Sports Cardiology of Cross Country Skiing: Sochi Postcard

Feb 19, 2014   |  Christine E. Lawless, MD, FACC

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Editor's Note: Dr. Christine Lawless is Co-Chair of the ACC Sports and Exercise Cardiology Council/Section.

Pushing the limits of cardiac performance more than any other winter sport, perhaps even more than any other endurance sport, cross-country (XC) skiing athletes personify the Olympic motto, "Citius, Altius, Fortius" (Faster, Higher, Stronger). Inspired by athlete performances at the XXII Olympic Winter Games, and assisted by Team USA Nordic Skiing medical staff stationed at Endurance Village in Sochi, we explore the extraordinary cardiovascular (CV) demands placed upon these athletes, and the cardiac issues that may present to sports cardiologists asked to evaluate them.

Exercise Physiology of Cross-Country Skiing

Any sports cardiology assessment begins with awareness of the unique CV demands imposed by the athlete's sport, CV adaptations to training, and sports-specific appearance of the heart on cardiac testing. XC skiers may face the greatest CV demand of any sport, owing to the combined demands of endurance, altitude, movement of upper and lower extremities, and need for sprinting.1 Competitive XC skiing requires high maximal oxygen (VO2max) uptake, fast force production, and ability to resist fatigue. Distances of XC ski races vary between 1.5-30 km for women, and 1.5-50 km for men. During exercise, chronic training can result in VO2max as high as 87ml/kg/min, heart rate of 185 bpm, stroke volume 200ml, and cardiac output 37-40 L/min, Table 1.1

There are some notable outliers. Retired Norwegian XC skiing superstar, Bjorn Daehlie, was reported to have the highest VO2max for a skier at 96ml/kg/min.1a The single most important determinant of performance is VO2max, which is determined by cardiac output and circulatory function, lung function, red blood cell O2 carrying capacity, and peripheral O2 utilization. VO2 max is directly proportionate to maximal cardiac output, the most important single determinant of VO2max. Skier performance can be influenced by arterial blood O2 de-saturation at submaximal and maximal exercise, made profoundly worse by altitude. This is felt to be related to high transit times of blood through the lungs. The majority of XC skiing athletes collapse at the finish line, due to a combination of exhaustion, cessation of the muscular pump, arterial oxygen (O2) desaturation, and respiratory fatigue.1

Table 1: Maximal values at variable levels of training 1
  VO2max
(ml/kg/min) 		Cardiac
output (L/min) 		Stroke
volume (ml) 		Heart
rate (bpm)
Sedentary adult 30 15 75 198
Trained adult 53 23 120 195
Elite XC skier 87 37 200 185

Cardiac Testing in XC Skiers

Sports cardiologists may be called upon to interpret cardiac testing in XC skiing athletes, especially if there are symptoms that may represent underlying CV disease. Therefore, knowledge of sport-specific normative data is crucial. Male XC skiers demonstrate highest VO2max, 77 +/- 1 ml/kg/min, compared to 72 +/- 2 and 75 +/- 2 ml/kg/min in endurance cyclists and long distance runners respectively;2 while VO2max is 10-15% lower in female XC skiers.3 On cardiac imaging, adaptations appear as four chamber enlargement, with up to 40% increases in RV and LV mass, RV and LV volumes, and stroke volume compared to control non-athletes.2-4 LVEF is generally in the normal range in XC skiers;4 whereas it may be mildly reduced in up to 11.5% of endurance cyclists 5. Although information on ambulatory monitoring in XC skiers is scant, in other groups of endurance athletes, rate of complex ventricular arrhythmias is quite low;6 but, if present, may indicate a worse prognosis.7

Training an XC Skier and Preparing for Competition

As XC skiers are primarily "oxygen- users," training regimens for endurance athletes are designed to optimize MVO2. Invariably, this will involve some type of altitude training to take advantage of the physiologic adaptations to hypoxia: Live High-Train Low, Live High-Train High, or Artificial altitude.8 A variety of physiological changes occur at high altitude, which can impact VO2max, but, whether performance gains seen by altitude training are due to change in red blood cell volume, or a switch to a more economic mode of oxygen utilization is debated.9,10

Dr. Jim Stray-Gundersen serves as Sports Science Advisor to the US Ski and Snowboard Association (USSA). Currently in Sochi with Nordic Combined Team USA athletes, he is an authority on the merits of Live High-Train Low to optimize effect on VO2max in endurance athletes. He said, "There is not an endurance athlete today that is competitive at an international level that does not use hypoxia in their preparation for their most important competitions..... Altitude is divided into: sea level 0-1000m, low 1000-2000m, moderate 2000-3000m, high 3000-5000m and extreme >5000m. We have data supporting the idea that one needs three to four weeks at moderate altitude to have a positive effect in most athletes." This week, US XC athletes are living and competing at 1500m altitude in Sochi. This is low altitude and below all the athletes' altitude threshold; so, workouts are adjusted accordingly, to maintain optimal performance for each athlete. Stray-Gundersen said, "There is not a 'one size fits all' solution."

Medical Aspects of XC: Short and Long Term Cardiac Risks

ACC Sports and Exercise Section member, Dr. Larry Gaul, is also in Sochi, serving as a USSA Nordic Team Physician. Gaul's role is to evaluate and treat practice- and competition-related medical ailments, but also conditions that happen to athletes when they are living in tight quarters with a thousand of their closest friends. CV conditions are rare; but, over his career, Gaul has treated several instances of WPW-related arrhythmias which first became clinically manifest during a race event. As ECGs are now done routinely, he sees the WPW issue decreasing. Gaul said one of the more concerning issues in some of his skiing athletes is venous thromboembolism. In April 2008, Alaska resident and US Nordic star, Kikkan Randall, was diagnosed with deep vein thrombosis due to Factor V Leiden deficiency.11 Now, Gaul recommends Randall take an anticoagulant before long flights; in fact he takes one himself, and recommends it for selected athletes.

While XC skiing athletes from throughout the world were preparing to compete in Sochi, a young Dartmouth College XC skier named Torin Tucker suffered sudden cardiac arrest at the 42km point of a 50km race in Vermont on February 1st, 2014, and could not be resuscitated. Media reports indicated the cause of death was anomalous coronary artery.12 Sudden cardiac death (SCD) appears to be rare in the sport of XC skiing. In an older study among participants of the Swiss Engadine Ski Marathon, an incidence of SCD of 1:120,000 skiing hours was reported.13 Studies of Swedish Vasaloppet ski racers showed more than 7 X greater risk of SCD acutely during races compared to the non-skiing general population.14 But, longitudinal studies of 73,622 Vasaloppet race participants over a 10 year period yielded standardized mortality ratios of 0.48 [95% confidence interval (CI) 0.44-0.53], lowest in older participants and in those who participated in several races.15 Thus, while there is an increased risk of acute mortality during long-distance skiing, the short-term excess mortality appears to be outweighed by the long-term protective effects of XC skiing on mortality. Aside from mortality, long-term, there appears to be higher incidence of arrhythmias especially atrial fibrillation, in XC skiers,16, 17 similar to what has been observed in other endurance athletes.18

Summary

The Sochi XXII Olympic Winter Games provides ample opportunity to marvel in athleticism, drama and spectacle. But, for the cardiologist, the Games can also serve as the inspiration to advance one's knowledge of sports and exercise cardiology through closer analysis of an individual sport. We have presented the exercise physiology, training regimen, CV demands and adaptations, appearance of cardiac tests, and medical concerns of the XC skiing athlete. To properly provide consultation to these athletes, cardiologists require command of the information presented here.

Acknowledgements:

The authors would like to thank Dr. Jim Stray-Gundersen, Sports Science Advisor to the US Ski and Snowboard Association (USSA), and Dr. Larry Gaul, USSA Nordic Team Physician and member of the ACC Sports and Exercise Cardiology Section, for their contributions to this paper, and for taking time from their busy schedules in Sochi to correspond.

References

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Keywords: Exercise, Sports, Risk


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