Left ventricular (LV) mass is a strong predictor of cardiovascular disease (CVD) events; increased LV mass is common among US firefighters and plays a major role in firefighter sudden cardiac death. We aim to identify significant predictors of LV mass among firefighters. Cross-sectional study of 400 career male firefighters selected by an enriched randomization strategy. Weighted analyses were performed based on the total number of risk factors per subject with inverse probability weighting. LV mass was assessed by echocardiography (ECHO) and cardiac magnetic resonance, and normalized (indexed) for height. CVD risk parameters included vital signs at rest, body mass index (BMI)–defined obesity, obstructive sleep apnea risk, low cardiorespiratory fitness, and physical activity. Linear regression models were performed. In multivariate analyses, BMI was the only consistent significant independent predictor of LV mass indexes (all, p <0.001). A 1-unit decrease in BMI was associated with 1-unit (g/m 1.7 ) reduction of LV mass/height 1.7 after adjustment for age, obstructive sleep apnea risk, and cardiorespiratory fitness. In conclusion, after height-indexing ECHO-measured and cardiac magnetic resonance–measured LV mass, BMI was found to be a major driver of LV mass among firefighters. Our findings taken together with previous research suggest that reducing obesity will improve CVD risk profiles and decrease on-duty CVD and sudden cardiac death events in the fire service. Our results may also support targeted noninvasive screening for LV hypertrophy with ECHO among obese firefighters.
Despite the critical prognostic significance of left ventricular (LV) mass, its measurement and role in clinical practice have yet to be established. Echocardiography (ECHO) and cardiac magnetic resonance (CMR) are the 2 most commonly used imaging methods for the assessment of LV mass. Although, CMR is considered the gold standard for assessing LV mass, ECHO is a well validated, noninvasive method that is more widely used in clinical practice. In addition to considering different imaging methods, disagreement exists as to the most appropriate method of indexing LV mass to body size parameters. Current evidence suggests indexing by height to the allometric powers of 1.7 and 2.7 are the most accurate normalization techniques. This study identifies the most important predictors of LV mass after indexing for height among career male firefighters as assessed by both ECHO and CMR.
Methods
Male career firefighters, aged 18 years and older were recruited from the Indianapolis Fire Department. Eligible firefighters had a recorded fire department–sponsored medical examination in the last 2 years that included a submaximal exercise tolerance test and had no restrictions on duty.
From those eligible, we selected a total of 400 participants, using an “enriched” randomization strategy based on age at randomization, obesity, hypertension, and cardiorespiratory fitness status at last examination, where a larger number of higher risk participants could be selected. Thus, we randomly selected 100 participants from the entire eligible population; 75 low-risk participants (age <40, nonobese, free of hypertension, and high cardiorespiratory fitness) and 225 higher risk participants (at least 2 of the following: age ≥40 years, obese, hypertension, or low cardiorespiratory fitness) for further LV hypertrophy/cardiomegaly screening and imaging tests. Obesity was defined by standard criteria (body mass index [BMI] ≥30 kg/m 2 ). Hypertension was considered present if blood pressure at rest is ≥140/90 mm Hg. Low cardiorespiratory fitness was defined as the bottom tertile, as measured by the recorded treadmill time and the estimated maximal VO 2 during the last exercise test. Those selected were included in the study if they had no contraindication to CMR and signed informed consent to participate.
LV mass was assessed by both ECHO and CMR imaging. First, a transthoracic cardiac echocardiogram was done as a simple 2-dimensional study with limited m-mode recordings. An abbreviated CMR was performed as “function only” immediately after the ECHO. Images were obtained using a retrospectively electrocardiogram gated steady-state free precession cine sequence. In this fashion, a contiguous short-axis stack of 8-mm slices was obtained parallel to the atrioventricular groove to cover the entire length of the LV. Then, manual tracing of end-diastolic epicardial and endocardial borders was performed. Standard long-axis views were also obtained including horizontal long-axis, vertical long-axis, and 3-chamber views, facilitating the interpretation of ventricular function. Board certified specialists performed clinical interpretation of imaging. LV mass indexes were derived by dividing LV mass in grams with height to the allometric powers of 1.7 and 2.7 (in meters 1.7 and meters 2.7 , respectively).
Height was measured in the standing position with a clinic stadiometer. Body weight was measured with bare feet and in light clothes on a calibrated scale. BMI was calculated as the weight in kilograms divided by the square of height in meters. Blood pressure was measured using an appropriately sized cuff with the subject in the seated position. Heart rate and blood pressure were obtained in a resting state from the physical examination (and were not measured before the exercise test). Medical examination data were further supplemented by a preimaging questionnaire, which collected comprehensive information on smoking status, a history of heart rhythm problems, family history of cardiac problems, and moderate to vigorous physical activity level in minutes per week. Obstructive sleep apnea risk was assessed using the validated Berlin Questionnaire.
We performed a weighted analysis so as to account for our enriched randomization sampling strategy. Weights were calculated based on the total number of risk factors per subject with the technique of inverse probability weighting. Baseline characteristics were described using the mean (SD) in the case of quantitative variables and the frequency (%) for categorical variables. The effects of the different independent variables on the LV mass indexes were assessed with the use of linear regression models. Any independent variables that were significant in the univariate regression models were included in the multivariate regression models. In the multivariate analysis, we followed the backward stepwise elimination process with a removal criterion of alpha = 0.20. Then, considering the predictors that resulted from the backward elimination process and variables that we knew a priori to be important clinical predictors, we constructed the final multivariate regression models. The interaction effects between BMI with obstructive sleep apnea risk and age were also assessed in these models. Collinearity was evaluated using the variance inflation factor. Analyses were performed using SPSS, version 21.0 (IBM, Armonk, New York). A p value of <0.05 was considered statistically significant, and all tests performed were 2 sided.
Results
Of the 400 firefighters, we excluded 7 participants with missing measurements of LV mass, assessed by CMR. Baseline characteristics are summarized in Table 1 . The mean age of the study subjects was 45 (8.1) years, their mean BMI was 30 (4.5) kg/m 2 and 45% were obese.
| Variables | Study Sample (N = 393) | Study Sample Unweighted |
|---|---|---|
| Age (years) ∗ | 47 ± 8.2 | 45 ± 8.1 |
| Height (cm) ∗ | 179± 6.4 | 179 ± 6.6 |
| Heart rate (bpm) ∗ | 81 ± 13 | 80 ± 13 |
| Body weight (kg) ∗ | 99 ± 17 | 97 ±17 |
| Resting systolic blood pressure (mm Hg) ∗ | 126 ± 9.7 | 125 ± 9.4 |
| Resting diastolic blood pressure (mm Hg) ∗ | 82 ± 8.1 | 81 ± 7.4 |
| High risk of obstructive sleep apnea † | 112 (38%) | 254 (32%) |
| Body mass index (kg/m 2 ) ∗ | 31 ± 4.6 | 30 ± 4.5 |
| Smoker † | 50 (13%) | 135 (13%) |
| History of heart rhythm problems † | 60 (16%) | 153 (15%) |
| Family history of cardiac problems † | 153 (40%) | 426 (41%) |
| Age ≥ 40 (years) † | 301 (78%) | 770 (73%) |
| Body mass index ≥ 30 (kg/m 2 ) † | 260 (56%) | 474 (45%) |
| Low cardiorespiratory fitness † | 178 (47%) | 363 (34%) |
| Moderate to vigorous physical activity (min/week) ∗ | 177 ± 117 | 187 ± 118 |
| LV mass by echocardiography (g) ∗ | 189 ± 38 | 187 ± 37 |
| LV mass by cardiac magnetic resonance (g) ∗ | 139 ± 24 | 138 ± 23 |
| LV mass by echocardiography indexed to height 1.7 (g/m 1.7 ) ∗ | 70 ± 13 | 70 ± 13 |
| LV mass by echocardiography indexed to height 2.7 (g/m 2.7 ) ∗ | 40 ± 7.6 | 39 ± 7.4 |
| LV mass by cardiac magnetic resonance indexed to height 1.7 (g/m 1.7 ) ∗ | 52 ± 8.3 | 51 ± 8.1 |
| LV mass by cardiac magnetic resonance indexed to height 2.7 (g/m 2.7 ) ∗ | 29 ± 4.7 | 29 ± 4.6 |
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