Alterations in cardiac structure and function at high altitudes

By Tommy Kirkham,

As winter continues in British Columbia, more and more athletes will venture up to the mountains to enjoy various Nordic sports from skiing and snowboarding to ice climbing and mountaineering. An important aspect to remember about higher elevation is the reduced atmospheric oxygen pressure. That is to say that high altitudes are relatively hypoxic environments. In response to the decreased oxygen being delivered to the system, especially during exercise, certain changes occur to the cardiovascular system at higher altitudes. These changes consist of acute structural and functional adaptations of the heart and its mechanics in order to facilitate appropriate oxygen delivery at rest and during exercise (Stembridge et al. 2015).

At higher altitudes, around 4000-5000m above sea level, hypoxic pulmonary vasoconstriction (HPV) can occur (Stembridge et al. 2015). HPV results in the blood vessels of the lungs and the rest of the respiratory system constricting. This constriction will result in less blood being returned to the heart from the lungs; therefore, the stroke volume (SV), the amount blood ejected from the heart at each beat, will be reduced (Stembridge et al. 2015). This means that during exercise at high altitude the left ventricle will pump less blood to the body; however, the cardiovascular system compensates for the lower stroke volume by increasing the average heart rate in order to keep a constant cardiac output (CO) (Stembridge et al. 2015). The higher heart rate is achieved by an increase in sympathetic nerve activity and an increase in vagal tone (the vagus nerve is a major heart rate regulator) (Stembridge et al. 2015) (Vizzardi et al. 2015). It is important to note; however, that the maximal heart rate during exercise at higher elevation is reduced (Stembridge et al. 2015). Furthermore, due to the hypoxic environment and the increase in sympathetic activity, a decrease in plasma volume (the liquid component of blood) is evident (Stembridge et al. 2015). The reduced plasma volume results in a lower left ventricle end diastolic volume (LVEDV), which is the amount of blood in the left ventricle before it is pumped out to the rest of the body (Stembridge et al. 2015). The lower LVEDV also contributes to the lower SV at high altitude. These changes are evident even after short-term high altitude exposure.

One of the major implications of all this cardiac remodeling is that lowlanders venturing up to higher elevations will have a lower functional reserve volume of blood in their heart (Stembridge et al. 2015). The functional reserve volume is normally used during exercise to meet the increased oxygen demand of the contracting muscles. Because of the reduced reserve volume, there is less oxygen availability in the body, which can result in lower muscular power output (Garvican-Lewis et al. 2015). Furthermore, if the environment is more severely hypoxic and the oxygen demand of the cardiomyocytes (heart muscle cells) is not met, then the contractility of the heart will decrease (Stembridge et al. 2015). In other words, at very high elevations the heart is stiffer. Additionally, those individuals with existing coronary artery disease (CAD) are at a higher risk at high altitude because the hypoxic environment exacerbates cardiac and pulmonary diseases (Vizzardi et al. 2015), and diseased coronary arteries do not dilate with increasing hypoxemia as healthy vessels do (Vizzardi et al. 2015). Therefore, only patients with stable CAD, as shown clinically and/or electronically, should risk an adventure at high altitude (Vizzardi et al. 2015).

Considering these cardiovascular changes when exercising at high altitude, you should always be aware of your heart rate and any level of light-headiness, dizziness and shortness of breath (also known as dyspnea) as they can be signs of insufficient oxygen perfusion and a possible cardiovascular problem. Also be sure to stay adequately hydrated and avoid diuretics like caffeine and alcohol in order to perform your given activity and to keep plasma volumes at a sufficient level so your SV doesn’t decrease any further.

References:

1) Garvican-Lewis L, Clark B, Martin D, Schumacher Y, McDonald W, Stephens B, Ma F, Thompson K, Gore C, Menaspa P. Impact of altitude on power output during cycling stage racing. PLOS One 10(12): 1-15, 2015.

2) Stembridge M, Ainslie P, Shave R. Short-term adaptation and chronic cardiac remodeling to high altitude in lowlander natives and Himalayan Sherpa. Exp Physiol 100.11: 1242-1246, 2015.

3) Vizzardi E, Berlendis M, Sciatti E, Bonadei I, Quinzani F, Tassi G, Metra M. Risk assessment for high altitude alpinist with coronary artery disease. Heart, Lung and Vessels 7(3): 268-270, 2015.