Background
Inferior vena cava (IVC) diameter and its respiratory change, as determined using echocardiography, are commonly used to assess right atrial pressure (RAP). Despite the widespread use of the IVC approach for RAP assessment, the relations among body surface area (BSA), IVC diameter, and respirophasic change remain unclear. The aim of this study was to investigate the impact of BSA on IVC parameters for predicting elevated RAP.
Methods
Ninety consecutive patients undergoing right-heart catheterization or central venous catheter insertion were prospectively included. To investigate the impact of BSA on IVC parameters, patients were divided into higher and lower BSA groups by comparing individual BSA measurements with the median value. Optimal cutoff points of IVC parameters for detecting RAP of ≥10 mm Hg were defined using receiver operating characteristic curves.
Results
The median RAP and BSA were 8 mm Hg (range, 1–25 mm Hg) and 1.61 m 2 (range, 1.23–2.22 m 2 ), respectively. In all patients, the optimal cutoff point for maximal IVC diameter (IVCD max ) and IVC collapsibility for the detection of RAP ≥ 10 mm Hg were 20 mm and 49.0%, respectively. The optimal cutoff point of IVCD max for predicting RAP of ≥10 mm Hg was significantly larger in patients with higher BSAs than in those with lower BSAs (21 vs 17 mm, P = .0342). No differences in collapsibility indices were detected between the two groups. IVCD max was larger in men (19 ± 5 vs 17 ± 5 mm in women, P = .0347) and weakly correlated with BSA ( r = 0.35, P = .0007), whereas no relation was found between IVCD max and age. However, the partial correlation coefficient of the entire cohort demonstrated that only BSA was still associated with IVCD max after adjusting for age and gender (partial correlation coefficient = 0.32, P = .0020).
Conclusions
Body size, measured as BSA, is important to consider when IVC diameter is used to assess RAP. The optimal cutoff point of IVCD max was 21 mm for patients with larger BSAs and 17 mm for those with smaller BSAs. However, the cutoff point of IVC collapsibility was not influenced by the difference of BSA.
Highlights
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The Impact of body size on IVC parameters to predict RAP was studied.
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Patients were divided into two groups according to BSA.
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The optimal diameter cutoff was smaller in smaller patients than in larger patients.
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The optimal collapsibility index cutoff did not differ by body size.
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Body size may need to be considered when IVC diameter is measured.
Assessment of volume status represents a pivotal component in the care of patients with heart failure. In particular, right atrial pressure (RAP) is a simple and objective index of intravascular volume status and ventricular function and has recently been reported as an independent prognosticator for overall survival in patients with heart failure.
The measurement of inferior vena cava (IVC) diameter and degree of inspiratory IVC collapse determined by two-dimensional echocardiography are commonly used to estimate RAP in the clinical setting. The current guidelines proposed by the American Society of Echocardiography suggest a maximal IVC diameter of >21 mm in conjunction with an IVC collapse of <50% as cutoff thresholds for predicting RAP of ≥10 mm Hg. To date, the IVC approach has been studied mainly for American and European populations. In addition, because the size of organs is considered to vary with body size, many echocardiographic variables are indexed to body surface area (BSA). Despite the widespread use of the IVC approach for RAP assessment, the relation between BSA and IVC parameters remains unclear.
In the present study, we investigated the impact of BSA on several IVC parameters and attempted to determine the optimal cutoff values for predicting elevated RAP in patients on the basis of BSA.
Methods
Study Patients
The study protocol was approved by the institutional ethics committee, and written informed consent was obtained from all patients. In this single-center study, we prospectively evaluated patients with heart disease who were scheduled for right-heart catheterization or central venous catheter insertion between December 2012 and June 2013. Patients on mechanical ventilation were excluded. Echocardiography was performed within 3 hours before catheterization. For patients hospitalized for scheduled examinations, echocardiography was performed at the bedside before catheterization, and subsequently we determined central venous pressure in the catheterization laboratory. For patients hospitalized urgently with acute decompensated heart failure, the pulmonary artery catheter was indwelled at the catheterization laboratory for accurate hemodynamic monitoring, and thereafter central venous pressure measurement and echocardiography were performed simultaneously at the bedside. No patients were sedated for either procedure in our study.
Invasive RAP Measurement
RAP was measured in the end-expiratory period using a flow-directed pulmonary artery catheter or central venous catheter inserted via the internal jugular vein. The observer was blinded to the echocardiographic data. Pressure calibration was performed before and after pressure measurements. All readings were referenced to the midaxillary line with the patient in a supine position. Pressure measurements were performed during three to five respiratory cycles, and the mean value was calculated.
Echocardiographic Measurements
Echocardiography of the IVC was performed with patients in the supine position using a Vivid q device (GE Healthcare, Milwaukee, WI) by an experienced sonographer blinded to patients’ data. IVC images were recorded in a standard echocardiographic subcostal longitudinal view ( Figure 1 ). The intrahepatic segment of the IVC was visualized as it entered the right atrium. Taking care to measure IVC dimensions at the same cranial-caudal level throughout the respiratory cycle, the maximal IVC diameter (IVCD max ) and minimal IVC diameter (IVCD min ) were measured just proximal to the junction of the hepatic veins that lie approximately 0.5 to 3.0 cm proximal to the ostium of the right atrium. Patients were then asked to sniff to estimate the smallest IVC size (IVCD sniff ). The IVC collapsibility index with passive respiration was computed using the formula [(IVCD max − IVCD min )/IVCD max ] × 100. The IVC collapsibility index with sniff inspiration was calculated by substituting IVCD sniff for IVCD min in the equation used to estimate IVC collapsibility index with passive respiration. All values presented are the average of three consecutive measurements.
Calculation of BSA
The BSA of each patient was calculated using the Shintani method (modified formula of Du Bois method for Japanese ) as follows:
BSA ( m 2 ) = weight ( kg ) 0 . 425 ×height ( cm ) 0 . 725 ×0 . 007358 .
Statistical Analysis
Categorical variables are presented as percentages, symmetrically distributed continuous variables are presented as mean ± SD, and asymmetrically distributed continuous variables are presented as medians and interquartile ranges. The Welch t test or Wilcoxon rank sum test was used for continuous variables to compare between groups, while the χ 2 test or Fisher exact test was used for categorical variables.
Receiver operating characteristic (ROC) curves were estimated to determine the optimal cutoff points of IVC variables for detecting RAP ≥ 10 mm Hg. The optimal cutoff points were determined on the basis of the Youden method. The 95% CIs of the areas under the ROC curves (AUCs) were estimated using the bootstrap method with 100,000 resamplings. To compare correlated AUCs, we used the method proposed by DeLong et al . Spearman correlation coefficients, optimal cutoff points, and AUCs of IVC parameters and those indexed to BSA were assessed in the whole cohort. We also examined the use of alternative indexes, including powers of height and BSA 0.5 .
The interaction between IVCD max and BSA was explored using linear regression models with RAP as the dependent variable. In particular, BSA was treated both as a continuous variable and as a categorical variable dichotomized by its median. The difference of optimal cutoff points in higher and lower BSA subgroups was tested using the bootstrap method with 100,000 resamplings.
The associations of IVCD max with age and BSA were evaluated on the basis of Spearman correlation coefficients, while that with gender was assessed by examining the difference between men and women. To eliminate the effects of age and gender on the relation between IVCD max and BSA, the partial correlations were estimated.
Statistical analyses were performed using JMP version 10.0 (SAS Institute Inc., Cary, NC) and R version 3.1.1 (R Development Core Team, Vienna, Austria). The significance level for statistical hypothesis testing was set at .05.
Results
Patient Characteristics
Of the 99 consecutive patients screened for the study, nine (9.1%) were excluded because of insufficient image quality for assessing IVC parameters. The patients’ demographic data and clinical diagnoses are presented in Table 1 . The median age of our cohort was 62 years (range, 17–91 years), and 66.7% were men. The mean height was 163.3 ± 8.8 cm, and the median BSA was 1.61 m 2 (range, 1.23–2.22 m 2 ). The histogram of BSA in our cohort is displayed in Figure 2 .
| Variable | Value |
|---|---|
| Patient demographics | |
| Age (y) | 62 (45–72) |
| Men | 60 (66.7%) |
| Height (cm) | 163.3 ± 8.8 |
| Weight (kg) | 56.4 ± 12.6 |
| BSA (m 2 ) | 1.64 ± 0.2 |
| BMI (kg/m 2 ) | 21.0 ± 3.6 |
| Clinical diagnosis | |
| Nonischemic cardiomyopathy | 32 (35.6%) |
| Ischemic cardiomyopathy | 10 (11.1%) |
| Post–valvular surgery | 9 (10.0%) |
| Post–cardiac transplantation | 8 (8.9%) |
| Pulmonary hypertension | 8 (8.9%) |
| Valvular | 7 (7.8%) |
| Hypertensive heart disease | 4 (4.4%) |
| Acute coronary syndrome | 2 (2.2%) |
| Others | 10 (11.1%) |
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