Continuous thermodilution cardiac output measurement in sheep.

A technique has been developed to continuously measure cardiac output by means of the principles of thermodilution. Pulmonary artery catheters were modified by placing a 10 cm filament near the usual injectate port. Small amounts of heat were infused according to a randomly repeating binary on-off sequence. The distal blood temperature was recorded and cross-correlated with the heat waveform to produce a dilution curve and calculate cardiac output. The technique was compared with bolus thermodilution in seven sheep. Cardiac output ranged from 1.5 to 13.2 L/min, and heart rate varied from 59 to 180 beats/min. The linear regression between the data obtained by the two methods is represented by the equation y = 1.00x + 0.13; the correlation coefficient, R, is 0.97, and the p value is less than 0.0001.

Evaluation of a continuous thermodilution cardiac output catheter.

The authors evaluated a thermodilution catheter designed to continuously measure cardiac output (CO). A 10 cm long surface heating element is positioned in a Swan-Ganz catheter corresponding to a right atrial-ventricular site. Heat is repetitively deposited into flowing blood in a unique, pseudorandom binary form. Small temperature fluctuations are sensed with a high performance thermistor and correlated with the heat input pattern, from which CO is determined. Seven adult sheep were anesthetized and instrumented for both continuous and standard cold bolus injection thermodilution (COM1) flow measurements. Heart rate and blood volume were adjusted to vary CO from 1.5 to 13.2 L/min. Continuous measurements correlated well with triplicate COM1 determinations (Sy,x = 0.56, r = 0.967) that improved with experience (Sy,x = 0.38, r = 0.99 for the last three animals). The surface heat transfer coefficient was measured in water (catheter parallel to flow). Results agreed well with a standard cylinder-in-crossflow correlation. The right ventricle heating element surface temperature was predicted for several CO and heating combinations. Worst case results yielded a 5.8 degrees C surface temperature elevation, suggesting that thermally induced damage is unlikely. Results suggest this catheter provides accuracy at least comparable to that of standard cold bolus injection methods, with no heat induced damage to blood.

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 Syx 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.

Peritubular capillary control of proximal tubule reabsorption in the rat.

Changes in peritubular capillary hydrostatic and oncotic pressures, which probably affect net interstitial pressure and, thus, the force on fluid movement across the tubule basement membrane, can modulate absolute proximal reabsorption rate (APR). To examine the relationship between APR and net interstitial pressure, we measured peritubular capillary hydrostatic and oncotic pressure, single nephron filtration rate, APR, absolute distal reabsorption (ADR), and tubular hydrostatic pressure in hydropenic, saline-loaded, and plasma-loaded rats. Net interstitial pressure in saline loading was estimated from subcapsular hydrostatic pressure and lymph protein concentration measurements. The surface area-hydraulic conductivity product of the peritubular capillary network was estimated from these measurements with a model of capillary fluid exchange in which fluid uptake was defined to be APR plus ADR. The estimated value was assumed to remain constant in all three states, and was then used to estimate net interstitial pressure in hydropenic and plasma-loaded rats. APR and net interstitial pressure correlated strongly, a finding consistent with the hypothesized role for net interstitial pressure in regulating proximal reabsorption.