurements. A review of current techniques and research

Cardiac output is the volume of blood ejected by the heart per unit time. It is a useful measurement in that it can be used to evaluate overall cardiac status in both critically ill patients and patients with suspected cardiovascular disease. An ideal cardiac output measurement system would have automated continuous output capability, be minimally invasive, accurate, fast, small, low cost and clinically adaptable. This paper presents a theoretical and practical description of the variety of clinical techniques in use today and lists their advantages and shortcomings with respect to the ideal system. Included are the Fick method, indicator dilution techniques, velocity measurements and transthoracic impedance and combined Doppler ultrasound as noninvasive techniques. In addition, several experimental methods are described along with their desirable features and possible constraints. These include intravascular heating/recording, thermistor tracking of cardiac output, ejection fraction measurements and magnetic susceptibility plethysmography.

Measurement of flow in large veins by the continuous thermodilution method

Simple methods for measuring percutaneously blood flow in veins characterized by a large oscillatory component of flow are lacking. Therefore, a thermodilution technique with a constant infusion rate was used for the measurement of inferior vena cava flow in anesthetized dogs. The accuracy of the method was studied both in an artificial circuit and in in vivo experiments. The thermal catheter was introduced upstream in a flow of water ranging from 50 to 1,200 ml/min produced in an artificial circuit maintained at 37 degrees C. With a volume of cold injectate of 43 ml/20s, the correlation between the values obtained by direct measurement and by thermodilution technique was highly significant (r = 0.993; n = 37) with a slope practically equal to 1.0 (Y = 1.03 X + 2.42). With the thermal catheter introduced through the jugular vein and an electromagnetic flowmeter probe placed around the exposed vessel, volumetric flows were registered in the subhepatic vena cava, infrarenal vena cava, as well as renal and iliac veins of eight dogs. Comparison of the values obtained by the two techniques yielded a regression equation of Y = 0.96 X + 34.31 with a correlation coefficient r = 0.943 (range 50-1,000 ml/min; n = 40). The continuous injection method was as accurate at low flows as at high. Qualitatively, thermodilution curves were comparable to electromagnetic flowmeter curves, reproducing instantaneously the cyclic respiratory variations. The method is thus particularly suitable for use in those veins in which there is a large oscillatory component of flow.

The effects of dobutamine, nitroprusside, or volume expansion on cardiac output and lung water after CPPV

Pulmonary microemboli can create an ARDS-like state in dogs (high pulmonary vascular resistance, pulmonary oedema and arterial hypoxemia). CPPV can correct the hypoxemia of pulmonary microemboli but reduces cardiac output (Q) and tissue oxygenation. This paper compares the effect of improving Q by infusing volume, reducing afterload, or increasing myocardial contractility. Four groups of seven dogs were studied. All had 0.125 g . kg-1 of starch microemboli (63-74 microns) infused and then CPPV at 15 cm H2O applied. The control group had no further treatment applied. In three other groups volume (dextran) or dobutamine or nitroprusside (NTP) was infused to return Q to the level before CPPV was applied. All treatments (volume, dobutamine and NTP) improved Q and O2 transport. Only the volume group had a significant increase in pulmonary microvascular pressure, Pmv = PLA + 0.4 (PPA - PLA) from 2.53 +/- 0.27 to 3.35 +/- 0.13 kPa, p less than 0.05. Only the volume group demonstrated a significant increase in lung water above (double) the control group as measured by a double indicator dilution technique (ETVL) and post mortem lung weights. We conclude volume infusions to improve a CPPV depressed Q may increase lung water and that better treatment would be to infuse NTP or dobutamine, thus maintaining a lower Pmv and therefore lung water. As a corollary the least CPPV should be applied to maintain adequate oxygenation and create the least need for interventions to improve Q.

Method of assessing the reproducibility of blood flow measurement: factors influencing the performance of thermodilution cardiac output computers

Measurements of blood flow by three different makes of thermodilution cardiac output computer in an artificial circulation were analysed by linear regression against absolute flow measured by timed blood volume collection. For each computer the horizontal distance between the 95% confidence limits for a single prediction was calculated at a standard flow rate of 5 litres per minute. This measurement represents the range of flow rates that could give rise to an identical measurement and provides a summary of the reproducibility of the computer's results and its ability to detect a change of flow rate. This measurement was used to evaluate the effect on each computer's performance of pulsatile or continuous flow, injectate volume, and injectate temperature. With continuous flow the optimum results were 1.8, 0.85, and 0.85 litres per minute and with pulsatile flow they were 1.3, 1.05, and 1.65 litres per minute. There was generally a deterioration in performance when pulsatile flow was evaluated. Under the conditions of the experiment optimum performance in both flow modes was obtained with 5 ml of ice cold injectate, but these findings cannot necessarily be extrapolated to the clinical situation. With pulsatile flow the overall range of blood flows that could give rise to identical measurements were for each computer 2.0, 1.5, and 3.1 litres per minute, corresponding to 40, 30, and 62% changes of the standard flow rate of 5 litres per minute.

Maximal perfusion of skeletal muscle in man

Five subjects exercised with the knee extensor of one limb at work loads ranging from 10 to 60 W. Measurements of pulmonary oxygen uptake, heart rate, leg blood flow, blood pressure and femoral arterial-venous differences for oxygen and lactate were made between 5 and 10 min of the exercise. Flow in the femoral vein was measured using constant infusion of saline near 0 degrees C. Since a cuff was inflated just below the knee during the measurements and because the hamstrings were inactive, the measured flow represented primarily the perfusion of the knee extensors. Blood flow increased linearly with work load right up to an average value of 5.7 l min-1. Mean arterial pressure was unchanged up to a work load of 30 W, but increased thereafter from 100 to 130 mmHg. The femoral arterial-venous oxygen difference at maximum work averaged 14.6% (v/v), resulting in an oxygen uptake of 0.80 l min-1. With a mean estimated weight of the knee extensors of 2.30 kg the perfusion of maximally exercising skeletal muscle of man is thus in the order of 2.5 l kg-1 min-1, and the oxygen uptake 0.35 l kg-1 min-1. Limitations in the methods used previously to determine flow and/or the characteristics of the exercise model used may explain why earlier studies in man have failed to demonstrate the high perfusion of muscle reported here. It is concluded that muscle blood flow is closely related to the oxygen demand of the exercising muscles. The hyperaemia at low work intensities is due to vasodilatation, and an elevated mean arterial blood pressure only contributes to the linear increase in flow at high work rates. The magnitude of perfusion observed during intense exercise indicates that the vascular bed of skeletal muscle is not a limiting factor for oxygen transport.

Determination of heat gain in the inferior vena cava during thermodilution measurements

The measurement of blood flow in the inferior vena cava (IVC) by thermodilution may be useful in the determination of hepatic venous blood flow (HVBF), but is subject to major errors. The most serious error is gain of heat from the abdominal viscera by the cool thermodiluent bolus as it moves up the IVC. The determination or elimination of this heat gain is necessary for accurate IVC blood flow measurement. By comparison of the area under the curve (AUC) for cardiac output following femoral vein injection of the thermodiluent bolus to the AUC following right ventricular injection of the thermodiluent, it is possible to quantitatively determine the unidirectional heat gain by the bolus at it moves up the IVC. The heat gain varied from 0 to 40% of the thermodiluent injected. The factors that aggravated the heat gain were a low cardiac output (below 4000 ml/mn), a large thermodiluent calorie load (over -40 calories), and possibly a slow transit time. The error due to heat gain may be effectively eliminated by keeping the size of the thermodiluent bolus below 40 negative calories.

Increase in rat spinal cord blood flow with the calcium channel blocker, nimodipine

Nimodipine, a calcium channel blocker, is known to increase cerebral blood flow. In the present study, the authors investigated the effect of nimodipine on spinal cord blood flow in normal rats. Cardiovascular parameters, including mean systemic arterial blood pressure, cardiac output, and heart rate, were recorded during infusion of nimodipine in a dose-response fashion. The experiment was a randomized blind study in which four groups of five rats received different doses of nimodipine (0.001, 0.01, 0.05, and 0.10 mg/kg) intravenously over 30 minutes, and a control group of five rats received only the diluent. The hydrogen clearance and thermodilution techniques were used to measure spinal cord blood flow and cardiac output, respectively. The 0.05-mg/kg dose of nimodipine caused the largest increase in spinal cord blood flow, with a 40% increase over the preinfusion level, although there was a 25% reduction in mean arterial pressure. The 0.10-mg/kg dose did not increase spinal cord blood flow more than the 0.05-mg/kg dose, most likely due to the concomitant 37% reduction in mean arterial pressure. Cardiac output was significantly increased by the 0.05- and 0.10-mg/kg doses secondary to the drop in total peripheral resistance. The increase in spinal cord blood flow produced by nimodipine lasted approximately 20 minutes after the termination of the infusion. Thus, nimodipine at a dose of 0.05 mg/kg markedly increased blood flow in the normal spinal cord even though there were major changes in mean systemic arterial pressure and cardiac output. Further research is required to determine whether this drug might be beneficial in treating ischemic states of the spinal cord, such as posttraumatic ischemia.

Percutaneous measurement of portal blood flow in the piglet by continuous thermodilution

The continuous thermodilution method was applied to the percutaneous measurement of portal blood flow in the piglet by inserting a thermodilution probe into the portal vein via the transhepatic transvenous route. The method was compared to electromagnetic flowmetry in three piglets; the two methods yielded statistically identical results. The coefficient of variation of the continuous thermodilution measurements was 4%. The method allows portal blood flow measurements that are instantaneous, reliable, reproducible, and repetitive. It was applied to a study of rapid flow variations in piglets receiving an infusion of posterior pituitary extract.

Indicator dilution approach to NMR signal-flow curves

An indicator dilution model for the dependence of NMR signal on flow rate is presented. The model explains the enhancement in NMR signal observed at low flow rates and the reduction in NMR signal at high flow rates. It also produces an excellent fit to original NMR flow data, which are also presented.

Cardiac output and tissue blood flow in the abalone, Haliotis cracherodii (Mollusca, Gastropoda)

Little is known about the characteristics of hemolymph (blood) flow in animals with open circulatory systems. We measured cardiac output and blood flow to specific tissues in the black abalone, Haliotis cracherodii, a gastropod mollusk. The use of thermodilution allowed us to make repeated measurements of cardiac output and cardiac stroke volume over relatively short time intervals (5-10 heart beats) in resting, unrestrained abalone while disturbing the animals minimally. Anatomical studies of the abalone circulation showed that the arterial system terminated in small diameter (approaching 10-20 micron in some cases) lacunar tissue spaces. Because of this, we used radioactive microspheres (which must be trapped in the tissue vasculature) to measure blood flow rates to selected tissues. The major findings of our study were that 1) cardiac output in the black abalone ranged from about 100 to 150 ml X kg-1 X min-1, and was highly correlated with body size; 2) weight-specific cardiac stroke volume was about 5 ml X kg-1, considerably larger than that of a mammal; 3) tissue blood flow rates ranged from 10 ml X 100 g-1 X min-1 (foot muscle) to 80 ml X 100 g-1 X min-1 (nephridial tissue), similar to typical tissue blood flow rates in mammals. Our data suggest that the blood in the abalone is directed to the tissues not in proportion to percent total body weight the tissues represent (as might be expected in an open vascular system), but apparently in proportion to tissue metabolic rate.

An evaluation of thermodilution cardiac output measurement using the Swan-Ganz catheter

Errors in thermodilution cardiac output measurement were quantitated to determine the order of accuracy of routine measurements performed by unskilled personnel. In vitro and in vivo studies were undertaken to examine factors affecting the volume and temperature of the injectate, catheter thermistor and computer performance, effect of respiration, use of cold (0-4 degrees C) versus ambient temperature (20-25 degrees C) injectate, and the interpretation of measurements. Ambient temperature injectate incurred unacceptably large errors; cold injectate (injections were timed with respiration) produced variations in performance by equipment and personnel which accounted for only 2% of the variation between successive measurements. Real changes in cardiac output and inherent variability of the downslope of the thermal curve, necessitating an empirically based calculation, account for up to 10% variation between successive measurements. When cold injectate was used, and the average of three corrected measurements taken, thermodilution cardiac output measurements were within 10% of a simultaneous dye dilution measurement.

Thermodilution cardiac output--a systematic error

The purpose of this study was to examine the mechanism and magnitude of a systematic error in thermodilution cardiac output measurement. One hundred and seventy-one thermodilution cardiac output measurements in dogs using a Swan-Ganz catheter were compared with simultaneously made dye dilution measurements under different conditions over a wide range of cardiac outputs. A systematic error with the thermodilution technique was confirmed and was almost identical to that observed in the literature. It is proposed that its mechanism is loss of thermal indicator between the injectate orifice and detection. Application of a further correction factor for thermal indicator loss is suggested.

Determination of catheter wall heat transfer in cardiac output measurement by thermodilution

1. A model system of the thermal dilution technique was used to determine in vitro the loss of thermal indicator across different sizes of polyethylene catheters. 2. Thermal indicator loss (caloric gain by the indicator fluid or caloric loss by the catheter) per unit length of catheters (H/L) increased as a function of the outer radius (OR) of the catheters. Linear regression analysis gave the following equation: H/L=0.337OR+0.073 (r=0.88, P<0.05). 3. Thermal indicator loss per unit surface area (H/SA) decreased almost linearly with the following regression equation: H/SA=-1.860 OR+0.364 (r=0.97, P<0.97, P<0.01). 4. Thermal indicator loss per unit volume contained in the catheter (H/vol) decreased exponentially according to the following equation: H/vol =89.le-25.4(OR) (r=0.97, P<0.01). 5. This study indicates that there is a measurable loss of thermal indicator during injection corresponding to a loss between 9 and 16% (for a 11.5 cm length of PE 50 and PE 240) of the required calories for warming the injectate to 37 degrees C. Data is provided for correction of heat loss across the wall of a given length and diameter of polyethylene tubing.

Haemodynamic responses to stimulation of the cardiac autonomic nerves in the anaesthetized cat with closed chest

1. The changes in cardiac output and mean right atrial pressure (R.A.P.) evoked by stimulation of the cardiac autonomic nerves were investigated in cats under chloralose anaesthesia, with unopened chests and spontaneous respiration, and with active vascular reflexes. Cardiac output was measured by thermal dilution; the technique used was calibrated against the direct Fick method.2. The initial values of R.A.P. and output were varied by infusion of dextran-saline solution followed by withdrawal of blood. At positive values of R.A.P. withdrawal of blood caused a fall in R.A.P. with no change in cardiac output. At negative R.A.P. blood withdrawal caused a fall in output with little change in R.A.P.: the linear regression coefficient for output on R.A.P. was 48.2 ml./min.kg.mmHg (S.E. 2.06, n = 63, nine cats).3. Stimulation of the right cardiac sympathetic nerve increased heart rate by 69.2 beats/min (S.E. 4.0) from the resting rate of 158 beats/min (S.E. 6.3, ten cats). The acceleration was accompanied in most instances by a rise in cardiac output and a fall in R.A.P. and the magnitude of the rise in output was related to that of the fall in R.A.P.4. In no experiment could R.A.P. be reduced below -2.5 mmHg either by withdrawal of blood or by sympathetic stimulation. At negative values of R.A.P. the fall in R.A.P. and rise in output evoked by sympathetic stimulation were small; substantial changes could be obtained only from positive initial values of R.A.P. The proportional increase in output evoked by a given proportional increase in heart rate during near-maximal sympathetic stimulation had a linear relationship to the initial value of R.A.P. over the range -2 to +8 mmHg. The output increment was less than proportional to the rate increment at all values of R.A.P. below +3 mmHg.5. In five experiments stimulation of the left cardiac sympathetic nerve evoked a greater increase in output for a given increase in heart rate than did stimulation of the right nerve; on the other hand both nerves gave similar increments of output for a given fall in R.A.P.6. Stimulation of the distal end of the right vagus nerve slowed the heart and caused a fall in cardiac output and a rise in R.A.P. The change in output associated with a given change in R.A.P. was significantly greater (P = 0.05) during sympathetic than during vagal stimulation in 14 out of 18 tests; the difference increased as circulating volume was reduced.7. It is concluded that the relationship between cardiac output and R.A.P. during sympathetic and vagal stimulation is consistent with the hypothesis that neurally evoked changes in cardiac performance vary output mainly, but not exclusively, by inducing changes in R.A.P. which alter the pressure gradient for the return of blood to the right atrium from the periphery.

Thermal dilution measurement of cardiac output in dogs using an analog computer

Thermal dilution cardiac output determinations in dogs were compared to simultaneously performed Fick oxygen measurements. The purpose of this study was to validate in dog experiments a method for thermal dilution measurement which employs a double-thermistor catheter combined with an automatic computer as described by Olsson et al. Dilution and injectate temperature are entered directly into the calculation. The method does not employ logarithmic extrapolation, integration of the dilution signal being terminated when a preset cut-off level is reached. Errors due to recirculation, thermal capacitance of the right heart and heat exchange with the catheter's dead space require the use of an empirically derived correction factor, which in dogs was found to be significantly different from the factor used for human thermal dilution curves. With the appropriate cut-off level and correction factor a good agreement was found between the results of the thermal dilution and the Fick method. The regression equation for 47 experiments was found to be COtd = 0.95 COFick + 0.08; the correlation coefficient was 0.94.

Comparison of the cardiovascular effects of phentolamine, sodium nitroprusside and nitroglycerin in anaesthetized cats (comparison of vasodilators)

The effects of phentolamine (PH), sodium nitroprusside (NP) and nitroglycerin (GTN) were compared in experiments on anaesthetized cats. All three agents diminished vascular resistance and lowered blood pressure, although they differed in the intensity of their effects (NP greater than PH = GTN) and in the steepness of their dose-response curves (NP greater than PH greater than GTN). NP and GTN did not cause consistent changes in heart rate and dP/dt, whereas PH produced marked, dose-dependent increases in these two variables and in cardiac output, which was augmented to a lesser extent by NP and not affected by GTN. It is concluded that whereas the beneficial therapeutic effects of NP and GTN are presumably due exclusively to the reduction of preload and afterload, PH has an additional cardiostimulant action.

Thermodilution measurement of human umbilical blood flow at delivery

A local thermal dilution technique was used to estimate blood flow in the human umbilical vein during the first minute after delivery of the neonate. Estimates of flow were attempted at 101 deliveries and thermal dilution curves obtained at 52. The mean estimation of all flows recorded up to 20 seconds after delivery was 171 ml. per minute reducing to 58 ml. per minute between 40 and 60 seconds.

Assessment of thermodilution cardiac output in small subjects

Thermodilution cardiac output determinations were compared with dye-dilution measurements in normovolemic and hypovolemic puppies. Good correlation was achieved with small volumes of cold and warm saline. Injectate volumes and significant caval displacement of the injectate port did not significantly affect thermodilution output measurements. This study demonstrates that thermodilution is a reliable and practical method of cardiac output determination in small subjects and suggests that this technique may be useful in the clinical management of neonates and small infants.

[Determination of cardiac output by means of an automatized thermal dilution technique (author's transl)]

Cardiac output was measured in 55 patients by the direct Fick method and an automatized thermodilution method. The results showed a good correlation (correlation factor r = 0.87, regression equation y = 0.91 x + 0.59). For the thermodilution method ice-cold normal saline solution as indicator substance was injected into the right atrium. The temperature-time-curve was measured by a thermistor located in the pulmonary artery. The area of indicator dilution was determined by electronic integration, the calculated cardiac output appeared as 1/min in digital display.

Error in thermodilution cardiac output measurement caused by variation in syringe volume

It is important to measure the exact volume of 5% dextrose injected when thermodilution cardiac output determinations are made. The marked volume on plastic syringes and their variability may contribute to a small but significant error in the results.

Use of balloon flotation catheters in critically ill patients

In summary, balloon flotation catheterization of the central circulation provides data in patient management which are meaningful and important. It has allowed the application of sound physiologic principles to the understanding of the circulatory abnormalities characterizing an illness in an individual patient, and provides a rational basis for selection of therapy with objective, quantitative assessment of patient response. The procedures are simple, the complication rate is low, and the information highly relevant to clinical care.

Measurement of cardiac output by thermal dilution in infants and children

The accuracy of the thermal dilution technique for measuring cardiac output in infants and children has been assessed by comparison with the direct Fick technique. Satisfactory correlation between the two methods has been demonstrated, and it is concluded that a commercially available monitor can be used to determine cardiac output by thermal dilution in acutely sick infants and children. The volume of injectate should be selected according to the weight of the child, and the value displayed by the monitor should be adjusted by the appropriate fraction.