Heat Stress and the Immune Response

by Mary Boudreau Conover BSNed

Welcome to part 2 of STRESS. In part 1 we covered prolonged excessive exertional stress and psychological stress. Both heat stress and intense exercise reduce blood flow to the intestines in order to provide the skin with an increased perfusion of blood for heat dissipation. This causes release of endotoxins (liposaccharides) that damage the intestinal lining and elicit, by cell-signaling, the immune response, a production of proinflammatory cells such as monocytes and macrophages.^{1 2 3 4 5 6}

Damage from Heat Stress

The damage from heat stress during competition in endurance sports is initially the same as that from the psychological and exertional stress, covered in Stress-Induced Breach of the Gut Lining. However, when heat is a major factor the path to death is shortened by multiple organ failure, and occurs more often in unprepared, unfit, runners.

Life-Threatenng Events During Endurance Sports

Is heat more important than arrhythmic deaths? From March 2007 to November 2013 a study was done by Lior Yankelson MD PhD and his associates in Tel Aviv involving 137,580 long distance runners to determine if more life-threatening events during endurance sports were caused by heat stroke or directly related to events of the heart.⁷ The Tel Aviv study was followed by a Medscape journalist’s review with an eye-catching, misleading title: “Heat, Not Heart, More Likely Culprit When Runners Collapse.”⁸

Thankfully, two brilliant, experienced and respected Cardioloists, Brian Olshansky M.D. and David S. Cannom M.D., wrote an editorial on the Yankelson Tel Aviv study.⁹ They entitled their editorial to acknowledge both dangers — ” Neither too fast nor too hot: Keeping Marathoners’ hearts alive during the race.”

The stats. For the 137,580 runners in the combined 14 Tel Aviv marathons, there were two non-fatal cardiac events, two heat stroke fatalities (multiple organ failure) and 19 heat stroke survivors. All were aggressively treated.

No comparison. For those endurance athletes who repeatedly run intense marathons, the heart itself can kill at any time, irrespective of gut integrity and the temperature. Life-ending arrhythmias due to cellular electrophysiological and anatomical remodeling of the heart muscle secondary to repeated and intense cardiac overload and stretching of that unique muscle time after time, race after race, provides the initiating cause as well as the sustaining deadly mechanism; and that death need not be while actually participating in a marathon. The athlete’s history counts!¹⁰

The Links between Endurance Training, Gut Barrier Integrity and Heat Tolerance

A group of Canadians published a well documented study looking at the links between cardiac output, gut barrier integrity, and heat tolerance with the conclusion that a higher core temperature can be tolerated longer by endurance trained athletes than by untrained individuals. Their study is well worth the read (on-line) and is summarized here.

Aerobic fitness accounts for the differences between endurance trained athletes and untrained individuals in both gut barrier integrity and its effect on thermotolerance. There is “a functional link between the cardiovascular adaptations that occur with endurance training and the resultant anti-inflammatory cytoprotective changes.

For the soldier, maintaining a high level of aerobic fitness enables them to maintain their operational state of readiness for longer periods of time when exposed to high environmental temperatures while wearing protective clothing and carrying heavy loads.”¹¹

References

1. Bouchama A, Al-Sedairy S, Siddiqui E, Rezeig M: Elevated pyrogenic cytokines in heatstroke. Chest 1993,104:1448-1502.
2. Bouchama A, Parhar S, El-Yazigi A, et al: Endotoxemia and release of tumor necrosis factor and interleukin 1-alpha in acute heatstroke, 1991 J Appl Physiol 70:2840-2844.
3. Lambert GP: Characterization of hyperthermia-induced intestinal permeability and the role of oxidative and nitrosative stress, 2001 PhD Diss. Univ. Iowa, Iowa City.
4. Kregel, K. C., P. T. Wall, and C. V. Gisolfi. 1988. Peripheral vascular responses to hyperthermia in the rat. J. Appl. Physiol. 64:2582 – 2588.
5. Sakurada, S., and J. R. S. Hales. 1998. A role for gastrointestinal endotoxins in enhancement of heat tolerance by physical fitness. J. Appl. Physiol. 84:207 – 214.
6. Lambert GP: B J Anim Sci 2009, 87: E101-E108.
7. Yankelson L, Sadeh B, Gershovitz L, et al: Life-threatenng events during endurance sports: Is heat stroke more prevalent than arrhythmic death? J Am Coll Cardiol 2014; 64:463-469.
8. Michael O’Riordan (a senior journalist for theheart.org and a Medscape Cardiology journalist), Heat, Not Heart, More Likely Culprit When Runners Collapse, July 29, 2014.
9. Olshansky B, Cannom DS: Neither too fast nor too hot: Keeping Marathoners’ hearts alive during the race. J Am Coll Cardiol 2014; 64:470. Ed
10. O’Keefe JH, Patil HR, McCullough PA: Potential Adverse Cardiovascular Effects From Excessive Endurance Exercise http://www.mayoclinicproceedings, Jun 2012;87(6):587-595
11. McLellan, TM, Selkirk GA, Wright HE, Rhind SC: Importance of aerobic fitness in extending thermotolerance in extreme environments: connecting molecular biology to the whole body response.