Jos Jansen

Improvement of cardiac output estimation by the thermodilution method during mechanical ventilation

The reliability of cardiac output estimation by thermodilution during artificial ventilation was studied in anesthetized pigs at the right side of the heart. The estimates exhibited a cyclic modulation related to the ventilation. The amplitude of the modulation was independent of the level of positive end-expiratory pressure, ventilatory pattern and volemic loading of the animals. However, a non-constant phase relation existed between the ventilatory cycle and the modulation. Single observations at a fixed moment in the ventilatory cycle are therefore not appropriate for estimation of mean cardiac output nor for studying its relative changes. The averaging of estimates spread equally over the ventilatory cycle led to a much larger reduction in the deviation of the averages from the mean cardiac output than an averaging procedure of randomly selected estimates. The accracy of estimation of mean cardiac output by two estimates equally spread in the ventilatory cycle was equal to the accuracy obtained by averaging five randomly selected estimates. Averaging four estimates, equally spread in the cycle, appeared to be the optimal procedure. For 89% of all averages an accuracy of 5% around the mean was obtained and for 99% an accuracy of ±10%.

An adequate strategy for the thermodilution technique in patients during mechanical ventilation

The application of the thermodilution method in conditions associated with variations in blood flow implies a misuse of the Stewart Hamilton equation. Therefore, we studied the reliability of the thermodilution method for the estimation of mean cardiac output (CO) during mechanical ventilation in patients (n=9). Variation of the injection moment in the ventilatory cycle elicited a cyclic variation of CO estimates. This variation was not the same for all patients neither in phase nor in amplitude. Therefore, no specific phase in the ventilatory cycle could be selected for an accurate estimation of mean CO. Averaging CO estimates randomly distributed in the ventilatory cycle led to an improvement of accuracy with the square root of the number of observations. The averaging of CO estimates spread equally over the ventilatory cycle led to a much better result, e.g., the variation in the average of two estimates equally spread in the ventilatory cycle was similar to the variation in the average of four random estimates. We conclude that averaging of 3 or 4 estimates spread equally over the ventilatory cycle is an adequate strategy to estimate mean cardiac output in patients reliably.

The thermodilution method for the clinical assessment of cardiac output

ConclusionThe thermodilution technique has been shown to measure cardiac output accurately if flow is stationary. During mechanical ventilation the technique can be liable to gross errors. However, if certain precautions are followed mean cardiac output can be accurately estimated, even with a theoretical misuse of the Stewart-Hamilton equation. This can be done best by averaging three or four measurements equally spread over the ventilatory cycle. For this approach manufacturers of cardiac output computers have to be persuaded to include a phase selector and an automatic injector.

Mean systemic filling pressure as a characteristic pressure for venous return.

Guytons theory on venous return, implying a linear relationship between blood flow and central venous pressure, was tested in an intact circulation after thoracotomy and airtight chest closure. In eleven Yorkshire pigs (approx. 10 kg) we measured flow in the pulmonary artery and aorta and pressure in the central veins and aorta during pentobarbital anesthesia and mechanical ventilation. To change central venous pressure different lung volumes were randomly applied at intervals of 5 min in a series of inspiratory hold procedures of 7.2 s. During these short periods hemodynamic steady state circumstances were met without involvement of cardiovascular control mechanisms.We confirmed the linear relationship between venous return and central venous pressure and derived mean systemic filling pressure from the regression equation. Mean systemic filling pressure was on average 10.5±2.3 (SD) mm Hg.The time dependent changes during the inspiratory hold procedure showed that the increase in central venous pressure was the primarily dependent variable, followed by a decrease in venous return and right ventricular output. After a delay of 2–4 heart beats also a decrease in left ventricular output and aortic pressure occurred. Subsequently, the lower venous return during inspiratory hold was mainly sustained by the lower aortic pressure, but nevertheless fulfilled the linear relationship mentioned above.For analysis of flow and pressure changes in the systemic circulation during changes of central venous pressure a tube of constant flow resistance was used as a conceptual model. Consequently, the point where mean systemic filling pressure exists during normal flow conditions was predicted at a characteristic location in the peripheral venous system. Downstream from this point blood pressure will rise and vessel capacity will be filled up during increases in central emptying vessel capacity partially.

Near continuous cardiac output by thermodilution

A new thermodilution method for frequent (near continuous) estimation ofcardiac output, without manual injection of fluid into the blood, was tested.The method utilizes a pulmonary artery catheter equipped with a fluid filledheat exchanger. The technique is based on cyclic cooling of the blood in theright atrium and measurement of the temperature changes in the pulmonaryartery. Using this technique, a new estimate of cardiac output can be obtainedevery 32 s. Cardiac output estimates, obtained for a running mean of threemeasurements with this method, were compared to the mean of three conventionalthermodilution measurements. The measurements were obtained during shortperiods of stable respiration and circulation.In six pigs, we made 46 paired measurements of conventional thermodilution(TD) and near continous (TDc) thermodilution. The cardiac output(CO TD) ranged from 2.4–13.7 l/min (mean 5.4 l/min). Thebest linear fit through the paired data points was CO TDc =−0.57 + 1.01 CO TD. The mean difference between themethods was −0.50 l/min (S.D. = 0.39). The mean coefficient of variationof repeated measurements with the near continuous thermodilution was3.6%.Considering changes of more than 0.25 l/min to be significant, all changes incardiac output measured by conventional thermodilution were followed by therunning mean of three near continuous thermodilution estimates.This study demonstrates the feasibility of the new method to monitorcardiac output, and to detect all changes greater than 0.25 l/min.

A stable model of respiratory distress by small injections of oleic acid in pigs

AbstractObjectiveDevelopment of a stable model of respiratory distress in pigs with oleic acid, fulfilling clinical criteria of the adult respiratory distress syndrome (ARDS).DesignEight pigs (9.1±0.7 kg) were anesthetized with pentobarbital, paralyzed with tubocurarine and mechanically ventilated with an

Negative effect of insufflation on cardiac output and pulmonary blood volume.

F_{IO_2 }

A Non-Invasive Determination of Lung Perfusion Compared with the Direct Fick Method

of 0.6, an I∶E ratio of 2∶3 and a PEEP of 0.2 kPa. Oleic acid (dissolved 1∶1 in 96% alcohol) was administered in a series of multiple injections of 0.1 ml until