Increased cardiac troponin I (cTnI), a marker for cardiac damage, has been reported after strenuous exercise in young subjects. However, little is known about changes in cTnI after moderate-intensity exercise in a heterogenous population or which factors may contribute to this change in cTnI levels. We examined cTnI levels before and immediately after each day of a 4-day long-distance walking event (30 to 50 km/day) in a heterogenous group (67 men, 42 women), across a broad age range (21 to 82 years), with known cardiovascular pathology or risk factors present in many subjects (n = 24). Walking was performed at a self-selected pace. Cardiac TnI was assessed using a standard system (Immulite) with high values (≥0.20 μg/L) cross-checked using a high-sensitive cTnI assay (Centaur). Mean cTnI levels increased significantly from 0.04 to 0.07 μg/L on day 1, with no further increase thereafter (p <0.001, analysis of variance). Backward linear regression found a weak, but significant, association of age (p <0.001), walking speed (p = 0.02), and cardiovascular pathology (p = 0.03) with postexercise cTnI level (combined r 2 = 0.11, p <0.001). In 6 participants (6%), cTnI was increased above the clinical cut-off value for myocardial infarction on ≥1 day. These participants supported the regression analysis, because they were older, walked at higher relative exercise intensity, and reported a high prevalence of cardiovascular pathology. In conclusion, prolonged, moderate-intensity exercise may result in an increase in cTnI levels in a broad spectrum of subjects, especially in older subjects with pre-existing cardiovascular disease or risk factors.
The presence of intracellular cardiac troponin subunits T and I (cTnT and cTnI) in the blood is a sensitive and specific indicator for myocardial injury. In recent years, increased circulating cTnT/cTnI concentrations have been reported after prolonged exercise. The purpose of our study was to examine baseline and postexercise cTnI levels in a large, heterogenous group of participants undertaking the Nijmegen Marches (The Netherlands), an annual 4-day walking event involving ∼40,000 participants. Participants walked 30, 40, or 50 km on 4 consecutive days. Specifically, we examined changes in absolute cTnI levels, identified those with an increase in cTnI above the cut-off value for acute myocardial infarction (AMI), and examined whether postexercise cTnI levels were related to age, gender, body mass index (BMI), relative exercise intensity, core temperature, walking speed, training status, or underlying cardiovascular pathology.
Methods
One hundred nine participants (21 to 82 years of age) were randomly selected 4 weeks before the event ( Table 1 ). All participants completed a minimum distance of 30, 40, or 50 km on 4 consecutive days. The medical ethical committee of the Radboud University Nijmegen Medical Center approved the study and all participants provided written informed consent before participation. This study was conducted in line with the Declaration of Helsinki.
| Overall | 30 km | 40 km | 50 km | |
|---|---|---|---|---|
| Characteristics | ||||
| Men/women | 67/42 | 22/13 | 28/17 | 17/12 |
| Smoking | 20 (18%) | 3 (9%) | 9 (20%) | 8 (28%) |
| Age (years) | 57 ± 15 | 69 ± 6 | 54 ± 16 | 48 ± 15 |
| Length (cm) | 174 ± 10 | 171 ± 9 | 175 ± 10 | 176 ± 10 |
| Weight (kg) | 77 ± 15 | 73 ± 12 | 78 ± 15 | 80 ± 17 |
| Body mass index (kg/m 2 ) | 25.2 ± 3.3 | 24.9 ± 2.6 | 25.0 ± 3.2 | 25.7 ± 4.1 |
| Systolic blood pressure (mm Hg) | 138 ± 18 | 144 ± 19 | 135 ± 16 | 137 ± 19 |
| Diastolic blood pressure (mm Hg) | 84 ± 10 | 86 ± 11 | 83 ± 9 | 84 ± 11 |
| Previous participation | 94 (87%) | 35 (100%) | 35 (78%) | 25 (86%) |
| Distance trained (km) | 491 ± 661 | 541 ± 458 | 509 ± 866 | 398 ± 476 |
| Exercise (hours/week) | 3.3 ± 4.3 | 3.9 ± 6.0 | 3.3 ± 3.3 | 2.6 ± 3.2 |
| ≥5 times/week ≥30 minutes exercise | 91 (83%) | 29 (83%) | 37 (82%) | 25 (86%) |
| Cardiovascular disease | 24 (22%) | 15 (43%) | 8 (18%) | 1 (3%) |
| Hypertension | 22 (20%) | 13 (37%) | 8 (18%) | 1 (3%) |
| Hypercholesterolemia ⁎ | 19 (17%) | 6 (17%) | 7 (16%) | 6 (21%) |
| Myocardial infarction/cerebrovascular infarction | 4 (4%) | 4 (11%) | 1 (2%) | 0 (0%) |
| Diabetes mellitus type 2 | 2 (2%) | 2 (6%) | 0 (0%) | 0 (0%) |
| Depression/asthma/rheumatoid arthritis | 5 (5%) | 2 (6%) | 1 (2%) | 2 (7%) |
| Medication | ||||
| Medication use | 35 (32%) | 17 (49%) | 13 (29%) | 5 (17%) |
| Antihypertensive drugs | 20 (18%) | 14 (40%) | 6 (13%) | 0 (0%) |
| Statins | 14 (13%) | 6 (17%) | 6 (13%) | 2 (7%) |
| Diuretics | 8 (7%) | 6 (17%) | 2 (4%) | 0 (0%) |
| Anti-inflammatory drugs | (9%) | 0 (0%) | (15%) | (10%) |
⁎ Total cholesterol levels >6.5 mmol, as previously diagnosed by a physician.
One day or 2 days before the start (day 0), venous blood was drawn and serum was stored for later analysis, and general demographic data were obtained. Before the start of each walking day (which varied from 4 to 7 a.m. depending on the walking distance) and at every 5-km point, heart rate and core body temperature were measured. Immediately after finishing, all measurements from day 0 were repeated.
All measurements were performed in the same laboratory located at the finish area. Measurements were performed from 11 a.m. to 5 p.m. on day 0, and directly after finishing (which varied from 12 to 5 p.m. ) for subsequent walking days. The sequence of measurements was similar each day.
On day 0, body weight (Seca 888 Scale, Seca, Hamburg, Germany) and height were measured. In addition, heart rate and blood pressure at rest were measured using an automated sphygmomanometer (M5-1 Intellisense, Omron Health Care, Hoofddorp, The Netherlands) after 5 minutes of seated rest.
Core body temperature was assessed using a portable telemetry system (CorTemp system, HQ Inc., Palmetto, Florida), which has been demonstrated to be safe and reliable. Participants ingested an individually calibrated telemetric temperature sensor the evening preceding day 1. Before the start of each walking day (days 1 to 4) the core temperature of each participant was measured using an external recorder. Baseline core body temperature was defined as the average of 3 consecutive measurements. Similarly, core body temperature was measured every 5 km along the route. Participants ingested a new telemetric sensor when the sensor was eliminated from the body or the transmitted signal was too weak to record. Mean core body temperature during each day was calculated as the average of all measurements, excluding the values derived before the start and after the finish. In addition, the highest value of these measurements was presented as the peak core body temperature.
Heart rate was measured with a 2-channel electrocardiographic chest band system (Polar Electro Oy, Kempele, Finland) simultaneously with core body temperature, using the same data recorder. Mean heart rate during each walking day was calculated as the average heart rate, excluding the values derived directly before the start and after the finish. Mean heart rate during exercise was presented in absolute values (beats per minute) and as a percentage of the predicted maximal heart rate (percentage from 208 to 0.7 × age).
Ten milliliters of blood was drawn from an antecubital vein. After exercise on days 1 to 4, this was performed 10 to 20 minutes after the finish. Whole venous blood was collected in serum-gel Vacutainer tubes and allowed to clot for ∼45 minutes. After centrifugation, serum was aliquoted, frozen, and stored at −80°C for later analysis.
Cardiac TnI was analyzed using the STAT troponin I assay for the Immulite 2500 system (Siemens Healthcare Diagnostics, Breda, The Netherlands). Total assay imprecisions were 7.8% at 2.3 μg/L and 7.9% at 29.1 μg/L. The detection level of this assay was set at 0.1 μg/L. However, quality control analysis of our data <0.1 μg/L revealed coefficients of variation of 8.1% and 14.7% at 0.08 and 0.02 μg/L, respectively, which then increased to 24.1% at 0.01 μg/L. Analysis was performed on a single day using the same calibration and setup to minimize variation. After identifying participants with a cTnI >0.2 μg/L, which is used as the clinical cut-off value for diagnosis of AMI, values were cross-checked using a highly sensitive cTnI assay (Centaur TnI-Ultra, Siemens Healthcare Diagnostics). Assay imprecisions of the highly sensitive cTnI assay were 5.3% at 0.08 μg/L and 3.0% at 27.2 μg/L, with a detection limit of 0.006 μg/L.
Another 2 ml of blood was drawn from the antecubital vein for immediate analysis from the collecting syringe for plasma levels of hematocrit (liters per liter) using a Rapidpoint 400 (Siemens Healthcare Diagnostics, Inc., Tarrytown, New York). In addition, hemoglobin (millimoles per liter) was determined using a B-hemoglobin analyzer (Hemocue AB, ängelholm, Sweden). Relative changes in plasma volume (percentage) were calculated from blood hematocrit and hemoglobin concentrations using Dill and Costill’s equation.
Dry bulb, wet bulb, and globe temperatures were measured every 30 minutes during the 4 days using a portable climate monitoring device (Davis Instruments, Inc., Hayward, California) positioned at the start/finish area. The wet bulb globe temperature index was calculated using the formula: wet bulb globe temperature index = 0.1 (T dry bulb ) + 0.7 (T wet bulb ) + 0.2 (T globe ), where T represents temperature.
Statistical analyses were performed using SPSS 16.0 (SPSS, Inc., Chicago, Illinois). All data are reported as mean ± SD unless stated otherwise, and statistical significance was assumed at a p value <0.05. When data demonstrated a non-Gaussian distribution, natural logarithmic transformation was applied. Repeated measures analysis of variance (with day as the independent factor) was used to assess differences across the 5 testing days for cTnI. Post hoc t tests with the least square difference correction for multiple comparisons were performed when the analysis of variance reported a significant main or interaction effect. Backward stepwise linear regression analysis was used to identify factors that significantly relate to postexercise cTnI levels. Age, gender, BMI, walking speed, core body temperature, distance trained, and cardiovascular pathology were examined as potential determinants of postexercise cTnI level.
Results
Due to orthopedic problems, 3 participants did not complete day 1, and 2 participants did not finish on day 2. Another participant was excluded from further participation because he exceeded the time limit on day 2. As a result, 103 participants completed the event (94.5%), which was slightly greater than the overall completion rate (89.4%).
Apart from age, baseline characteristics were not significantly different among the 3 groups that walked 30, 40, or 50 km ( Table 1 ). Based on medical history, 22% was diagnosed a priori with cardiovascular disease, with hypertension reported most frequently ( Table 1 ). The 30-km group, including the oldest participants, reported the highest prevalence of cardiovascular disease ( Table 1 ). Participants with prescribed medication predominantly used antihypertensive drugs, statins, and/or diuretics ( Table 1 ).
Exercise was performed under mild ambient conditions, which did not differ across the 4 days ( Table 1 ). Mean walking speed and mean finish time did not differ across days ( Table 2 ). Core body temperature and heart rate increased significantly during exercise ( Table 2 ). When presented as the relative intensity, exercise intensity on the 4 days varied from 72% to 65% of their predicted maximal heart rate. Plasma volume did not change after day 1 or 2, but showed a significant increase on consecutive days (p <0.001, analysis of variance; p <0.05, post hoc analysis; Table 2 ).
| Variable | Day | |||
|---|---|---|---|---|
| 1 (n = 106) | 2 (n = 103) | 3 (n = 103) | 4 (n = 103) | |
| Walking | ||||
| Time (hours:minutes) | 8:37 ± 1:38 | 8:57 ± 1:33 | 8:36 ± 1:47 | 9:08 ± 2:09 |
| Speed (km/hour) | 4.6 ± 0.6 | 4.4 ± 0.6 | 4.6 ± 0.6 | 4.4 ± 0.6 |
| Ambient conditions | ||||
| Minimum wet bulb globe temperature (°C) | 13.2 | 14.2 | 12.3 | 12.5 |
| Maximum wet bulb globe temperature (°C) | 20.6 | 20.4 | 19.8 | 19.4 |
| Physical parameters | ||||
| Heart rate during exercise (beats/min) | 115 ± 18 | 108 ± 16 | 105 ± 15 | 104 ± 14 |
| Heart rate during exercise (percent maximum heart rate) | 72 ± 10 | 68 ± 9 | 66 ± 10 | 65 ± 9 |
| Peak core body temperature (°C) | 38.2 ± 0.4 | 38.3 ± 0.5 | 38.0 ± 0.5 | 38.1 ± 0.4 |
| Plasma volume change compared with day 0 (%) | 0.2 ± 8.7 | 2.1 ± 8.1 | 9.2 ± 10.7 | 10.8 ± 10.8 |
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