Significance of continuous blood gas monitoring in cardiac surgery with cardiopulmonary bypass.

EDITOR: Standard non-invasive monitoring during anaesthesia usually includes pulse oximetry saturated pressure of oxygen (SPO2) and end-tidal carbon dioxide (end-tidal CO2). They frequently fail to monitor blood gas changes during cardiac surgery due to reduction in perfusion, ambient light, hypothermia and the use of vasoconstrictors [1]. It is also impossible to monitor SPO2 during cardiopulmonary bypass (CPB) because of the lack of pulsatile flow. Intermittent blood gas monitoring, the rule in cardiac surgery, often fails to detect the rapid changes that are frequent in the beginning and end of bypass. Continuous blood gas monitoring with the Paratrend 7 (Diametrics, Manchester, UK) could therefore be an interesting option. The aim of this prospective observational study was to compare conventional intermittent blood gas monitoring with the continuous Paratrend system in patients undergoing cardiac surgery with bypass. The study was approved by the hospital Ethics Committee. Ten consecutive patients (age 70 5 yr) were included with written informed consent. Anaesthesia was induced with etomidate, midazolam and sufentanil with cisatracurium or rocuronium for muscle relaxation. Anaesthesia was maintained with isoflurane. The inspired oxygen fraction during anaesthesia was 50% and this increased if needed to keep SPO2 97%. Monitoring included invasive blood pressure (BP), eight-lead ECG, central venous pressure (CVP), SPO2 and end-tidal CO2. Blood gases were assessed intermittently in the laboratory and continuously with the Paratrend 7 system. Two anaesthetists took part in the study. One was responsible for the standard patient care according to the departmental guidelines. He had access to the conventional blood gas analysis, SPO2 and end-tidal CO2. However, he was blinded to the Paratrend data. A second anaesthetist monitored the continuous Paratrend blood gas data. Abnormal Paratrend data were classified as Level 1 (abnormal, but not lifethreatening values) or Level 2 (dangerous). Level 1 and 2 limits are defined in Table 1. Level 2 abnormalities, with the exception of arterial pressure of oxygen (PaO2) 26 kPa, were immediately communicated to the anaesthetist in charge of patient care. The number of therapeutic actions performed on the basis of the results of conventional blood gas analysis, SPO2 and end-tidal CO2 were noted. An average of 6.6 blood gas values per patient was outside the predefined limits. The mean duration of Level 1 and 2 abnormalities for arterial pressure of oxygen and carbon dioxide (PaO2 and PaCO2, respectively), and pH in absolute time and as a percentage of operating time are shown in Table 1. The main abnormalities found with the Paratrend system were hyperoxaemia and hypocapnia. These deviations were also discovered by intermittent blood sampling but with a significant delay. According to the Paratrend data, the patients were hyperoxaemic 31% of the operating time. PaO2 often exceeded 26 kPa (average duration 57 min). Hyperoxaemia has been implicated in microcirculatory deterioration [2] and exacerbation of the inflammatory process post-bypass [3]. The risk of hypoxaemia is substantial during cardiac surgery with CPB. It can be due to the patient’s condition or to mechanical failure. However, episodes of intraoperative hypoxaemia found with the Paratrend were much less frequent than hyperoxaemia, representing only 8% of operating time. These episodes were revealed with a delay by traditional intermittent blood gas sampling, requiring the blind to be lifted on five occasions (Table 1). It seems that patients are voluntarily maintained hyperoxaemic, as anaesthetists fear hypoxaemia more than hyperoxaemia. The information provided by pulse oximetry played only a minor part in the management of the patients. In 10% of operating time (bypass excluded), the SPO2 Correspondence to: Andy Musat, Department of Anaesthesia, University Hospital Le Bocage – Dijon, Bd. Marechal de Lattre de Tassigny, 21000 Dijon, France. E-mail: [email protected]; Tel: 33 3 80 29 35 28; Fax: 33 3 80 29 35 57