(Circulation. 2006;113:1597-1604.)
© 2006 American Heart Association, Inc.
Imaging |
From the Donald W. Reynolds Cardiovascular Clinical Research Center, Division of Cardiology, Department of Internal Medicine, University of Texas Southwestern Medical Center, Dallas.
Reprints requests to Dr Mark Drazner, University of Texas Southwestern Medical Center, 5323 Harry Hines Blvd, Dallas, TX 75390-9047. E-mail Mark.Drazner{at}utsouthwestern.edu
Received July 11, 2005; revision received January 18, 2006; accepted January 20, 2006.
| Abstract |
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Methods and Results Using cardiac magnetic resonance imaging in a probability-based sample of Dallas County residents aged 30 to 65 years (1435 women and 1183 men), we compared LVEF in women and men. The association of gender with stroke volume independent of end-diastolic volume (EDV) or other potential confounders was assessed by multivariable analysis. Gender-specific thresholds for a low LVEF were defined at the 2.5th percentile in women and men from a healthy reference subpopulation. The median (25th, 75th percentile) LVEF was higher in women than in men (75% [70%, 79%] in women versus 70% [65%, 75%] in men, P<0.001). Left ventricular EDV and end-systolic volume indexed to body surface area were smaller in women than in men (P<0.001 for both). Gender remained significantly associated with stroke volume, independent of EDV and other potential confounders in multivariable analysis. A low LVEF was defined as below 61% in women and below 55% in men.
Conclusions Women have a higher LVEF than men in the general population, secondary to a higher stroke volume for a given EDV independent of known potential confounders.
Key Words: cardiac volume epidemiology heart failure magnetic resonance imaging myocardial contraction
| Introduction |
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Clinical Perspective p 1604
To further elucidate this issue, we examined the following 2 hypotheses: (1) LVEF as measured by cardiac MRI will systematically differ between women and men in the general population, and (2) this gender difference will be independent of potential confounders, including differences in left ventricular volume.
| Methods |
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In the present study, we included subjects who completed all 3 visits of the DHS and had a measurable LVEF (n=2705). From those, we excluded participants who self-reported valvular abnormalities (n=81), congenital heart defects (n=4), or both (n=2), leaving a final study sample of 2618.
Cardiac MRI
MRI was performed by using 2 comparable 1.5-T MRI systems (Philips Medical Systems, Best, the Netherlands) as previously described.2 Briefly, short-axis breath-hold electrocardiographic-gated cine magnetic resonance images were obtained from the apex to the base of the left ventricle with use of the following parameters: slice, 6 mm; gap, 4 mm; field of view, 36 to 40 cm; acquired pixel size at 36-cm field of view, 1.29x2.58; and temporal resolution, 40 ms. A turbo field echo sequence (TFE) was used in 93.5% of the cohort. A steady-state free precession sequence (SSFP) was used in the remaining 6.5%, and these values were adjusted by using regression equations derived from a subset of subjects (n=60) who had both sequences performed. Thus, all values are reported for a TFE sequence to permit comparison with prior studies. The regression equations were as follows: EDVTFE (mL) = 0.8020xEDVSSFP+11.904; ESVTFE (mL) = 0.8172xESVSSFP1.0188; LVMTFE (g) = 0.9588xLVMSSFP+17.53, where EDV is end-diastolic volume, ESV is end-systolic volume, and LVM is left-ventricular mass. MASS software (Medis Medical Imaging Systems, Leiden, the Netherlands) was used to process the images. End-diastolic and end-systolic endocardial borders were traced manually to measure the left ventricular cavity in each short-axis image. Portions of the papillary muscles in continuity with the left ventricular wall were included in the myocardial mass and excluded from the left ventricular volume. The software also permitted inclusion of the intracavitary papillary muscle components as needed. Measurements from each slice were summed by the method of disks. LVEF was calculated from the endocardial volumes: EF=100(EDVESV)/EDV. Interobserver difference for left ventricular ESV was 2.6±2.7 mL, intraobserver difference was 2.7±0.4 mL, and interscan variability was 0.3±3.5 mL. Interobserver difference for left ventricular EDV was 3.0±9.4 mL, intraobserver difference was 2.7±3.2 mL, and interscan variability was 2.3±8.9 mL. Interobserver difference for LVEF was 4±4%, intraobserver difference was 1±1%, and interscan variability was 1.3±2.5%.
Blood Pressure
Four measurements of blood pressure were obtained by use of a nonferromagnetic blood pressure cuff for the arm and a Welch Allyn automatic blood pressure monitor:11 Measurements were taken (1) before the start of the scan outside the magnet, (2) before the scan with the patient inside the magnet, (3) after the scan with the patient inside the magnet, and (4) immediately after the scan with the patient outside the magnet. We averaged the blood pressure from measurements 2 and 3 (n=2355) when both were available; otherwise, we used whichever of these 2 measurements was available (n=132), we averaged measures 1 and 4 (n=63), or we used either measure 1 or 4 (n=42). Given the influence of blood pressure and afterload on LVEF, these blood pressure measurements obtained in proximity to the MRI scan were used in analysis rather than those obtained at a different time during the third clinic visit.11
DEXA and Electron BeamComputed Tomography
DEXA scans were performed with the use of a dual-beam absorption energy unit (Delphi W unit, Hologic, Inc, Bedford, Mass) bone densitometer in array mode with Oasis software to measure body composition, including fat mass and fat-free mass.2 Coronary artery calcium was measured with the use of an Imatron C-150XP electron beamcomputed tomography (EBCT) scanner (Imatron, Inc, San Bruno, Calif). Presence of coronary artery calcium was defined as an EBCT score of >10 Agatston units.12
Study Definitions
Gender, ethnicity, and age were self-reported. Heavy alcohol use was defined as >255 g of alcohol consumption per week, 2 standard deviations above the mean. Cocaine use was self-reported. Body surface area (BSA) was calculated with the use of gender-specific equations derived by Tikuisis et al.13 Hypertension was defined as blood pressure
140/90 mm Hg or taking antihypertensive medication. Diabetes mellitus was defined as a fasting serum glucose
126 mg/dL, nonfasting serum glucose
200 mg/dL, or self-reported diabetes with use of hypoglycemic medication. History of myocardial infarction was determined by a yes answer to "Has a doctor or other health professional ever told you that you had a heart attack? This includes myocardial infarction, coronary occlusion, or coronary thrombosis." History of congestive heart failure was determined by a yes answer to "Has a doctor or other health professional ever told you that you have congestive heart failure, an enlarged heart, a weak heart, or cardiomyopathy?"
Data Analysis
Baseline characteristics were compared between men and women by the Student t test for continuous variables and
2 test for discrete variables. Smoothed histograms of LVEF and left ventricular EDV or ESV (indexed to BSA) were constructed for each gender. The relationship of EDV and ESV with LVEF was found to be nonlinear, so we log-transformed ventricular volumes and logit-transformed LVEF. For similar reasons, we log-transformed stroke volume when we assessed its relationship with log EDV. To determine whether gender was associated with stroke volume independent of EDV, linear regression was performed with the dependent variable log stroke volume and independent variables of log EDV and gender. The P value associated with the ß coefficient of the gender term was used to determine whether the intercept of the EDVstroke volume relationship differed by gender. An interaction term (gender x log EDV) was entered into this model to determine whether the slope of the EDVstroke volume relationship also differed by gender. To determine whether our findings were independent of confounders, multivariable linear regression was performed (dependent variable, log stroke volume; independent variables, log EDV and gender) with the following covariates incorporated: mean arterial pressure, height, weight, and self-reported history of heart failure or myocardial infarction, diabetes mellitus, or alcohol or cocaine use. An interaction term (gender x log EDV) was then entered into these models to determine whether there were slope differences between genders in the EDVstroke volume relationship. A series of additional models was constructed with alternative potential confounders to determine the robustness of our conclusions. A similar series of models was constructed with log-transformed stroke work (stroke volume x mean arterial pressure) as the dependent variable. Additionally, we performed a subgroup analysis excluding subjects with prior myocardial infarction, congestive heart failure, or positive coronary artery calcification (CAC) score (n=2060, including 1204 women and 856 men). We also performed a second subgroup analysis excluding only subjects with hypertension (n=1788, including 958 women and 830 men).
To establish gender-specific threshold values for low LVEF, we selected a healthy reference subpopulation (n=718, including 308 men and 410 women) from our overall study cohort by excluding participants with a body mass index >30 kg/m2, those with a self-reported history of valvular heart disease, congenital heart disease, hypertension, heart failure, myocardial infarction, diabetes, or heavy alcohol or cocaine use, and those with left ventricular hypertrophy as measured by cardiac MRI (LVM, >89 g/m2 for women and >112 g/m2 for men) or CAC as measured by EBCT. A gender-specific low LVEF was defined as being below the 2.5th percentile of women or men in this reference healthy subpopulation.
SUDAAN (Research Triangle Institute, Research Triangle Park, NC) was used to create population-weighted histograms of LVEF in male and female residents of Dallas County. For all other statistical analyses, SAS 9.13 (SAS Institute, Inc, Cary, NC) was used. A value of P<0.05 was considered to be statistically significant.
The authors had full access to the data and take full responsibility for its integrity. All authors have read and agreed to the article as written.
| Results |
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Comparing Ejection Fraction and Ventricular Volumes in Men and Women
The distribution of LVEF was shifted to the right (ie, higher LVEF) in women as compared with men (Figure 1A), whereas the distributions of left ventricular ESV indexed to BSA (Figure 1B) and EDV indexed to BSA (Figure 1C) were shifted to the left in women as compared with men (ie, lower volumes in women). The median (25th, 75th percentile) LVEF was 75% (70%, 79%) in women versus 70% (65%, 75%) in men (P<0.001), and the median ESV indexed to BSA was 12 (9.7, 15) mL/m2 in women versus 16 (13, 20) mL/m2 in men (P<0.001). Similarly, the median indexed EDV was smaller in women than in men: 49 (43, 55) mL/m2 versus 54 (48, 61) mL/m2, respectively (P<0.001). A low LVEF defined as below the 2.5th percentile of a healthy subset of the DHS was 61% in women and 55% in men.
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Relationship Between Left Ventricular Volumes and Ejection Fraction, Overall and by Gender
Scatterplots demonstrated that the relationships of indexed ESV (Figure 2A) or indexed EDV (Figure 2B) with LVEF were nonlinear. Log transformation of ventricular volume and logit transformation of LVEF, which improved the linear fit (especially for ESV), are shown for ESV (Figure 2C) and for EDV (Figure 2D). In both cases, an increased indexed left ventricular volume was associated with a decreased LVEF. However, the percentage of variation of LVEF explained by indexed ESV was substantially higher (r2=0.71, P<0.001) than that explained by indexed EDV (r2=0.05, P<0.001). The relationship between ESV and LVEF in men and women overlapped (Figure 3), and in a gender-combined model, neither the intercept nor slope differed significantly between men and women, demonstrating that for any given ESV, men and women would have a comparable LVEF. Because of this finding and the poor correlation of EDV and LVEF (Figure 2D), we focused on comparing the relationship of EDV and stroke volume in men and women, reasoning that this analytic approach would be a more robust method to determine whether there were gender differences in LVEF independent of EDV (inasmuch as LVEF=stroke volume/EDV).
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Relationship of EDV to Stroke Volume and Stroke Work in Men and Women
Log stroke volume and log EDV were highly correlated (r2=0.76, P<0.001). For a given EDV, women had a higher stroke volume (leading to a smaller ESV and higher LVEF) than did men (Figure 4A). The intercept but not slope of the relationship of log stroke work (stroke volume x mean arterial pressure) versus log EDV also differed significantly between women and men (Figure 4B).
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The correlation of LVEF with blood pressure (either mean arterial pressure or systolic blood pressure) or with age was poor in men and women (r2<0.01 for all). In multivariable linear regression analysis, the EDVstroke volume relationship had a higher intercept and significantly different slope in women than in men, independent of other potential confounders (Table 2). To determine whether the association of gender and stroke volume was robust, a series of alternative multivariable regression models were constructed in which the following covariates were substituted: CAC for myocardial infarction; body mass index, BSA, or fat mass and fat-free mass for height and weight; and systolic blood pressure or pulse pressure for mean arterial pressure. In each case, the slope and intercept of the EDVstroke volume relationship remained significantly different in men and women (P<0.05 for various slopes, except P=0.06 when fat mass and fat-free mass were substituted for height and weight). In subgroup analysis excluding participants with myocardial infarction, congestive heart failure, and positive CAC score as well as in subgroup analysis excluding for hypertension, the EDVstroke volume relationship had a higher intercept in women than men (P<0.001), but there was no significant slope difference. The slope of the relationship between log stroke work and EDV (Figure 4B) remained the same in women and men in multivariable analysis (P=0.9).
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As crude indexes of contractility, we divided systolic blood pressure by ESV indexed to BSA. This ratio was significantly higher in women than in men (3.3±1.6 versus 2.1±0.9 mm Hg/mL per m2, respectively; P<0.001). The gender difference in this ratio persisted whether ESV was not indexed or indexed to fat-free mass or height2.7. We also divided stroke work (stroke volume x mean arterial pressure) by EDV to give a single point estimate of preload recruitable stroke work. This ratio was also significantly higher in women than in men (73±12 versus 69±12 mm Hg, P<0.001).
| Discussion |
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Others have used echocardiography to address whether the LVEF differs in women and men. Early studies of small cohorts have shown LVEF to be higher in women than in men,15 with some finding gender differences in fractional shortening only in subjects over 55 years of age.16 A more recent study from Brazil has reported that EF is not different in men and women.17 In contrast, most studies with larger cohorts have shown that the prevalence of systolic dysfunction (ie, low LVEF) is more than 2-fold higher in men than in women.3,1822 Additionally, the mean LVEF in women has been found to be 1% to 5% higher36 and to persist in multivariable models.3,4,6
LVEF in men and women has also been compared by cardiac MRI. Some studies have reported LVEF to be
3 EF points higher in women than men,23,24 but others have reported that the EF is similar in both genders.7,8 In the Framingham Heart Offspring Study, the largest MRI study (63 men and 79 women) of which we are aware,9 there were no significant differences in LVEF between men (69%) and women (70%).
When purported gender differences in LVEF are assessed, it is also important to consider the impact of gender differences in ventricular volume. By use of pressure-volume loop analysis in a study of 30 subjects, women were shown to have higher left ventricular systolic function independent of ventricular volume.25 In a study of 517 subjects, the mean LVEF was higher in women than in men by 1 (hypertensive) or 2 (normotensive) LVEF percentage points. Accounting for body size decreased differences in systolic function in normotensive but not hypertensive individuals.26 We found that the differences in stroke volume between men and women were independent of differences in ventricular EDV and body size.
Women, as compared with men, are overrepresented among patients with heart failure who have a preserved LVEF.1 The basis of this observation is not yet well understood. Interestingly, the right-shifted distribution of LVEF in female, as compared with male, patients with heart failure demonstrated in the EuroHeart Survey27 is similar to our findings in the general population (Figure 1). Women whose LVEF falls in the range between the male and female gender-specific criteria for low LVEF (eg, in the present study, between 55% and 61%) would be misclassified as having a preserved LVEF, when in fact their LVEF is reduced. In the EuroHeart Survey,27
10% of women with heart failure would have their classification changed from heart failure with preserved LVEF to reduced LVEF if the LVEF threshold criteria for women were increased from 55% to 60%. We recognize that other mechanisms besides misclassification are undoubtedly operative for the association of female gender and heart failure with a preserved LVEF.
The higher stroke volumes in women than in men for a given EDV were likely due to either higher contractility or lower afterload. Findings consistent with the latter were that women versus men had lower blood pressures, that the stroke volume differences between the genders were no longer significant when subjects with hypertension were excluded (Table 2), and that the slope of the stroke work versus EDV relationship was the same in women and men (Figure 4B). In contrast, there were other data supporting the contention that contractility was higher in women than in men, including a higher systolic blood pressure/ESV ratio,28 a higher stroke work/EDV ratio, and persistence of higher stroke volume for a given EDV in multivariable analysis that adjusted for blood pressure, although the utility of these indexes as measured in a population-based study is uncertain. The possibility that the genders differ in cardiac contractility and response to cardiovascular stress has been well documented previously, including elegant studies of patients with aortic stenosis in which women had a higher LVEF and were less likely to have low LVEF than men.29 A variety of animal studies also suggest that male versus female animals were more likely to develop cardiac dysfunction and/or ventricular dilation in response to stress, including pressure overload.3037 Nevertheless, the data in the present study do not allow us to determine with certainty whether the higher stroke volume for a given EDV in women versus men was secondary to differences in contractile state or loading conditions.
Limitations of the Study
Although interpretation of cardiac MRI was performed in absence of knowledge of other study information and typically performed with the use of 150% to 200% magnification centered on the heart, structures outside the heart could not be totally masked, and this may have led to an unintentional bias in interpretation. Also, we cannot exclude the possibility of residual confounding in the association of gender and LVEF. In particular, hematocrit was not measured, and the effect of anemia in premenopausal women in contributing to the observed gender LVEF differences is therefore unknown. The poor correlation of LVEF with age, which would reflect menopausal status, and the similar cardiac index between men and women suggest that anemia is unlikely to be the major explanation of our findings. LVEF is load dependent and is not an intrinsic measure of contractility. Nevertheless, LVEF is widely used in making therapeutic decisions; thus, the gender differences in LVEF demonstrated in the present study have clinical relevance irrespective of their underlying cause. The ascertainment of a healthy reference subpopulation to define a low LVEF may have been biased by differential reporting of medical illness by women and men, although the exclusions of subjects with left ventricular hypertrophy by MRI or CAC by EBCT should have reduced this bias. We were unable to calculate ventricular stress in the absence of measured ventricular dimensions in the DHS. LVEF was assessed with cardiac MRI, and our conclusions may not be applicable to assessments by other imaging modalities.
Conclusions
In the general population of Dallas County, women had higher LVEF than did men, reflecting a higher stroke volume for a given EDV. This difference in stroke volume was independent of potential confounders and may reflect underlying gender differences in native cardiac function or response to hemodynamic stress. These data raise the provocative concept of whether gender-specific criteria are needed in clinical decision making, with use of a threshold of
5 LVEF points higher in women than in men. Furthermore, these data suggest that women with LVEF toward the lower limits of normal have more severe contractile dysfunction than previously appreciated and that this may be an explanation for their apparent predisposition to develop symptomatic heart failure.
| Acknowledgments |
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Disclosures
None.
| References |
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5 LVEF points higher in women than in men, an approach that would need to be tested in future studies.
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