Journal of clinical monitoring
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This article is the first segment of what we hope will become a regular feature in the Journal of Clinical Monitoring. The Monitoring Dilemmas series is designed for clinicians who are struggling with the information presented by new monitoring. The past decade has seen an explosion in the number and type of clinical monitors in daily use. ⋯ We welcome your comments on this new section. We also welcome your own Monitoring Dilemmas. Please contact the Section Editor, Dr Partridge, for more details.
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We developed a noninvasive computer-based system for estimating continuous cardiac output by a modified pulse contour method using a finger pressure waveform. The method requires no individual patient calibration or baseline cardiac output. First, we calibrated the system in a "learn" group of 20 patients. ⋯ The program demonstrated that information about cardiac output can be obtained by using the Finapres device (Ohmeda, Boulder, CO). The cardiac output values obtained by this continuous noninvasive technique were within +/- 20% of the simultaneous thermodilution values 87% of the time. This was true over the narrow range of cardiac outputs (2.9 to 6.4 L/min) and wide range of heart rates (45 to 140 beats/min).
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A thermodilution catheter and computer system has been developed to measure right ventricular ejection fraction and volumes. To evaluate the performance of this method, the thermodilution system was evaluated in an in vitro pulsatile flow model. Thermodilution measurements of ejection fraction (EF), cardiac output (CO), stroke volume (SV), end-diastolic volume (EDV), and end-systolic volume (ESV) were compared with known values in a pulsatile flow bench. ⋯ The standard deviations of the error for EDV and ESV were 11.0% and 16.4%. The thermodilution measurements were repeatable, with CO, SV, and EF coefficients of variation of 3.2%, 3.3%, and 4.7%, respectively. EDV and ESV were slightly more variable, with coefficients of variation of 5.5% and 7.2%, respectively.
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Simultaneous superior vena caval (Scvo2) and mixed venous (Svo2) oxyhemoglobin saturation values in 15 children recovering from open heart surgery were compared to assess the value of superior vena caval blood samples in monitoring systemic oxygen supply/demand balance. Samples were obtained immediately following the operation and postoperatively every morning for 4 days. During the 4-day study period, the patients' cardiopulmonary functions improved, allowing partial weaning from respiratory and cardiovascular support. ⋯ This difference may be secondary to residual intracardiac left-to-right shunting of blood or to altered distribution of systemic blood flow. The saturation difference between the two venous samples decreases during postoperative recovery, making a superior vena caval blood sample an inadequate substitute for a mixed venous blood sample in calculating derived cardiopulmonary variables intended to reflect the function of the body as a whole. Because Scvo2 was frequently subnormal while Svo2 was in the normal range, monitoring of Svo2 could not be reliably used to rule out oxygen supply/demand imbalance during the early postoperative period in these patients.
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Mixed venous oxygen saturation (SvO2) monitoring has been advocated for some critically ill patients. Patients with end-stage hepatic failure have oxygen consumption rates that are lower than normal. Using the Fick equation, oxygen consumption may be calculated if mixed venous and arterial oxygen tensions (and saturations), hemoglobin, and cardiac output are determined simultaneously. ⋯ The oxygen consumption rates decreased to the lowest point during the anhepatic phase and rose above baseline by the end of the case. The SvO2 and oxygen consumption data reported here follow the presence of presumed hepatic metabolic activity (increased CO2 and ionized calcium). Further research must be completed to determine whether these measurements indicate early hepatic nonfunction.