Comparison of Cardiac Index Measurements in Intensive Care Patients using Continuous Wave versus Pulsed Wave Echo-Doppler compared to Pulse Contour Cardiac Output

Abstract

Purpose:Cardiac index (CI) assessments are commonly used in critical care to define shock aetiology and guide resuscitation. Echocardiographic assessment is non-invasive and has high levels of agreement with thermodilution assessment of CI. CI assessment is derived from the velocity time integral (VTI) assessed using pulsed wave (PW) doppler at the level of the left ventricular outflow tract (LVOT) divided by body mass index (BMI). Continuous wave (CW) doppler offers an alternative means to assess VTI and may offer better assessment at high velocities. Methods:We performed a single centre, prospective, observational study in a 15-bed intensive care unit (ICU) in a busy district general hospital. Patients had simultaneous measurements of cardiac index by Pulse Contour Cardiac Output (PiCCO) (thermodilution), transthoracic (TTE) LVOT PW-VTI and TTE AV CW-VTI. Mean differences were measured with Bland-Altman (B-A) limits of agreement (LOA) and Percentage Error (PE) calculations.Results:Data was collected on 52 patients. Mean CIs were: AV CW 2.7 L/min/m2 (range 0.78 – 5.11, SD 0.92). LVOT PW 2.33 L/min/m2 (range 0.77 – 5.40, SD 0.90) and PiCCO 2.86 L/min/m2 (range 1.50 – 5.56, SD 0.93). AV CW VTI and PiCCO mean difference was -0.16 L/min/m2 PE 43.5%. LVOT PW VTI and PiCCO had a mean difference of -0.54 L/min/m2 PE 38.6%. AV CW VTI and LVOT PW VTI had a mean difference of 0.38 L/min/m2 PE 46.0%. Conclusions:CI derived from both AV CW-VTI and LVOT PW-VTI methods underestimate CI compared to PiCCO, with the AV method having closer values overall to PiCCO. AV CW-VTI may offer a more accurate assessment of CI.