In the present study, we have demonstrated an influence of NE dosage on agreement of PCCO, as only during high NE dosage the criteria of interchangeability with COTCP were met. Time elapsed between calibrations did not affect agreement between methods.
Goal-directed therapy in high-risk patients has been shown to improve outcomes [
4,
5]. One essential observation in these studies was that the earlier treatment was started, the better the outcome. Therefore, continuous CO monitoring in critically ill patients is needed. However, PCCO needs to be validated in a large number of patients and during relevant conditions to gain more insight into the mechanisms influencing this variable. The present study compared PCCO and CO
TCP in 73 ICU patients with several comorbidities. Most previous studies compared PCCO with CO
TCP in small series of patients during cardiac surgery [
6,
8,
9,
22]. Data from larger patient samples, however, are scarce. The percentage error between PCCO and CO derived by a thermodilution method varied between 26% and 50% in earlier studies [
14,
23]. Critchley and Critchley [
20] defined a percentage error of less than 30% to indicate interchangeability. Accordingly, we found an acceptable agreement of PCCO with CO
TCP only in data subsets obtained with high NE dosage, although a percentage error of 28% is still reasonably high. However, the results of the present study tend to refute our first hypothesis. Increasing NE dosage does not seem to be associated with decreased agreement between PCCO and CO
TCP, but rather with improved interchangeability. PCCO further showed a better performance in tracking changes in CO during increased NE dosage because the coefficient of correlation between ΔPCCO and ΔCO
TCP was higher. Vascular tone seems to be an important issue regarding the agreement of PCCO methods with a reference method such as transcardiopulmonary thermodilution. Rodig
et al. [
12] described an increased bias between PCCO and CO measured by thermodilution after administration of phenylephrine. The observed change of SVR >60% between calibrations may explain their findings. A recent publication applying the same PCCO software used in our study concluded that agreement was not influenced by changes in SVR due to better adaptation of the newer algorithm [
14]. In the present study, SVR was not different between NE subgroups. Therefore, we hypothesize that despite a comparable SVR, a differing compliance of the vascular tree between subgroups of different NE dosages may explain the different level of agreement. A higher NE dosage may result in an increased central arterial stiffness and therefore reduced arterial compliance [
24], as recently reported by Wittrock
et al. [
16]. In agreement with these findings, high NE dosage resulted in a significantly higher PP/SV relationship as an indicator of arterial stiffness. Increasing arterial stiffness leads to a more rigid vascular system and therefore may result in better agreement between methods. It is conceivable in this context that the vasculature of patients on high NE has less oscillatory capacity, which limits changes in arterial compliance and consequently on the deviation from the compliance obtained upon calibration. In clinical practice, however, many patients may be treated with either a low dose of NE or no NE, and according to our results, PCCO is not interchangeable with CO
TCP in these patients.
Our results do not show a time-related effect on the agreement between PCCO and CO
TCP, thus refuting the second hypothesis. The percentage error was above 30% in all calibration interval subgroups. The manufacturer recommends recalibration every 8 hours. Godje
et al. [
9] reported an overall acceptable agreement up to 44 hours; however, they did not indicate the bias and percentage error of subsets regarding different calibration intervals. Hamzaoui
et al. [
14] reported a percentage error below 30% only within the first hour after calibration of PCCO, but up to 37% within a 6-hour calibration interval. These authors concluded that PCCO is stable during a 1-hour period, and even changes in SVR did not alter the agreement. These results would prompt one to use hourly recalibration. Regarding our results, time elapsed from preceding calibration did not determine the level of agreement, as individually good agreement was observed up to 24 hours and individually poor agreement occurred within a period of 2 hours after calibration. Moreover, we found acceptable agreement in patients who were administered a high NE dosage, and thus had higher arterial stiffness, who had mean calibration periods of 7 hours.
This study also examined the clinical use of calibrations by using PiCCO technology. Our institutional guidelines recommend a recalibration of the PiCCO system every 8 hours (three times daily), as well as before and after any major change in therapy. We found that in only 54% of recordings were institutional guidelines of recalibration met. We did not observe a correlation of calibration frequency with APACHE II score or NE dosage, indicating that calibration of PCCO may not be dependent on the severity of critical illness. These findings are surprising, since recalibration may increase agreement between methods [
13]. However, our results indicate that the time interval between calibrations may not to be the most important factor in determining PCCO accuracy; moreover, therapy during calibrations seems to be important.