I was recently asked by a rather intimidating medical student,
"What's the point of measuring the respiratory rate?"
Fortunately I had recently read Respiratory Rate: The Neglected Vital Sign and had an answer ready. If you haven't read it I would strongly recommend that you do. The authors make a very good case for using RR, ahead of other vital signs, to help identify critically ill patients. Amongst several thought provoking references that were quoted by the authors, one really stands out. Back in 2002, a team from the Royal National Orthopaedic Hospital at Stanmore studied 1047 patients who were referred to their intensive care outreach service for assessment. Whilst a number of vital signs were recorded, it was the respiratory rate (RR) that most closely mirrored hospital mortality. At a RR of 6 to 24, the mortality rate was 9% but this increased consistently with RR - 21% (RR: 25-29), 28% (RR: 30-34) and 41% (RR 35+)*.
The reason why respiratory rate mirrors hospital mortality shouldn't come as a big surprise. Life threatening conditions - whether they be medical or surgical - either starve the body of oxygen or generate enough acid from damaged tissues to trigger a significant rise in carbon dioxide. Sometimes they do both. Either way, peripheral and central chemoreceptors respond and the rate and depth of breathing increases. For a short while this may improve oxygen delivery and remove excess carbon dioxide. However without the right intervention the patient will eventually deteriorate and die. RR may help identify patients early and treat them before they reach the latter stages of their illness. We would therefore do well to follow the conclusions of "Respiratory Rate..." and remember,
"Any adult with a respiratory rate of over 20 breaths per minute is probably unwell and any adult with a respiratory rate of over 24 breaths per minute is likely to be critically ill."
Unfortunately there's a problem with this advice for mountain medics. Whilst the dangers of a RR of 20 or 24 might hold true at sea level, it cannot be applied at altitude. Indeed, a rise in RR is a healthy response to hypoxia and is part of the normal acclimatisation process. An increase in RR is to be encouraged. Nevertheless the science still holds - the parameters just need to change. RR's of 20 and 24 are comparable to 50% and 100% increases in resting sea level values. When examining a patient at altitude the key is to use the vital signs of a healthy, well acclimatised member of your group as a guide. Hopefully, this will be you! If the trekker you're examining has a RR that is 50% greater than yours they're likely to be unwell, if it's 100% greater then there's a real chance that the trekker is critically ill.
The prominent pinnacle of the Dent Du Geant can be seen directly above the Torino Hut
I did my best to follow this advice recently whilst staying at the Torino Hut (3375m). During the previous day's approach my partner and I had met two Swiss climbers who were planning to do the same route as us - the South West Face of the Dent Du Geant. Thanks to an early start we managed to avoid them but on their return we shared a celebratory drink. Unfortunately, one of the pair was diabetic and had misjudged the amount of insulin that had been needed for the route. As a result his blood sugar measurements were high and he had begun to feel tired and unwell during the descent. His partner had put this down to dehydration and had encouraged his friend to drink more fluids and increase his dose of insulin. Unfortunately, this did not lead to any improvement in symptoms and I was asked to examine him a few minutes before the last cable car of the day was due to descend! Suffice to say, all his vital signs were normal except a RR of 36 (mine was 16!). But this, together with his symptoms and medical history, led me to think that he was suffering from diabetic ketoacidosis (DKA) and I urged him to descend. With the help of the hut guardian we were able to escort the climber and his partner into the cable car and arranged for an ambulance to pick him up in the valley.
A week or so later an email arrived confirming that the climber had been diagnosed with DKA and had made a full recovery. A rather worrying "off the scale" set of blood results were also included. Looking back, I think the RR measurement had been the deciding factor. With this example the medical student seemed satisfied and headed off to his next teaching session. I breathed a sigh of relief.
*Goldhill DR, et/al. A physiologically-based early warning score for ward patients: the association between score and outcome. Anaesthesia 2005;60:547-553.
Many thanks to Dr Edi Albert for reminding me about this article and inspiring this post!