Technology, Computing, and Simulation: Case Report
Cardiac output monitoring by the carbon dioxide (CO2) rebreathing technique (COCR) has been developed and evaluated in several studies in the past decade. In previous studies, there was a relatively strong correlation between COCR and cardiac output, as measured by the thermodilution method (COTH) in postoperative patients and in critically ill patients (1,2). Pneumoperitoneum by CO2 during laparoscopy induces endocrine and physiological changes that have hemodynamic consequences. A transient increase in end-tidal CO2 tension (ETco2), such as at the time of tourniquet release, influences the accuracy of COCR. An increase in CO2 content in the blood may interfere with accurate monitoring by the COCR. We simultaneously monitored the COCR and COTH during laparoscopic adrenalectomy in a patient to examine whether pneumoperitoneum with CO2 affects the accuracy of COCR.
A 68-yr-old woman (height, 154 cm; weight, 70 kg) with a nonfunctional adrenal tumor was scheduled to undergo laparoscopic adrenalectomy. Her preoperative examinations were unremarkable. Continuous cardiac output monitoring by COCR and also by COTH during the surgery were scheduled. This procedure was approved by the ethics committee of the Second Hospital of Nippon Medical School. Written informed consent was obtained from the patient.
Premedication consisted of IM injection of atropine sulfate 0.5 mg and hydroxyzine chloride 25 mg 30 min before the induction of anesthesia. The patient received both epidural and general anesthesia. In the operating room, the left antecubital vein was secured, and an epidural catheter was placed through the T10-11 interspace. General anesthesia was induced by thiopental 4 mg/kg. Vecuronium bromide 0.2 mg/kg facilitated tracheal intubation. General anesthesia was maintained by isoflurane 0.5%–1.5% and oxygen 35% with a nitrous oxide mixture. An epidural injection of mepivacaine 2% 6–10 mL was performed every 30 min. Respiration was controlled using a semiclosed breathing circuit and an anesthesia machine (Aestiva 5™, Datex Ohmeda, Helsinki, Finland) with a tidal volume of 12 mL/kg; the respiratory rate was initially set at 10 breaths/min, and then it was adjusted to maintain the ETco2 at 35–45 mm Hg. Arterial blood pressure (BP) was monitored via the left radial artery.
The flow sensor and expandable dead space of the monitoring system using COCR (NICO2™, Novametrix™, Wallingford, CT) were positioned between the endotracheal tube and Y piece of the breathing circuit. The length of the expandable dead space was adjusted according to the tidal volume. The rebreathing valve was controlled to add 150–200 mL of volume to the breathing circuit over a 50-s period every 3 min, which caused an increase in ETco2 of 3–5 mm Hg. Cardiac output was calculated from the changes in ETco2, breath by breath, after the rebreathing phase. Data were acquired, calculated, and displayed every 3 min.
A thermodilution pulmonary artery catheter (Oximetry CCO Thermodilution catheter™, Edwards™, Irvine, CA) was inserted into the right internal jugular vein and guided by waveform to the pulmonary artery. Cardiac output was calculated by a cardiac output computer (Vigilance™, Edwards™). The cardiac output over every 1-min interval was calculated, and an average value over every 3 min was determined.
Samples of arterial and mixed venous blood were obtained at three time points during the nonrebreathing phase of the surgery (one before insufflation, and two during insufflation) and were subjected to blood gas analysis by a blood gas analyzer (IL Synthesis 25™, Instrumentation Laboratory™, Barcelona, Spain). The COCR, COTH, peak airway pressure, noninvasive BP, heart rate (HR), CO2 output (Vco2), and ETco2 were recorded every 9 min throughout the surgery. These variables were recorded 5 min after completion of intraabdominal insufflation. The insufflation pressure was maintained at approximately 10 mm Hg. During the surgery, the patient was kept in the head-up position at approximately 10 degrees from the supine position.
We examined the relationship between COCR and COTH using linear regression analysis. The degree of agreement between the two methods was assessed by Bland-Altman analysis.
There were no remarkable complications during the laparoscopic adrenalectomy. BP, HR, and cardiac output varied during the surgery depending on the type of surgical manipulation and depth of anesthesia; however, the BP and HR were maintained within relatively narrow ranges. The results of blood gas analysis before and during insufflation are shown in Table 1. The Paco2 values in the 2 blood samples that had been obtained at 54 min and 135 min after completion of insufflation were larger than that in the blood sample obtained before insufflation. The changes in COCR, COTH, and Vco2 before, during, and after insufflation of CO2 are shown in Figure 1. Twenty measurements of cardiac output were obtained by each of the two methods simultaneously during insufflation. The correlation coefficient between COCR and COTH was 0.78 (P < 0.05; Fig. 2). Bland-Altman analysis revealed that the COCR overestimated the CO compared with the COTH by an average of 0.3 L/min with sd of 0.14 L/min (Fig. 3). The Vco2 varied during the surgery. COCR and COTH were each correlated with Vco2 on linear regression analysis (R2 = 0.52 and 0.58, respectively; P < 0.05).
In our patient, there was a strong correlation between COCR and COTH during insufflation. The correlation coefficient between COCR and COTH was nearly identical to those obtained in studies on postoperative patients or critically ill patients (1,2). Although Schmid et al. (3) found that the COTH was not as accurate as cardiac output as measured by the bolus thermodilution reference technique, with a difference in cardiac output ranging from 0.1 to 0.3 L/min, they concluded that the continuous thermodilution technique may be acceptable in clinical settings because of the advantages of automated and continuous cardiac output monitoring. In the present study, the mean difference between COCR and COTH in Bland-Altman analysis, i.e., 0.3 L/min, is clinically acceptable. The CO2 COCR monitoring creates a periodical increase in ETco2, and cardiac output is calculated by the differential Fick equation. In the past decade, studies comparing cardiac output, as calculated by several prototype machines using the indirect Fick method and COTH, have been performed (4,5). The NICO2™ system, which was marketed in 1999, uses the ratio of the changes in ETco2 and Vco2 in response to a 50-second period of rebreathing to calculate the pulmonary artery blood flow. Cardiac output is then estimated by adding a correction factor for the pulmonary shunt using the Nunn iso-shunt nomogram (6). There have been few studies on the relationship between cardiac output as measured by the CO2 COCR and that measured by the COTH (1,2,7). While performing the COCR, the pulmonary capillary blood flow can be calculated for ETco2 values ranging from 25 to 80 mm Hg (8,9). During measurements of COCR, several theoretical assumptions and practical considerations must be satisfied including a steady-state environment, adequate recirculation time, and negligible pulmonary shunt.
For calculation of COCR, the mixed venous CO2 tension (Pvco2) should remain unchanged during the rebreathing phase. In the present case, recording of the variables, including the COCR and COTH, were recorded every nine minutes during the surgery; they were recorded five minutes after completion of abdominal insufflation when the Paco2 and Pvco2 were assumed to have reached the steady-state. Although CO2 insufflation induced mild increases in ETco2 and Pvco2 compared with the respective values before insufflation, the Pvco2 may have been stable during the rebreathing phase. The tidal volume in the present case was 12 mL/kg, which is within the recommended range for cardiac output monitoring by the NICO2™ system. Because Vco2 is highly dependent upon the tidal volume, the Vco2 did not significantly change during insufflation and was correlated with each of COCR and COTH. There may have been a correlation between COCR and COTH because CO2 insufflation did not affect the Vco2. The changes in ETco2 caused by CO2 insufflation may not interfere with accurate COCR as long as CO2 is constantly being eliminated.
In conclusion, we simultaneously measured cardiac output by two noninvasive methods during laparoscopic surgery in a patient. We found that CO2 insufflation did not interfere with COCR.
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© 2005 International Anesthesia Research Society
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