Using Ultrasound to Evaluate Acute Mountain Sickness

©Outdoor EM Acute mountain sickness (AMS) is the bane of many hikers and mountain climbers, affecting persons of all different ages, BMI and fitness levels.  Its debilitating symptoms of headache, insomnia, fatigue and GI disturbance can ruin a much anticipated trip, as although we have some pharmacologic therapy available to help treat it, the best treatment remains descent.  The pathophysiology behind it is not fully understood, but it is postulated that an increase (or insufficient loss) in intra- and extra-vascular fluid volume is at least partially behind the syndrome, but investigations into the cause have been limited due to the fact that, by definition, those who experience it are typically far from immediate medical attention and are generally successfully treated without the care of a physician anyway.  Ultrasound is an ideal technology for investigations into subjects such as this given its increasing utility and miniaturization.

Pitman et. al. recently published a paper on their research into the use of field ultrasound to evaluate central volume status as a marker of AMS symptom severity.  The variables they used to determine central volume status were the inferior vena cava collapsibility index (IVC CI) and left ventricular outflow tract velocity-time integral (LVOT VTI) before and after passive leg raise.  For the IVC CI measurement, a collapse of >50% was determined to be suggestive of low intravascular volume status.  The subjects underwent an altitude gain of 2759m (ending at an altitude of 3519m) and were excluded if they had been exposed to altitude >2500m for more than 7 days.  They were included in the AMS cohort if they had a headache and at least one other symptom (including nausea, vomiting, fatigue, weakness, dizziness, lightheadedness or poor sleeping), defined by the Lake Louse AMS questionnaire.  They had a total of 69 subjects enroll: 18 in the AMS group and 51 in the control group.

Ultrasound was performed while the subjects were seated and a screening medical exam was performed (heart rate, BP, respiratory rate, pulse ox).  The results of the study demonstrate no significant difference in the IVC CI between the control group and the AMS group with passive leg raise.  Of note, there was also no difference in either heart rate or cardiac output between the control and AMS groups.  However, they did find a significant difference in the increase in LVOT VTI before and after passive leg raise between the two groups.

 

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These results suggest that the use of ultrasound, which has already found such a niche in field medicine, may have a role in monitoring changes in intravascular volume through serial examination of climbers.  The individual variability limits its usefulness as a point-of-care test, but it may have a role in climbers who are known to be particulary susceptible to AMS.  The difference between the LVOT VTI in the control and AMS groups (with the AMS groups demonstrating less change in LVOT), supports the hypothesis laid down by previous studies that AMS is at least contributed to by pathologic fluid retention which doesn’t allow the individual to adapt to the hypoxic hypobaric environment.  This modality is not ready for primetime, as this was an initial study with a small number of subjects and large variability.  But it lays the groundwork for future studies using ultrasound to monitor for fluid loss or retention in the field which may provide an early warning sign for dehydration in austere environments, as well as conditions such as AMS.

Reference:

Pitman JT, Ghan BT, Harris NS.  Field Ultrasound Evaluation of Central Volume Status and Acute Mountain Sickness.  Wild Environ Med. 2015; 26: 319-326.