R. B. P. de Wilde

An evaluation of cardiac output by five arterial pulse contour techniques during cardiac surgery.

The bias, precision and tracking ability of five different pulse contour methods were evaluated by simultaneous comparison of cardiac output values from the conventional thermodilution technique (COtd). The five different pulse contour methods included in this study were: Wesselings method (cZ); the Modelflow method; the LiDCO system; the PiCCO system and a recently developed Hemac method. We studied 24 cardiac surgery patients undergoing uncomplicated coronary artery bypass grafting. In each patient, the first series of COtd was used to calibrate the five pulse contour methods. In all, 199 series of measurements were accepted by all methods and included in the study. COtd ranged from 2.14 to 7.55 l.min−1, with a mean of 4.81 l.min−1. Bland‐Altman analysis showed the following bias and limits of agreement: cZ, 0.23 and − 0.80 to 1.26 l.min−1; Modelflow, 0.00 and − 0.74 to 0.74 l.min−1; LiDCO, – 0.17 and − 1.55 to 1.20 l.min−1; PiCCO, 0.14 and − 1.60 to 1.89 l.min−1; and Hemac, 0.06 and − 0.81 to 0.93 l.min−1. Changes in cardiac output larger than 0.5 l.min−1 (10%) were correctly followed by the Modelflow and the Hemac method in 96% of cases. In this group of subjects, without congestive heart failure, with normal heart rhythm and reasonable peripheral circulation, the best results in absolute values as well as in tracking changes in cardiac output were measured using the Modelflow and Hemac pulse contour methods, based on non‐linear three‐element Windkessel models.

Performance of three minimally invasive cardiac output monitoring systems

We evaluated cardiac output (CO) using three new methods – the auto‐calibrated FloTrac–Vigileo (COed), the non‐calibrated Modelflow (COmf ) pulse contour method and the ultra‐sound HemoSonic system (COhs) – with thermodilution (COtd) as the reference. In 13 postoperative cardiac surgical patients, 104 paired CO values were assessed before, during and after four interventions: (i) an increase of tidal volume by 50%; (ii) a 10 cm H2O increase in positive end‐expiratory pressure; (iii) passive leg raising and (iv) head up position. With the pooled data the difference (bias (2SD)) between COed and COtd, COmf and COtd and COhs and COtd was 0.33 (0.90), 0.30 (0.69) and −0.41 (1.11) l.min−1, respectively. Thus, Modelflow had the lowest mean squared error, suggesting that it had the best performance. COed significantly overestimates changes in cardiac output while COmf and COhs values are not significantly different from those of COtd. Directional changes in cardiac output by thermodilution were detected with a high score by all three methods.

Monitoring cardiac output using the femoral and radial arterial pressure waveform

This study was performed to determine the interchangeability of femoral artery pressure and radial artery pressure measurements as the input for the PiCCO system (Pulsion Medical Systems, Munich, Germany). We studied 15 intensive care patients following cardiac surgery. Five‐second averages of the cardiac output derived from the femoral artery pressure (COfem) were compared to 5‐s averages derived from the radial artery pressure (COrad). One patient was excluded due to problems in the pattern recognition of the arterial pressure signal. In the remaining 14 patients, 14 734 comparative cardiac output values were analysed. The mean sample time was 88 min, range [30–119 min]. Mean (SD) COfem was 6.24 (1.1) l.min−1 and mean COrad 6.23 (1.1) l.min−1. Bland‐Altman analysis showed an excellent agreement with a bias of − 0.01 l.min−1, and limits of agreement from 0.60 to − 0.62 l.min−1. If changes in CO were > 0.5 l.min−1, the direction of changes in COfem and COrad were equal in 97% of instances. We conclude that femoral artery pressure and radial artery pressure are interchangeable as inputs for the PiCCO device.

A comparison of stroke volume variation measured by the LiDCOplus and FloTrac-Vigileo system

The aim of this study was to compare the accuracy of stroke volume variation (SVV) as measured by the LiDCOplus system (SVVli) and by the FloTrac‐Vigileo system (SVVed). We measured SVVli and SVVed in 15 postoperative cardiac surgical patients following five study interventions; a 50% increase in tidal volume, an increase of PEEP by 10 cm H2O, passive leg raising, a head‐up tilt procedure and fluid loading. Between each intervention, baseline measurements were performed. 136 data pairs were obtained. SVVli ranged from 1.4% to 26.8% (mean (SD) 8.7 (4.6)%); SVVed from 2.0% to 26.0% (10.2 (4.7)%). The bias was found to be significantly different from zero at 1.5 (2.5)%, p < 0.001, (95% confidence interval 1.1–1.9). The upper and lower limits of agreement were found to be 6.4 and −3.5% respectively. The coefficient of variation for the differences between SVVli and SVVed was 26%. This results in a relative large range for the percentage limits of agreement of 52%. Analysis in repeated measures showed coefficients of variation of 21% for SVVli and 22% for SVVed. The LiDCOplus and FloTrac‐Vigileo system are not interchangeable. Furthermore, the determination of SVVli and SVVed are too ambiguous, as can be concluded from the high values of the coefficient of variation for repeated measures. These findings underline Pinsky’s warning of caution in the clinical use of SVV by pulse contour techniques.