In 2005, the National Fire Protection Association reported that 44% of on-duty firefighter fatalities were due to sudden cardiac death.1 Two years later, Kales and colleagues2 confirmed that 45% of on-duty firefighter deaths were cardiac-related rather than fire-related injuries. On-duty firefighters have twice as many cardiovascular deaths as police officers and 4 times as many as emergency medical responders.2 The exact reason for this relatively higher risk for cardiovascular events is unknown. We previously reported on the prevalence of metabolic syndrome, coronary artery disease, sleep deprivation, binge drinking behaviors, and overall poor mental well-being among professional firefighters,3-5 all of which are known to increase the risk for cardiovascular events. To date, despite the high cardiac risk for firefighters and its societal importance, there are no available high-resolution field recordings of the electrocardiogram (ECG) during firefighting activities. Consequently, the aim of this study was to evaluate the prevalence of clinical and ECG risk factors among on-duty professional firefighters.
Methods
Subjects for this study were recruited from Surveying & Assessing Firefighters Fitness & Electrocardiogram (SAFFE) study between 2008 and 2010. The SAFFE study was a cross-sectional descriptive study that recruited professional firefighters from the Upstate New York area. Of 13 accessible firehouses in Upstate New York area, the research team travelled across 7 of these firehouses, and professional firefighters were approached while on duty to participate in the study. Research equipment and supplies were moved from 1 firehouse to the other to allow all eligible firefighters an equal chance for voluntary participation. Because professional firefighters have physician clearance to be on duty, no exclusion criteria were applied. Firefighters in the Upstate New York area undergo a 1-time physical assessment before becoming firefighters and then are enrolled in an optional occupational health monitoring program in a county hospital. The firefighters who consented to participate anonymously completed paper-and-pencil surveys, and then, in a private bunk room, research personnel obtained anthropometric measures (ie, height, weight, abdominal circumference, and abdominal height) and vital signs (ie, baseline blood pressure and heart rate [HR]). Two blood pressure readings were taken 5 minutes apart after resting in a chair for 5 minutes. Each firefighter then completed a symptom-limited standard Bruce exercise treadmill test (ETT), followed by ambulatory 12-lead ECG holter recording for 24 hours. The ECG monitoring was done continuously during all firefighting, exercising, eating, sleeping, and leisure activities. A registered nurse was made available in the firehouse during the ETT and later 24/7 by telephone to respond to all technical difficulties firefighters might encounter with the equipment during the holter monitoring. The nurse also accompanied the firefighters during numerous fire and medical responses to keep a log of all response activities. The firefighters received grocery gift cards for completing both the ECG recording and ETT and returning the equipment. The study was approved by the appropriate institutional review board of the State University of New York.
Electrocardiogram Recording and Analysis
Using H12+ Holter recorders (V3.12; Mortara Instruments, Milwaukee, Wisconsin), 12-lead ECGs were recorded. To optimize signal quality, the firefighters' skin was shaved, rubbed with alcohol wipes until thoroughly cleaned, and briskly dried with gauze to stimulate capillary flow. Disposable electrodes were applied in the Mason-Likar lead configuration under the firefighters' uniform t-shirts, and the Holter was secured to their uniform belt. All leads were simultaneously acquired at a high resolution (1000 samples per second), resulting in high-fidelity recordings with a frequency response of 0.05 to 60 Hz. All ECG recordings were then downloaded to a laptop computer equipped with H-Scribe 4 software (Mortara Instrument) for ECG processing and analysis. First, all ECGs were manually annotated to delete noise and artifact by a reviewer blinded to all clinical data. Second, the computer software automatically computed the necessary ECG metrics and exported them to an excel sheet for final analysis. High-frequency ECG waveform features such as notching in the QRS complex can be missed with the standard upper filter setting of 60 Hz; thus, the first 10-second ECG 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 Instruments) for subsequent analysis. Table 1 summarizes the high-risk ECG parameters that were computed in the current analysis.
Exercise Treadmill Testing
Eligible firefighters completed a standard Bruce protocol ETT using X-Scribe stress testing system (Mortara Instruments). The system wirelessly acquires diagnostic-quality ECGs during treadmill testing and displays these ECGs on a high-definition computer screen monitor. The system displays real-time 12-lead ECG signals during the exercise test. The system also computes a signal-averaged ST-T complex for each ECG lead for real-time, automated baseline comparison during the ETT. This highly sensitive feature allows clinicians to easily recognize even minor stress-induced ST segment and slope changes for real-time or offline analyses. The X-Scribe system also automatically measured blood pressure readings at 5-minute intervals and continuously measures oxygen saturation during test and recovery. Of note, firefighters were kept on the treadmill as long as they could tolerate, even if they exceeded their maximum HR (symptom-limited exercise). All analyses were completed by a reviewer blinded to research data. According to the current clinical guidelines,16 the presence of exercise-induced, horizontal ST segment depression of 2 mm or greater in 2 or more contiguous leads for 30 seconds or longer at peak or recovery of ETT was considered indicative of underlying coronary artery disease and positive ETT test. An ST segment depression of 1 mm or greater but less than 2 mm was considered a nonconclusive ETT test and indicative of further evaluation.
Statistical Analysis
All analyses were conducted using the Statistical Package for the Social Sciences (version 21.0 for Windows), and p < 0.05 was considered statistically significant. Dynamic variables are reported as mean (SD); and categorical variables, as number (percentage). Comparison between groups was completed using independent samples t test for dynamic variables and [chi]2 for categorical variables.
Results
One hundred twelve (n = 112) firefighters were enrolled in the SAFFE study (age, 43.6 [7.7] years; men, 96%; white, 81%) (Table 2). Almost half (49%) of the firefighters were overweight, and more than 40% were obese (body mass index [BMI], >=30 kg/m2), with more than half (55%) having a waist circumference of greater than 100 cm, a predictor of insulin resistance. Systolic and diastolic blood pressure readings were slightly elevated but within prehypertensive stage. One third of the firefighters were in the diagnostic range of hypertension, but less than 20% were being treated. In addition, 13% were active smokers, 3% had a history of coronary artery disease or a coronary intervention, and 9% had a respiratory disease (ie, asthma, chronic obstructive pulmonary disease, or sleep apnea).
Electrocardiogram Characteristics
Confirming our pilot study,17 the quality of the ECG data was ample for arrhythmia and myocardial ischemia interpretation (Figure 1). All firefighters were in normal sinus rhythm, and on-duty HRs ranged from a minimum of 47 (7) beats per minute to a maximum of 143 (21) beats per minute. Although no lethal arrhythmias occurred, nonsustained ventricular tachycardia (NSVT) did occur once. Many firefighters exceeded the cutoff point for cardiovascular risk for many ECG risk factors (Figure 2). Among the 12 high-risk ECG markers analyzed in this study, NSVT occurred the least frequently, whereas more than half of the firefighters had accelerated mean HR during a 24-hour monitoring period. There were no interactions between firefighters' demographic (ie, age and sex) and clinical (ie, BMI, waist circumference, blood pressure, medical history and current smoking) characteristics and the incidence of any high-risk ECG parameters.
Exercise Treadmill Testing Characteristics
Most firefighters (n = 110, 98%) successfully completed a symptom-limited ETT using a standard Bruce protocol. The other 2 firefighters did not do the test because of some medical concerns (ie, recent surgery and pregnancy). On average, it took each firefighter 12 minutes (fourth stage of the Bruce protocol, speed of 4.2 mph, with 16% grade) to achieve 93% of the maximum HR. Findings were diagnostic of myocardial ischemia in nearly 13 (11.6%) firefighters, and inconclusive findings that warrant further investigation were seen in approximately 21 (19%) more firefighters (Table 3). There were no age, sex, or clinical differences between those who passed or failed the ETT. On average, trends of blood pressure peak and recovery were normal, with most increase seen in the systolic blood pressure (+40 mm Hg) at peak. Oxygen saturation, measured with continuous pulse oximetry probe on the index finger, was normal (>=94%) during start and recovery, with 8 firefighters (7%) showing desaturation (<90%) at peak of ETT.
Discussion
Our findings demonstrate that high-risk ECG markers of fatal cardiac events and abnormal stress test results that warrant further evaluation are prevalent. We were successful in obtaining diagnostic-quality ECGs for arrhythmia and ischemia detection in this middle-aged, high-risk population. To our knowledge, this is the first study to record high-resolution 12-lead ECGs of professional firefighters while on duty. More than half of the firefighters had at least 1 high-risk ECG risk factor, and almost one-third had abnormal results of stress tests. Of note, more than 75% of on-duty fatalities in the United States happened to those older than 45 years,18 and nearly one-half (47%) of our sample was in that age group.
Electrocardiogram Markers of Mortality
An elevated mean 24-hour HR, an indicator of poor sympathetic tone,19 was seen in more than 50% of the sample. This finding is consistent with on-duty firefighter's activities that are strenuous and often require firefighters to work at their maximal HR for prolonged periods or even beyond, as seen in this study. However, the depressed HR variability (HRV) seen in nearly 10% of these firefighters provides more specific insights about how such abnormalities in HR fluctuation can increase the risk for cardiac death. Depressed HRV indicates autonomic dysfunction and increased risk for fatal arrhythmic events.20 A more ominous finding is the presence of a widened QRS-T angle in nearly 25% of the sample. The spatial angle between mean R and T vectors denote abnormal repolarization and underlying cardiac pathology.21 An abnormal angle has a larger hazard ratio for fatal cardiac events than other established risk factors.12 Another prevalent marker was fragmented QRS complexes seen in nearly 20% of our sample. This high-risk ECG pattern indicates myocardial scaring and is also strongly correlated with subsequent cardiac events in clinical populations.22 Of note, nearly one-fifth of our sample is known to have a cardiac history. Another known marker of death is dynamic ST depression, which was prevalent in more than 10% of our sample. ST depression indicates subclinical myocardial ischemia in asymptomatic adults and can be a key for identifying those at greater risk who might benefit from early treatment.23 Similarly, nearly 12% to 30% of firefighters had abnormal stress test results that might indicate subclinical coronary artery disease. Finally, nearly 8% of the firefighters had a prolonged corrected QT interval, a historically known risk factor of fatal arrhythmic events.24 Other high-risk ECG markers were not prevalent but remain a strong prognostic marker of subsequent cardiac events, including left bundle branch block (1%) and NSVT (1%).
Firefighters and the Risk for Cardiovascular Mortality
Compared with other first responders, firefighters are at increased risk for cardiovascular events.1 Previous retrospective studies showed that firefighters with a medical history of coronary artery disease, active smokers, and those who have hypertension are at excess risk for on-duty cardiac events.25 In this study, only 3% of the firefighters had a history of coronary artery disease, but nearly 10% had undiagnosed high blood pressure and 13% were active smokers. We did not find any correlations between these clinical characteristics and other high-risk holter or stress ECG findings. However, our results indicate that nearly 12% to 30% of firefighters might have subclinical coronary artery disease, which might explain the increased risk for on-duty cardiac events in this high-risk population. Of note, literature indicates that fire-suppression activities are associated with the highest risk for mortality among firefighters. These fire-suppression activities increase the risk approximately 10 to 100 times as that for nonemergency firefighting duties.2 This indicates that future efforts need to focus more on studying the relationship between thermal stress and the pathogenesis of coronary artery disease, which might provide interesting and important insights about this phenomenon. Finally, given that firefighters often undergo a 1-time physical assessment before becoming firefighters, primary prevention programs need to focus on mandatory annual physical assessments to screen for cardiovascular risk factors (ie, subclinical coronary artery disease and untreated hypertension) and other incentive programs that can encourage firefighters to quit smoking and maintain their physical fitness.
Conclusions
Firefighters are a unique population with many occupational health problems. In this study, we found that nearly 10% of firefighters had undiagnosed and untreated hypertension and approximately 30% had possible coronary artery disease, a rate higher than typically expected among middle-aged adults. More than half of these firefighters had at least 1 high-risk ECG risk factor present, including abnormal sympathetic tone, abnormal repolarization, myocardial scarring, and myocardial ischemia. Annual physical assessments with routine 12-lead ECG can identify those who might benefit from preventive cardiovascular services. This preliminary work may help develop a noninvasive risk stratification approach to better identify professional firefighters at risk for cardiovascular events. Over the longer horizon, algorithmic guidelines are needed to route at-risk firefighters for optimal and timely cardiac care.
What's New and Important
* Firefighters are a unique population known to have high rate of cardiovascular diseases.
* The 12-lead ECG provides prognostic information about cardiovascular mortality.
* Annual physical checkups with routine 12-lead ECG can identify those who might benefit from preventive cardiovascular services.
Acknowledgments
The authors thank the professional firefighters of Upstate New York for participating in the study and Fire Commissioner Michael Lombardo for his specific support.
REFERENCES