Study population and methods
The study population consisted of 35 consecutive adult patients admitted to our hospital for elective coronary artery bypass grafting. Exclusion criteria were: age <18 years, preoperative left ventricular ejection fraction (LVEF) <30 %, preoperative left ventricular dilatation (end-diastolic dimension ≥6 cm), preoperative severe tricuspid valve regurgitation, preoperative right ventricular dysfunction (tricuspid annular plane systolic excursion <16 mm), at least moderate aortic valve disease, at least moderate mitral valve disease and difficult acoustic window resulting in inability to obtain interpretable ultrasound images. Coronary artery bypass grafting was performed using cardiopulmonary bypass. In all patients full midline sternotomy was made. Mediastinal drainage (midline drain tubes) was placed in all patients. Additionally, those with arterial graft with left internal mammary artery had a drain placed in left pleural cavity. All patients were operated in moderate hypothermia (32–33 °C) and warmed up to 36.6 °C within two hours following the end of the operation. Total intravenous anesthesia with propofol, sufentanil and pancuronium was used during the procedure. Propofol infusion was continued for 1-hour in the ICU and morphine infusion was used for postoperative pain relief. Cardiac ultrasound was performed when the patients were ventilated (SIMV-mode, tidal volume: 8 ml/kg, PEEP: 4.5 cmH2O).
The following baseline data were recorded for each patient: age (years), gender, weight (kg), height (cm), number of bypassed vessels, aortic cross-clamping duration, cardiopulmonary bypass duration, and preoperative echocardiographic parameters (left ventricular ejection fraction, presence of left ventricular hypertrophy, right ventricular end-diastolic diameter and tricuspid regurgitation grade). Pulse pressure was calculated as the difference between systolic and diastolic pressure readings (expressed in mmHg). The use of the following vasoactive drugs was noted: nitroglycerine, dopamine, dobutamine, epinephrine, norepinephrine.
Transthoracic bedside echocardiography was performed by two trained investigators, both with at least 5 years of experience in emergency ultrasound. The examinations were conducted with portable ultrasound system equipped with a 1–5 MHz transthoracic phased-array transducer (CX 50 Philips, Eindhoven, Netherlands). All the studies were recorded as digital clips and reviewed independently by both sonohraphers.
Inferior vena cava was visualized longitudinally in the subcostal view. Maximal and minimal IVC diameters (IVCmax and IVCmin, retrospectively) were measured using 2D image, distally to the hepatic vein inlet, over a single respiratory cycle. A total of three measurements were obtained and averaged for each IVC diameter. The IVC collapsibility index (IVC-CI) was defined as: IVC-CI = IVCmax - IVCmin/IVCmax. The IVC distensibility index (IVC-DI) was calculated using the formula: IVC-DI = IVCmax - IVCmin/IVCmin. Both indices were expressed as a percentage.
Cardiac output (CO) was calculated from the left ventricular outflow tract (LVOT), using the previously described equation : CO = 0.785 x dLVOT2 x VTI LVOT x HR. The left ventricular outflow tract diameter (dLVOT) was measured in midsystole, in a parasternal long-axis view immediately adjacent to the aortic valve. LVOT velocity time integral (LVOT VTI) was recorded by pulsed Doppler imaging from a three-chamber or five-chamber apical view. Left ventricular ejection fraction (LVEF) was assessed by visual inspection.
Hemodynamic and ultrasound data were obtained at four sequential steps per enrollment: (1) at baseline, immediately after cardiac surgery procedure, when the patient was transferred to intensive care unit, in the semirecumbent position (45 °); (2) during passive leg raising (after 1 min), when the patient’s lower limbs were raised to a 45 ° angle while the patient’s trunk was lowered in supine position; (3) in the semirecumbent position (as a baseline data for the next stage); (4) after a 10-min infusion of 250 ml of saline, in the semirecumbent position. Fluid balance immediately after cardiac surgery was noted. The following parameters were recorded at each study step: heart rate (HR, bmp), systolic blood pressure (SPB, mmHg), diastolic blood pressure (DBP, mmHg), central venous pressure (CVP, mmHg), maximal (IVCmax, mm) and minimal (IVCmin, mm) diameters of inferior vena cava, left ventricular ejection fraction (LVEF, %), velocity time integral in left ventricular outflow tract (VTI LVOT, cm). Fluid responsiveness was defined as an increase in cardiac output ≥15 % after the fluid challenge that defined patients as responders and non-responders. Any clinically significant findings (e.g. severe left ventricular dysfunction, cardiac tamponade) were immediately reported to the treating physician.
Statistical analysis was performed using STATISTICA v 8.0 software. The required size of the study group was determined with power analysis. Using prior estimates of expected correlation, a sample size of 9 for each subgroup (responders and nonresponders) was determined, with α = 0.05 and β = 0.8. Numerical data were expressed as mean values ± SD. After determining the probability distribution with Shapiro-Wilk test, the comparisons between fluid responders and nonresponders were performed with U Mann–Whitney test. A p-value of 0.05 was considered significant. The correlations between different parameters were evaluated with r-Spearman correlation analysis. Scatterplots for the two variables were drawn with confidence interval 0.95. The area under the receiver operator characteristics curve (AUROC) was used to determine the diagnostic accuracy of the methods.