The electrocardiogram is a simple and noninvasive tool used to assess patients at risk for cardiac events. The spatial QRS-T angle is an electrocardiogram measure of the difference in mean vectors of depolarization and repolarization.1 Therefore, the spatial QRS-T angle is the vector sum of the cardiac ventricular gradient and has been postulated to represent the heterogeneity of cardiac action potential morphology.2 This heterogeneity in the action potential increases the susceptibility to dysrhythmias and poor ventricular function.3 With the emergence of computerized vectorcardiography, the spatial QRS-T angle may become more prevalent in clinical practice, so cardiovascular nurses need to be aware of its physiological underpinnings and its associated risks.1 The QRS-T angle is an electrocardiogram-derived vector sum of the ventricular gradient, which may represent heterogeneity in the cardiac action potential during depolarization and repolarization, and may be indicative of poor ventricular function.
A widened spatial QRS-T angle has been associated with an increased risk for all-cause mortality and cardiac mortality in a recent meta-analysis.4,5 Moreover, a widened spatial QRS-T angle may represent structural changes in the heart reflective of processes that contribute to left ventricular hypertrophy (LVH) confirmed by echocardiography.6,7 Left ventricular hypertrophy represents physiological adaptation to an increased cardiac workload and increased left ventricular wall stress, and hypertension is one known cause of LVH.8 Hypertension acts as a stressor on the left ventricular wall stimulating myocyte hypertrophy, collagen formation, and fibroblast-mediated myocardial remodeling, which subsequently reduces left ventricle compliance leading to diastolic dysfunction.8 The spatial QRS-T angle may be widened in patients with elevated blood pressure (BP) possibly because of structural changes in the heart reflective of LVH.9–12 Atsma et al9 reported that elevated BP among postmenopausal women led to ventricular depolarization and repolarization disturbance as measured by the spatial QRS-T angle prior to the development of LVH. In another study, authors reported that spatial QRS-T angle was associated with left ventricular performance among a group with type 2 diabetes.12 Furthermore, both LVH and the spatial QRS-T angle have been associated with increased susceptibility to poor cardiac outcomes.4,5,13 Therefore, spatial QRS-T angle is a predictor of poor cardiac outcomes possibly due to poor ventricular health caused by hypertension and the development of LVH.
Although the relationship between spatial QRS-T angle and LVH has been previously described, limited research has been published on spatial QRS-T angle and ventricular function during exercise, which may be impaired due to underlying processes such as LVH. As mentioned, authors of previous research suggest that adults with hypertension have widened spatial QRS-T angle possibly caused by the developing LVH.10,11 However, authors of previous studies have failed to look at this relationship during exercise testing, which requires greater ventricular stretch and ventricular compliance.10,11 It remains unknown how the spatial QRS-T angle, a known marker of overall cardiovascular health, relates to BP before, during, and after exercise testing. It is known that, in patients with LVH, left ventricular mass and ejection fraction measured using echocardiography during exercise are independent predictors of mortality.13 Unfortunately, only echocardiography and not electrocardiogram was used in this study, limiting the associations between echocardiographic and electrocardiogram measures. Because decent left ventricular health and adequate ejection fraction are required to maintain cardiovascular fitness, exercise testing among a population at risk for LVH due to hypertension may reveal an electrocardiogram-derived measure of ventricular health and overall ventricular stretch. Applying electrocardiogram and exercise testing in a patient population in which hypertension is prevalent would help establish spatial QRS-T angle as a measurement of ventricular function and ventricular stretch. In this secondary analysis, the relation between spatial QRS-T angle and BP recorded before, during, and after exercise testing and recovery from exercise was calculated to determine whether spatial QRS-T is a measurement of ventricular function.
Materials and Methods
Subjects for this secondary analysis were a part of the cross-sectional Surveying & Assessing Firefighters Fitness & Electrocardiogram study conducted in Western New York, and all firefighters were eligible to participate in the study. The primary results from the Surveying & Assessing Firefighters Fitness & Electrocardiogram study have been previously reported.14,15 Subjects included professional firefighters from 7 of the 13 city firehouses. Firefighters are a unique vulnerable workforce to study because they are at a substantial increased risk for cardiac events compared to other first responders and the general public.16 Furthermore, firefighters are generally hypertensive and overweight, adding to their cardiovascular burden.17 Given their increased cardiovascular burden, the effect size measured when evaluating spatial QRS-T angle and preexercise BP, maximum achieved BP during exercise, 2-minute postexercise BP, and BP recovery may be increased using this population. Only data recorded from a 12-lead 24-hour Holter electrocardiogram monitor for QRS-T angle measurement and BP recordings before, during, and after exercise testing were analyzed. For this secondary data analysis, all firefighters were included. The spatial QRS-T angle between the mean QRS and T vectors is relatively insensitive to variations in the estimates of T-wave onset and offset; therefore, there was little concern of repolarization abnormalities such as in bundle branch blocks impacting QRS-T angle measurement.18
The study was approved by the institutional review board of the State University of New York. All firefighters provided written consent after understanding the potential benefits and risks involved with this study. Self-report demographic data including age, number of years as a firefighter, ethnicity (white, black or African American, or other), current smoking status (yes/no), current medical diagnosis of sleep apnea (yes/no), and whether the firefighter had a pacemaker (yes/no) were collected. Afterward, a registered nurse obtained anthropometric measures including height, weight, and abdominal circumference. Weight was recorded once using a calibrated bathroom scale. Body mass index (BMI) was calculated and categorized in accordance with the Centers for Disease Control and Prevention standard BMI range categories for adults.19
To determine the severity of hypertension among this sample of firefighters, resting state BP was categorized in accordance with the new 2017 American Heart Association (AHA) BP guidelines.20 Firefighters were asked to sit and rest for 15 minutes prior to their BP being taken by a registered nurse. A second reading was taken 5 minutes later with the firefighter still resting and in the sitting position. The mean of the 2 BPs was considered the resting state BP. Firefighters being treated for hypertension or who reported a diagnosis of hypertension were still considered for this study because this study was conducted prior to the release of these new guidelines. The 2017 AHA BP guidelines are the following:
- Normal: Less than 120/80 mm Hg
- Elevated: Systolic between 120 and 129 mm Hg and diastolic less than 80 mm Hg
- Stage 1: Systolic between 130 and 139 mm Hg or diastolic between 80 and 89 mm Hg
- Stage 2: Systolic at least 140 or diastolic at least 90 mm Hg
12-Lead 24-Hour Holter Electrocardiographic Monitoring
In this study, we used 12-lead H12+ Holter electrocardiogram monitors that capture continuous data for 24 hours (V3.12; Mortara Instrument, Milwaukee, Wisconsin). The 12 leads were placed in the Mason-Likar lead configuration to measure QRS-T angle.21 The leads were placed on shaven and cleaned areas of the torso under the firefighters' uniforms, and monitors were securely fastened to their uniform belt. Firefighters were informed to continue their activities of daily living (ie, eating, sleeping, exercising) with the 12-lead Holter monitor attached. High-fidelity and high-resolution (1000 samples per second) recordings were obtained with a frequency of 0.05 to 60 Hz. Recordings were downloaded to a computer with H-Scribe 4 software (Mortara Instrument) for electrocardiogram processing and analysis. After semiautomatic annotation was performed to delete noise and artifact, all electrocardiogram were manually reviewed for quality assurance by a reviewer blinded to the study. High-frequency electrocardiogram waveforms such as QRS-T angle can be missed with the standard upper filter setting of 60 Hz. To resolve this potential issue, the first 10-second electrocardiogram tracing of the 24-hour monitoring period was exported into a portable document format with the standard filter setting at 0.05 to 150 Hz using the ELI LINK program (Mortara Instrument) for subsequent analysis. Lastly, using Super electrocardiogram software (Mortara Instrument), the average spatial QRS-T angle from the entire 24-hour monitoring session was calculated and reported. QRS-T angle is difficult to visualize and requires calculation. Current methods of measuring the spatial QRS-T angle include the generation of x-, y-, and z-axes between the QRS and T waveforms on the 12-lead electrocardiogram by using algorithms obtained from a matrix transformation.22 The mean x, y, and z values are calculated to obtain the mean QRS and T vectors, and the spatial QRS-T angle is calculated as the scalar product between these 2 vectors.22 Although, in this study, we used high-fidelity computerized calculations for spatial QRS-T angle, Rautaharju and colleagues23 have reported a simple estimation procedure for the spatial QRS-T angle from a standard 12-lead electrocardiogram. Lastly, LVH was also measured using the 12-lead H12+ Holter electrocardiogram monitors. Left ventricular hypertrophy was determined using Cornell criteria (lead SV3 + lead RaVL; cutoff for LVH, >20 mm in women or >28 mm in men). Su et al24 recently reported a sensitivity of 22.2% and a specificity of 95.2% for Cornell criteria for LVH.
Exercise Treadmill Testing
On-duty firefighters completed a standard Bruce protocol exercise treadmill testing X-Scribe stress testing system (Mortara Instrument). The Bruce protocol is a maximal exercise test in which a subject works to complete exhaustion as the treadmill speed and incline increase incrementally every 3 minutes.25 Firefighters underwent exercise treadmill testing for as long as tolerated, even if maximum heart rate was exceeded (symptom-limited exercise). Testing was only terminated if the firefighter reported symptoms such as dyspnea or palpitations; otherwise, the exercise test was completed in about 20 minutes. Blood pressure (mm Hg) was recorded 5 minutes prior to exercise treadmill testing (preexercise) and then at 5-minute intervals during exercise. The maximal BP achieved during exercise testing was recorded based on the values recorded during these 5-minute intervals. Postexercise BP was recorded 2 minutes after exercise treadmill testing. Authors of previous research informed us about the 2-minute cutoff.26,27 Recovery BP was calculated by subtracting the maximal BP by the 2-minute postexercise testing BP.
Descriptive data are presented as means (± standard deviation) or percentages where appropriate. Resting state BP means were used when determining hypertension classification. Bivariate correlations between QRS-T angle and all BP measures were reported using Pearson r coefficient. Spatial QRS-T angle was classified as normal (<100), borderline (100–139), and widened (≥140).28 One-way analysis of variance was conducted on classification of QRS-T angle on preexercise BP, maximal BP during exercise, postexercise BP, and recovery BP. All analyses were conducted using SPSS (version 21.0; IBM, Armonk, New York). Statistical significance was considered when P < .05.
In total, 111 on-duty firefighters were included in this analysis. The mean age was 43.6 (±7.7) years, and mean years as a firefighter was 15.5 (±7.0). The mean BMI was 29.5 (±4.1) kg/m2, and the majority were at least overweight (48.7%), if not obese (41.4%) (total, 90.1%). The mean resting state BP 129.3 (±14.9) mm Hg for was systolic and 81.8 (±10.6) mm Hg for diastolic. In accordance to the new 2017 AHA guidelines, the average firefighter had stage I hypertension. Nearly 40% of the on-duty firefighters had stage I hypertension, and 31.5% of the firefighters had stage II hypertension (total, 71.1%) (Table 1).
The mean spatial QRS-T angle among the on-duty firefighters was 78.1° (±37.3°). In this sample, 73% of the on-duty firefighters had a normal spatial QRS-T angle (<100°), 22% had a borderline spatial QRS-T angle (100°–139°), and 5% had a widened spatial QRS-T angle (≥140°) (Table 1).
Blood pressure was recorded preexercise, maximum during exercise, and postexercise, and additionally, BP recovery was calculated. In summary, for the entire sample of firefighters, the means for systolic BP were as follows: preexercise, 130 (±15.5) mm Hg; maximum during exercise, 168 (±27.5) mm Hg; postexercise, 152 (±23.9) mm Hg; and 2-minute recovery, 18 (±24.8) mm Hg. The means for diastolic BP were as follows: preexercise, 79 (±11.4) mm Hg; maximum during exercise, 77 (±13.9) mm Hg; postexercise, 75 (±15.4) mm Hg; and 2-minute recovery, 2 (±13.4) mm Hg.
Next, the relationships between QRS-T angle and BP were investigated. Between the 3 QRS-T angle groups, no statistically significant differences in the means were found for all BP measures, as seen in Table 2. Although the findings were not statistically significant, a trend was observed among the diastolic BP measures. The group with the widened QRS-T angle had lower diastolic BP compared with the group with the normal QRS-T angle. The mean maximum diastolic BP for the group with the widened QRS-T angle was 73.3 (±19.1) mm Hg compared with 77.8 (±13.8) mm Hg for the group with the normal QRS-T angle. The mean postexercise diastolic BP for the group with the widened QRS-T angle was 66.1 (±21.3) mm Hg compared with 76.4 (±14.6) mm Hg for the group with the normal QRS-T angle. Although these findings are not statistically significant, a trend was observed. Spatial QRS-T angle was nearly statistically significantly, negatively correlated with maximum diastolic BP (r = −0.190, P = .05) and statistically significantly, negatively correlated between spatial QRS-T angle and postexercise diastolic BP (r = −0.261, P = .008). Preexercise and recovery diastolic BP measurements were not statically significant, as seen in Table 3. Because of the small sample size in the group with the widened spatial QRS-T angle (n = 6), we combined the groups with borderline and widened spatial QRS-T angles and compared the means but did not find any statistical significance between the means for all BP measures (P > .05 for all).
This secondary analysis showed that a trend toward statistically significant negative correlation existed between spatial QRS-T angle and diastolic BP during maximal exercise and a statistically significant negative correlation existed between spatial QRS-T angle and diastolic BP 2 minutes postexercise among a sample of on-duty firefighters. Therefore, on-duty firefighters with a widened spatial QRS-T angle demonstrate lower maximal diastolic BP and lower postexercise diastolic BPs. This is evident in the reported means between QRS-T angle groups and BP measures. We conclude that a negative relation existed between spatial QRS-T angle and diastolic BP because of poor underlying ventricular health. Most firefighters in our sample were at least overweight, if not obese, and hypertensive. Obesity and hypertension may contribute to structural changes in the ventricles such as myocyte hypertrophy and cardiac remodeling, which reduces ventricle stretch.8 Authors of past research have suggested that diastolic BP is more closely related to left ventricular wall thickness reflecting pure pressure load.8,29 During diastole, the ventricles stretch and fill with blood to prepare for the next contraction. This process occurs more frequently with larger volumes of blood during exercise to meet metabolic demands. Therefore, on-duty firefighters with a widened spatial QRS-T angle demonstrated lower maximal diastolic BP during exercise and lower postexercise diastolic BPs possibly because of the ventricles' limited ability to stretch and fill with blood during diastole.
In our study, the spatial QRS-T angle was not associated with any measure of systolic BP during exercise testing, which further suggests that a widened QRS-T angle is associated with ventricular stretch. Left ventricular hypertrophy is a physiological adaptation to increased cardiac workload and increased left ventricular wall stress caused by hypertension.8 Furthermore, LVH reduces left ventricle compliance and is more closely related to left ventricular wall thickness contributing to poor ventricular stretch and diastolic dysfunction.30 These conclusions can only be confirmed with echocardiographic data to measure left ventricular wall thickness. the authors of one other study associated diastolic dysfunction with spatial QRS-T angle. In a 2016 study of patients with hypertrophic cardiomyopathy and echocardiographic evidence of diastolic dysfunction with a septal or posterior wall thickness greater than 15 millimeters, a widened spatial QRS-T angle was associated with sustained ventricular dysrhythmias due to poorer ventricular health.31 Therefore, a widened spatial QRS-T angle may resemble poor ventricular stretch and overall poor ventricular health. Ventricular stretch may be poor due to the development of LVH among populations with hypertension such as firefighters. Therefore, a widened spatial QRS-T angle may resemble poor ventricular stretch associated with the progression of LVH in populations with hypertension.
Although only 3.6% of firefighters had LVH per the Cornell Voltage Criteria, the contributing factors that lead to LVH, such as obesity and hypertension, are evident in this workforce. It may be concluded that a widened spatial QRS-T angle may be indicative of structural changes in the ventricles due to obesity and hypertension. Again, echocardiography needs to confirm these hypotheses. Authors of future prospective studies among on-duty firefighters may wish to examine spatial QRS-T angle and ventricular function using echocardiography to determine whether decreases in ventricular stretch are associated with widened spatial QRS-T angle. It should be noted that, in their previous research, Man et al7 suggested that a discriminant model equation combining body surface area and spatial QRS-T angle was a more accurate approach to the diagnosis of LVH compared to conventional electrocardiogram criteria. Although, in our study, we did not take body surface area into account, this demonstrates that authors of previous research have determined the clinical usefulness of QRS-T angle for evaluating LVH. Dilaveris and colleagues10 demonstrated that widened spatial QRS-T angle was a marker of ventricular repolarization in a sample of 110 patients with hypertension receiving treatment. In this study, hypertension was classified as systolic BP of 160 mm Hg or greater or diastolic BP of 95 mm Hg or greater. Our sample of on-duty firefighters who are generally overweight and hypertensive also suggests that a widened spatial QRS-T angle may be a marker of poorer ventricular stretch. In summary, our findings add to the body of existing evidence that a widened QRS-T angle is related to poorer ventricular stretch possibly due to LVH development.
Our study of QRS-T angle and BP measures collected before, during, and after exercise testing has strengths and weaknesses. In this study, we categorized BPs in accordance with the newly released 2017 AHA BP guidelines, which is one of the first to do so among firefighters. These results may also be generalizable to the population at large because on-duty firefighters are overweight and hypertensive, much like the general population.32,33 Limitations to this study include the small coefficient values obtained from the bivariate correlations and the small sample size of firefighters with borderline or widened QRS-T angles. Echocardiograms are necessary to determine the presence of LVH and left ventricular wall mass, which may impact diastolic function and relate to QRS-T angle. This is a significant weakness of our study. Authors of future research may wish to include imaging data when evaluating electrocardiogram-based markers and cardiac function to fully evaluate the relationship between QRS-T angle and diastolic function during exercise.
In this secondary analysis of on-duty firefighters, obesity and hypertension are prevalent medical conditions increasing cardiac burden and placing firefighters at a greater risk for LVH. A negative correlation existed between spatial QRS-T angle and maximal diastolic BP during exercise and between spatial QRS-T angle and 2-minute postexercise BP. We conclude that a negative relation that existed between spatial QRS-T angle and diastolic BP during and after exercise may be due to poor underlying ventricular stretch.
What's New and Important
- QRS-T angle was negatively correlated with maximal diastolic BP during exercise testing among on-duty firefighters potentially because of poor ventricular stretch.
- QRS-T angle was negatively correlated with postexercise diastolic BP among on-duty firefighters potentially because of poor ventricular stretch.
- Lower maximal and postexercise diastolic BPs suggest an inability for the ventricles to stretch and fill with blood during diastole, thus limiting the heart's ability to meet metabolic demands. QRS-T angle may be an electrocardiogram marker of poor ventricular stretch among populations with hypertension who are overweight and obese.
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Keywords:Copyright © 2019 Wolters Kluwer Health, Inc. All rights reserved
Electrocardiography; Exercise; Firefighters; Hypertension; Ventricular function