BJA Advance Access originally published online on July 24, 2007
British Journal of Anaesthesia 2007 99(4):484-492; doi:10.1093/bja/aem199
Continuous cardiac output during off-pump coronary artery bypass surgery: pulse-contour analyses vs pulmonary artery thermodilution
1 The Interventional Centre
2 Department of Biostatistics
3 Department of Anaesthesiology
4 Department of Cardio-Thoracic Surgery, Rikshospitalet-Radiumhospitalet Medical Centre, Oslo, Norway
5 Medical Faculty, University of Oslo, Oslo, Norway
* Corresponding author: The Interventional Centre, Rikshospitalet-Radiumhospitalet Medical Centre, NO-0027 Oslo, Norway. E-mail: per.steinar.halvorsen{at}rikshospitalet.no
Accepted for publication June 3, 2007.
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Abstract |
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Background: No gold standard method exists for monitoring continuous cardiac output (CO). In this study, the agreement between the two most frequently used methods, PiCCO pulse-contour analysis (PCCO) and STAT pulmonary artery thermodilution (STAT-CO), was assessed during multiple-vessel off-pump coronary artery bypass (OPCAB) surgery.
Methods: Thirty patients were enrolled in the study. Two time periods were defined during surgery; Period 1 included positioning of the heart and stabilizer device and Period 2 included the coronary occlusion. Measurements were obtained every minute during both periods. The agreement for the continuous CO and the change in CO (
CO) was estimated using the Bland–Altman method.
Results: Significant changes in mean arterial pressure (MAP), central venous saturation, PCCO and STAT-CO were seen only during Period 1. MAP correlated only with changes in PCCO, (P < 0.001, r = 0.60). The mean difference (2SD) between PCCO and STAT-CO ranged from – 0.29 (1.82) to – 0.71 (2.57) litre min–1, and the percentage error varied from 32 to 50%. For the CO measurements, the limits of agreements did not differ between Period 1 and Period 2. In contrast, for the CO measurements, the limits of agreements were wider in Period 1 than in the more haemodynamically stable Period 2.
Conclusions: PCCO and STAT-CO show large discrepancies in CO during OPCAB surgery. Clinically acceptable agreement was seen only for trends in CO during haemodynamically stable periods.
Keywords: anaesthesia, cardiovascular; heart, cardiac output; measurement techniques, pulse-contour analysis; measurement techniques, thermodilution; surgery, coronary artery bypass
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Introduction |
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During off-pump coronary artery by-pass (OPCAB) surgery, haemodynamic instability is common1–3 and continuous monitoring of cardiac output (CO) is advisable.4
There is no gold standard method for continuous CO monitoring. The most commonly used methods are the STAT pulmonary artery catheter (STAT-CO), which gives semi-continuous thermodilution measurements, and PiCCO pulse-contour analysis (PCCO) enabling almost real-time CO. Both methods have drawbacks. Because of a time delay of the CO calculations, the STAT method may miss rapid and transient haemodynamic changes.5–7 With PCCO, flow is estimated from the femoral artery blood pressure curve. The algorithm uses properties of the vascular system which are non-linear and differs from patient to patient.8 Thus, the reliability of PCCO has been questioned during haemodynamic instability.9 10
Continuous CO is used to monitor cardiac performance and to guide medical interventions. Therefore, these methods should be reliable and in agreement with each other. In this paper, we address the agreement between PCCO and STAT-CO during multiple-vessel OPCAB surgery. We hypothesize that the agreement depends on the patients' haemodynamic situation, that is, during haemodynamic instability, the agreement may be poorer than during more stable periods. For each coronary artery, the agreement was estimated in two periods; Period 1 included the positioning of the heart and the stabilizer, and Period 2 included the coronary occlusion and suturing of the anastomosis.
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Methods |
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Patients
The Regional Ethics Committee approved the study and informed consent was obtained from all patients. Exclusion criteria were unstable angina pectoris, left ventricle ejection fraction < 0.30, arrhythmias, valvular heart disease, intracardiac shunts, and severe peripheral arterial occluding diseases. The patients were premedicated with diazepam 5–15 mg orally. An arterial catheter was placed in the radial artery and anaesthesia was induced with fentanyl 2–5 µg kg–1, diazepam 0.1 mg kg–1, thiopentone 2–5 mg kg–1, and cisatracurium 0.15 mg kg–1. Anaesthesia was maintained with sevoflurane. Ephedrine or phenylephrine was administered when necessary in order to keep the mean arterial pressure (MAP) above 60 mm Hg, and atropine was given to maintain heart rate (HR) above 45 beats min–1. All patients received an infusion of glycerylnitrate (0.5–1.0 µg kg–1 min–1) except when MAP was below 60 mm Hg. The haemoglobin (Hb) concentration was kept above 9.0 g dl–1.
A 7.5 French pulmonary artery catheter (REF: 774HF75, Edwards Lifesciences, USA) was inserted via a 9.0 French triple lumen introducer (REF: m3L9FHSI, Edwards Lifesciences, USA) in the right internal jugular vein and advanced to the wedge position guided by waveform and pressure analysis. The pulmonary artery catheter was connected to a Vigilance monitor (version 6.2 Edwards Lifesciences, USA) with an inline temperature sensor (model 953 22) for intermittent pulmonary artery thermodilution CO. The mixed venous oxygenation (SvO2) was calibrated in vitro before the insertion of the pulmonary artery catheter. A 5 French arterial thermodilution catheter (PV 2025L20, PULSIOCATH, Pulsion Medical Systems, Munich, Germany) was inserted in the right femoral artery and connected to a pressure transducer (PV8115, PULSION Medical Systems, Munich, Germany), and the signals were computed in a PiCCO monitor (Pulsion Plus, software version 5.1, PULSION Medical Systems, Munich, Germany). A PiCCO inline injectate temperature sensor was fitted in the central venous line for transpulmonary thermodilution CO measurements. Three boluses of 10 ml of ice-cold glucose 5% were injected into the proximal port of the pulmonary artery catheter, and thermodilution CO for both the PiCCO and the pulmonary artery catheter were then obtained simultaneously. The injections were carried out by the same person to minimize variation. The mean value of CO obtained by transthoracic thermodilution was used to calibrate the pulse-contour analysis.
Measurements
For surgery on the anterior, lateral, and posterior coronary arteries, three time points were defined: T1 (baseline), T2 (immediately before coronary occlusion), and T3 (immediately before coronary reperfusion). Measurements of HR, MAP, SvO2 and PCCO were collected at all three time points. Because of the interruption of the thermodilution measurements, STAT-CO values could be obtained only at T2 and T3. Instead, at T1, the mean CO of the three pulmonary artery thermodilutions was used. All measurements were obtained with the table in the zero position and the heart untouched in the pericardial sack.
For each coronary artery, T1–T2 defined Period 1 and included positioning of the heart and the placement of the stabilizer device. Period 2 was defined from T2 to T3 and included the coronary occlusion and suturing of the anastomosis. PCCO was recalibrated before surgery on each coronary artery. The recalibrated pulse-contour value was used for baseline measurements (T1). Each minute, during the two time periods, both PCCO and STAT-CO were measured simultaneously. Since the STAT-CO value updates only every minute, a measurement interval of 1 min was used.
The software program ICUPilot (CMA, Sweden) was used for data collection.
Statistical analysis
The number of patients was based on a calculation of power. A clinically relevant mean difference was set to 0.20 litre min–1, with an SD of 0.20 litre min–1. Thirty patients were included to ensure power of at least 80%. For the T1, T2, and T3 measurements, the variables were not normally distributed, so the values are presented as median and inter-quartile range (25th and 75th percentiles). Differences between the time points were analysed with the Friedman test, and a P-value < 0.05 was considered statistically significant. The Wilcoxon rank sum tests with the Bonferroni correction were used for variables for which the Friedman test showed significant differences. For the correlation analysis, the Pearson correlation was used and a P-value < 0.01 was considered statistically significant. For the CCO and CO measurements, the agreement was assessed using the Bland–Altman method.11 The Bland–Altman method could be applied since these variables were normally distributed. Percentage error was calculated as follows: twice the SD for the difference between the two methods being compared was divided by the mean value obtained from both methods {2 x SD (difference between method 1 and method 2)/[(mean of method 1 + mean of method 2)/2]x100}. The statistical analysis was computed with SPSS (Version 13, SPSS Inc., Chicago, IL, USA).
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Results |
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Thirty patients were enrolled (26 men and 4 women). Median (range) age was 63 yr (39–86), body surface area 2.02 (1.72–2.56) m2, New York Heart Association functional class of angina pectoris 2.0 (1–4), and left ventricle ejection fraction 0.72 (0.37–0.88). Sixteen patients (53%) had a previous myocardial infarction and 27 (90%) of the patients received preoperative beta-blockers. The number of distal coronary anastomoses was 3.0 (1–4). End-tidal volume concentration of sevoflurane was 1.70 (0.80–2.50)%, fentanyl 1.70 (0.95–2.90) µg kg–1 h–1, ephedrine 10.0 (0.0–30.0) mg, phenylephrine 0.05 (0.0–0.55) mg, and atropine 0.50 (0.0–1.3) mg.
One patient was converted from OPCAB to on-pump surgery before grafting because of severe myocardial hypertrophy and fragile coronary arteries. Surgery on the anterior wall was performed in 29 patients, on the posterior wall in 15 patients, and on the lateral wall in 24 patients. Ventricular fibrillation occurred during reperfusion of the circumflex artery in one patient. He was successfully defibrillated and recovered without sequelae. Two patients had atrio-ventricular block during occlusion of the right coronary artery. Both patients spontaneously recovered to sinus rhythm after reperfusion. There were no deaths or myocardial or cerebral infarctions.
Erythrocyte concentrates were infused in seven patients (24%), with a median (range) volume of 400 ml (200–400). Twenty-two patients (76%) received autotransfusion; the median (range) volume given was 500 ml (0–2000). A small but significant (P = 0.005) change in the Hb was seen from the start to the end of surgery, mean (SD) 11.5 (1.3) and 10.4 (1.3) g dl–1, respectively. Blood gases were measured on average three times during surgery. The lowest mean Hb observed was 10.0 (1.1) g dl–1 (P < 0.001 towards the start, and P = 0.519 towards the end of surgery).
All patients were externally warmed, and the mean (SD) rectal temperature was 36.2 (0.5)°C at the start and 36.1 (0.4)°C at the end of the surgery (P = 0.497).
Figure 1 shows an example of haemodynamic data during surgery on the right coronary artery. There is a large discrepancy between PCCO and STAT-CO. The curves for SvO2, PCCO, and MAP decrease until the end of the period, then SvO2 increases, whereas PCCO and MAP continue to decrease. The MAP and PCCO curves look remarkably similar.
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Significant changes in MAP, SvO2, PCCO, and STAT-CO were observed from baseline (T1) to coronary occlusion (T2), but not from the start of occlusion (T2) to reperfusion (T3) (Table 1). Thus, in each subgroup of patients, Period 1 (T1–T2) is the most haemodynamically unstable period.
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For surgery on the anterior wall, median times (range) for Period 1 and Period 2 were 14 (6–25) and 12 (7–26) min, respectively. The corresponding times for lateral wall surgery were 12 (7–23) and 14 (8–20) min, and for posterior wall surgery 11 (4–20) and 14 (6–22) min, respectively. The mean difference and limits of agreements between PCCO and STAT-CO for the 1 min measurements during Period 1 and Period 2 are shown in Table 2 and Figure 2. For surgery on all heart regions, a significant discrepancy in the mean difference between the methods was observed for both periods.
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As shown in Table 2, wide limits of agreement were observed for both periods during surgery on the different coronary arteries. The limits of agreements in Period 1 and Period 2 were ± 2.20 and ± 2.21 litre min–1, respectively.
Trends in CO, that is, changes in CO from the previous value (CO), were also calculated using both methods, and their agreements are shown in Table 3 and Figure 3. For all regions, no significant discrepancy in the mean difference for CO was observed. However, the limits of agreement were markedly reduced in Period 2 compared with Period 1. Thus, when assessing trends in CO, the best agreement was seen during stable haemodynamics.
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In Table 4, the correlations between changes in MAP (
MAP), changes in SvO2 (SvO2), changes in PCCO (PCCO), and changes in STAT-CO (STAT-CO) for the 1 min measurements in Period 1 and Period 2 are presented. As seen, MAP was correlated only with PCCO (P < 0.001, mean r = 0.60) and not with SvO2 and STAT-CO. SvO2 was correlated to neither PCCO nor STAT-CO.
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Discussion |
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In this study, the agreement between the two most frequently used methods for monitoring CCO during OPCAB surgery was assessed. We found that the agreement between PCCO and STAT-CO was poor, with wide limits of agreements seen during surgery on every coronary artery. There were no differences in the limits of agreements between the two time periods. When measuring trends in CO, the methods agreed more closely, particularly during stable haemodynamics.
In the present study, the largest haemodynamic changes appeared during the positioning of the heart and the stabilizer. This is in accordance with other studies.1 3 No significant changes were observed during the coronary occlusion period. Thus, in our study, Period 1 is the most haemodynamically unstable.
Studies focusing on the agreement between different methods for assessing CO (both continuous and intermittent) usually compare measurements at clearly defined time points.4 12 13 Typically, the new methods are compared with intermittent thermodilution pulmonary artery CO, which is considered the most accurate. Since the thermodilution measurements take a few minutes, comparisons are generally performed in haemodynamically stable phases. This improves the precision and the agreement between the methods. In our view, it is more important to know how the different methods agree during haemodynamic instability.
Ideally, clinically acceptable mean difference and limits of agreement should be defined before starting the study. Instead, these are commonly based on a subjective judgement.14 In order to define objective criteria, Critchley and Critchley15 recommended accepting a new method where limits of agreements are up to ± 30%. In OPCAB surgery, trends in CO, that is, information on the direction and relative magnitude of changes in CO, may give more valuable clinical information than accurate intermittent measurements. Thus, one might accept reduced accuracy when using a continuous method. However, there is no consensus on the level of agreement, and therefore clinical judgement has to be used.
In our study, there was a poor agreement between PCCO and STAT-CO. For all heart regions, the methods differed significantly in Period 1 and Period 2, and the mean difference between STAT-CO and PCCO varied from –0.71 to 0.78 litre min–1. For both time periods and for every heart region, wide limits of agreements were observed (range ± 1.82 to ± 2.57 litre min–1). When applying the method described by Critchley and Critchley, our percentage error was in the range of 32–50%.
The two methods agree better when trends in CO were measured. For the CO values, no significant difference between the methods was observed. The best agreement was seen during stable haemodynamics (Period 2), with limits of agreements ± 0.64 litre min–1. For Period 1, the limits of agreements were ± 1.17 litre min–1. This is in contrast to the CO measurements, where no difference between periods was observed. Given these results, clinically acceptable agreement between PCCO and STAT-CO can be seen only for trends in CO during haemodynamically stable phases.
Our study allows no conclusion on which method is the most accurate. One of the main reasons for the lack of agreement is that STAT-CO is a semi-continuous method with a slow response time, whereas PCCO reflects CO in almost real time. Thus, rapid haemodynamic changes may be missed with the STAT method. This may explain why the mean difference changed from Period 1 to Period 2 for the absolute CO measurements. In Period 1, the mean differences were negative, showing that the STAT pulmonary artery was measuring higher values compared with PCCO. Since STAT-CO responds slowly, it continues to decrease into Period 2. In contrast, PCCO fell early in Period 1, but was stabilized during Period 2 (Table 1).
PCCO estimates flow from the femoral artery blood pressure curve and the algorithm is based on properties of the vascular system which are non-linear and differ from patient to patient.8 Vascular compliance is altered by myogenic intrinsic local regulation, sympathetic vasoconstriction, manipulation of the aorta, and changes in inotropic, vasoactive, or anaesthetic drugs during the time course of surgery. In a previous study, during stable haemodynamics, we found a percentage error of 33.5–35.5% between PCCO and intermittent pulmonary artery and transthoracic thermodilution.16 In a study including postoperative cardiac surgery patients, Østergaard and colleagues17 reported a percentage error of 49.5% between PCCO and intermittent pulmonary artery thermodilution. Reduced accuracy of PCCO during haemodynamic instability has been reported by others.10 18 Thus, in our study, since significant haemodynamic changes were seen and inotropes and vasoactive drugs were given in order to maintain acceptable MAP and HR, one should expect a high percentage error between PCCO and true CO.
In the present study, minor bleeding occurred and the patients were adequately oxygenated. Under such circumstances, SvO2 has been shown to correlate with CO,1 19 20 but small changes in MAP and CO may not alter SvO2 and the correlation between CO and SvO2 is better during haemodynamic instability. Thus, one should expect PCCO and STAT-CO to be correlated with SvO2 in Period 1 when the circulatory changes were the most prominent. However, this was not found, and the PCCO and MAP were the only haemodynamic variables that correlated. This may indicate a too strong mathematical coupling between MAP and PCCO, implying that PCCO does not reflect true CO precisely.
Another reason for the poor correlation found between PCCO, SvO2 and STAT-CO might be our short sampling interval. Although SvO2 can detect rapid and large haemodynamic changes, the time response is delayed compared with the blood pressure.7 In the present study, a major portion of the values for SvO2 and STAT-CO were small or even zero, and, since no linearity can be found on data clustered around zero, this may account for the poor correlations observed.
In our opinion, the reliability of PCCO during haemodynamic instability is uncertain, and if PCCO needs recalibration every time there are changes in vasoactive drugs or after manipulation of the heart and great vessels, we argue that the use of PCCO is limited during OPCAB surgery. For low-risk patients, we consider ECG, invasive arterial blood pressure, peripheral oxygen saturation, end-tidal carbon dioxide tension, and sufficient monitoring of central venous pressure. From the central venous line, intermittent central venous oxygenation can be measured, and a close correlation with SvO2 has been shown.21 In high-risk patients, we add a pulmonary artery catheter with continuous SvO2 monitoring and transoesophageal echocardiography (TOE). SvO2 provides valuable information on tissue oxygenation and, although slower than PCCO, rapid changes in CO can be detected. SvO2 adds no direct information about the different determinates of CO, such as preload, contractility, and afterload, and for that purpose, intermittent bolus thermodilution and TOE can be applied. In OPCAB surgery, the TOE also permits the monitoring of valvular function and regional myocardial ischaemia.22
A limitation in our study is the lack of a gold standard method for continuous CO measurements. Since no such method exists, we cannot conclude which method reflects CO most accurately.
In our study, we compared the CO values each minute during surgery. This interval is long enough to obtain a new STAT-CO value, but is too short to detect large changes because of the time delay of the calculations. Since CO might change considerably during 1 min in OPCAB surgery, we think that a measurement interval of 1 min is clinically relevant.
The duration of Period 1 and Period 2 was different for each patient. Thus, different numbers of measurements were obtained, implying unequal weighting for each patient. Our results show that there is less agreement between PCCO and STAT-CO during haemodynamic instability. Thus, if haemodynamic stability is associated with the time of surgery, this might have affected our results on the agreement between the methods. However, the measurements at different time points were compared for the same patients (each patients being their own control), and this may compensate for weighting.
In conclusion, we found a poor agreement between PCCO and STAT-CO during OPCAB surgery. Clinically acceptable agreement was seen only for trends in CO during haemodynamically stable periods. We could not conclude which method was the most accurate. In our opinion, intermittent bolus thermodilution is still the reference method for measuring CO, whereas PCCO and STAT-CO may be used only for monitoring trends. Particularly during haemodynamic instability, these trends have to be interpreted with caution.
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