Downloaded from http://jramc.bmj.com/ on September 12, 2015 - Published by group.bmj.com
Review
Physiology studies at high altitude; why and how Adrian Mellor,1,2,4,5 D Woods1,2,3,5 1
Defence Medical Services, Lichfield, UK 2 MDHU Northallerton, Friarage Hospital, Northallerton, UK 3 Royal Victoria Infirmary, Newcastle upon Tyne, UK 4 Cardiothoracic Anaesthesia, S Tees NHS Foundation Trust, Middlesbrough, UK 5 Carnegie Institute of Sport, Physical Activity and Leisure, Leeds Metropolitan University, Leeds, UK Correspondence to Surg Cdr Adrian Mellor, MDHUN, Friarage Hospital, Northallerton DL6 1JG, UK; [email protected]. uk Received 11 December 2013 Accepted 14 December 2013 Published Online First 21 January 2014
ABSTRACT The military has always had an important role in high altitude research. This is due to the fact that mountainous regions often span borders and provide a safe haven to enemies. Deploying troops rapidly into high altitude environments presents major problems in terms of the development of high altitude illness. This paper examines the rationale for carrying out research at high altitude and the opportunities within the UK Defence Medical Services for carrying out this research.
▸ Ascent to high altitude leads to significant physiological changes. ▸ Research at high altitude presents a significant challenge. ▸ The military needs to better understand adaptation to high altitude to be able to deploy troops safely and effectively in this environment.
INTRODUCTION Barometric pressure changes on ascent to high altitude (HA). Above 2500 m, oxygen saturations can be expected to fall below 90% and physiological changes occur in every body system. On ascent to altitude, there is an increase in RR which has the effect of reducing alveolar carbon dioxide (CO2) which in turn allows a small but crucial increase in alveolar oxygen. This would normally be limited due to an increase in pH slowing the respiratory drive; however, at altitude, bicarbonate is excreted maintaining a normal pH for a lower CO2. The diuresis that brings about increased bicarbonate excretion also creates relative haemoconcentration generating more oxygen carrying capacity per unit of blood. Through these mechanisms, individuals can tolerate profound hypoxia well. This process is termed acclimatisation. At the extremes of altitude, ABGs are quite remarkable with arterial samples drawn at 8400 m showing an arterial oxygen tension (PaO2) of 3.28 KPa, PaCO2 of 1.77 KPa and normal pH1 (Table 1). The physiological variables recorded during the 2012 Defence Medical Services (DMS) expedition to Bolivia are displayed in Table 2. However, in some individuals, acclimatisation fails resulting in a spectrum of altitude illness. Acute mountain sickness (AMS) is the most common and includes a headache plus a variety of other, multisystem symptoms (Table 3). HA pulmonary oedema and HA cerebral oedema are lifethreatening conditions occurring as a result of failure to acclimatise. These profound physiological changes in otherwise healthy individuals have led to an interest in investigating the effects of HA in the hope that clinically relevant lessons can be identified.2 3
Military importance
To cite: Mellor A, Woods D. J R Army Med Corps 2014;160:131–134.
Key messages
Since the end of the cold war, there has been a shift in conflict from large organised Armies to smaller ‘asymmetric’ threats from ideologically motivated groups rather than nation States. It stands to reason that these groups will seek shelter in inhospitable regions, away from easy surveillance and communication and mountains provide just
Mellor A, et al. J R Army Med Corps 2014;160:131–134. doi:10.1136/jramc-2013-000206
such a haven. During the initial war in Afghanistan, US forces fought at HA to displace the Taliban. An important battle during this phase was Op Anaconda, fought in the Shahi-Kot valley, where 2700 US and Afghan soldiers faced 1000 Taliban fighters at altitudes of 2500–3500 m. During this phase of the conflict, 8% of US casualties evacuated were as a result of altitude illness.4 The problem of identifying when troops are ready to deploy at HA is exacerbated by the fact that the only diagnostic criteria for AMS is the selfreported Lake Louise Score (LLS).5 A self-reported score may not be useful with a cadre of highly motivated and committed soldiers and the score also shares many features with anxiety questionnaires. A biochemical test or predictor for altitude illness would be useful. The NATO review of HA medicine in the Military6 identifies the need for a capability for the ‘early detection and diagnosis of altitude illness prior to onset of severe symptoms that is independent of the victim reporting their symptoms’ and this ‘will reduce altitude illness impact on mission and medical support and evacuation requirements’.
PROBLEMS IN CONDUCTING HA RESEARCH There are many problems with conducting high quality research at HA.
Subjects Many hundreds of thousands of people subject themselves to the potential risks of HA environments for leisure activities annually. To this extent, it is ethically justifiable to take a trekking team on a research expedition knowing that some will become ill. However, what becomes more difficult is tailoring a research protocol to an individual’s expectations. If subjects receive a fully funded trip to a beautiful and remote environment, should this be seen as unethical inducement or, alternatively, if subjects are paying a large amount of money to take part in the research trip, how far can protocols 131
Downloaded from http://jramc.bmj.com/ on September 12, 2015 - Published by group.bmj.com
Review Table 1 High altitude standard definitions and associated physiological changes
Intermediate altitude High altitude Very high altitude Extreme altitude
Altitude
Physiology
1500–2500 m 2500–3500 m 3500–5800 m >5800 m
Physiological changes occur, SpO2 >90%, altitude illness possible but rare Altitude illness common with rapid ascent Altitude illness common, SpO2
Review
Physiology studies at high altitude; why and how Adrian Mellor,1,2,4,5 D Woods1,2,3,5 1
Defence Medical Services, Lichfield, UK 2 MDHU Northallerton, Friarage Hospital, Northallerton, UK 3 Royal Victoria Infirmary, Newcastle upon Tyne, UK 4 Cardiothoracic Anaesthesia, S Tees NHS Foundation Trust, Middlesbrough, UK 5 Carnegie Institute of Sport, Physical Activity and Leisure, Leeds Metropolitan University, Leeds, UK Correspondence to Surg Cdr Adrian Mellor, MDHUN, Friarage Hospital, Northallerton DL6 1JG, UK; [email protected]. uk Received 11 December 2013 Accepted 14 December 2013 Published Online First 21 January 2014
ABSTRACT The military has always had an important role in high altitude research. This is due to the fact that mountainous regions often span borders and provide a safe haven to enemies. Deploying troops rapidly into high altitude environments presents major problems in terms of the development of high altitude illness. This paper examines the rationale for carrying out research at high altitude and the opportunities within the UK Defence Medical Services for carrying out this research.
▸ Ascent to high altitude leads to significant physiological changes. ▸ Research at high altitude presents a significant challenge. ▸ The military needs to better understand adaptation to high altitude to be able to deploy troops safely and effectively in this environment.
INTRODUCTION Barometric pressure changes on ascent to high altitude (HA). Above 2500 m, oxygen saturations can be expected to fall below 90% and physiological changes occur in every body system. On ascent to altitude, there is an increase in RR which has the effect of reducing alveolar carbon dioxide (CO2) which in turn allows a small but crucial increase in alveolar oxygen. This would normally be limited due to an increase in pH slowing the respiratory drive; however, at altitude, bicarbonate is excreted maintaining a normal pH for a lower CO2. The diuresis that brings about increased bicarbonate excretion also creates relative haemoconcentration generating more oxygen carrying capacity per unit of blood. Through these mechanisms, individuals can tolerate profound hypoxia well. This process is termed acclimatisation. At the extremes of altitude, ABGs are quite remarkable with arterial samples drawn at 8400 m showing an arterial oxygen tension (PaO2) of 3.28 KPa, PaCO2 of 1.77 KPa and normal pH1 (Table 1). The physiological variables recorded during the 2012 Defence Medical Services (DMS) expedition to Bolivia are displayed in Table 2. However, in some individuals, acclimatisation fails resulting in a spectrum of altitude illness. Acute mountain sickness (AMS) is the most common and includes a headache plus a variety of other, multisystem symptoms (Table 3). HA pulmonary oedema and HA cerebral oedema are lifethreatening conditions occurring as a result of failure to acclimatise. These profound physiological changes in otherwise healthy individuals have led to an interest in investigating the effects of HA in the hope that clinically relevant lessons can be identified.2 3
Military importance
To cite: Mellor A, Woods D. J R Army Med Corps 2014;160:131–134.
Key messages
Since the end of the cold war, there has been a shift in conflict from large organised Armies to smaller ‘asymmetric’ threats from ideologically motivated groups rather than nation States. It stands to reason that these groups will seek shelter in inhospitable regions, away from easy surveillance and communication and mountains provide just
Mellor A, et al. J R Army Med Corps 2014;160:131–134. doi:10.1136/jramc-2013-000206
such a haven. During the initial war in Afghanistan, US forces fought at HA to displace the Taliban. An important battle during this phase was Op Anaconda, fought in the Shahi-Kot valley, where 2700 US and Afghan soldiers faced 1000 Taliban fighters at altitudes of 2500–3500 m. During this phase of the conflict, 8% of US casualties evacuated were as a result of altitude illness.4 The problem of identifying when troops are ready to deploy at HA is exacerbated by the fact that the only diagnostic criteria for AMS is the selfreported Lake Louise Score (LLS).5 A self-reported score may not be useful with a cadre of highly motivated and committed soldiers and the score also shares many features with anxiety questionnaires. A biochemical test or predictor for altitude illness would be useful. The NATO review of HA medicine in the Military6 identifies the need for a capability for the ‘early detection and diagnosis of altitude illness prior to onset of severe symptoms that is independent of the victim reporting their symptoms’ and this ‘will reduce altitude illness impact on mission and medical support and evacuation requirements’.
PROBLEMS IN CONDUCTING HA RESEARCH There are many problems with conducting high quality research at HA.
Subjects Many hundreds of thousands of people subject themselves to the potential risks of HA environments for leisure activities annually. To this extent, it is ethically justifiable to take a trekking team on a research expedition knowing that some will become ill. However, what becomes more difficult is tailoring a research protocol to an individual’s expectations. If subjects receive a fully funded trip to a beautiful and remote environment, should this be seen as unethical inducement or, alternatively, if subjects are paying a large amount of money to take part in the research trip, how far can protocols 131
Downloaded from http://jramc.bmj.com/ on September 12, 2015 - Published by group.bmj.com
Review Table 1 High altitude standard definitions and associated physiological changes
Intermediate altitude High altitude Very high altitude Extreme altitude
Altitude
Physiology
1500–2500 m 2500–3500 m 3500–5800 m >5800 m
Physiological changes occur, SpO2 >90%, altitude illness possible but rare Altitude illness common with rapid ascent Altitude illness common, SpO2
