Addressing the risk of decompression sickness caused by high altitude airdrop missions

Introduction

High Altitude Parachute Drops are fast becoming the preferred strategy for advanced Special Forces (SF) Deployments. A growing number of high profile missions have successfully deployed SF units (and associated equipment), from an aircraft at altitudes up to 35,000 ft above sea level, putting High Altitude Deployment Missions firmly on the map as an effective and established SF capability reserved for the most advanced Special Forces units. The ability to deploy from high altitude permits an aircraft to make a drop from outside contested airspace or enter into contested airspace at high altitudes and thus stay outside of the range from enemy surface-to-air defence systems. This allows for covert insertion of personnel and equipment, such as Aerial Delivery, High Altitude High Opening (HAHO)/High Altitude Low Opening (HALO)- Parachuting and Aerial Reconnaissance. Whilst Armed Forces personnel complete extensive specialist training to prepare for exposure to decreased atmospheric pressure, including pre-oxygenation (breathing 100% oxygen prior to high altitude exposure) human physiological factors combined with environmental conditions are uncontrollable variables that can result in the formation of nitrogen bubbles inside the body and the onset of Altitude Related Decompression Sickness, leaving all crew members, including the pilots, at risk.

What is Altitude Decompression Sickness?

When the pressure of the gases dissolved in the body tissues exceed that of the atmospheric pressure the tissues become supersaturated with gas and there is a potential for the formation of bubbles within the tissues. These bubbles are thought to be the cause of a myriad of clinical disorders from mild limb pain to paresthesia through to more serious neurological or respiratory symptoms and ultimately to complete circulatory collapse and death (1).

Causal Factors

The major causal factor of DCS at altitude is altitude itself. It is widely recognised that the minimum altitude for bubble formation is 18,000ft, although there are reports of DCS suffered below that threshold. A study by the Defence Evaluation and Research Agency (DERA) for Human Sciences reported an 87% incidence of Venous Gas Embolism (Gas bubbles found in the deoxygenated blood traveling to the heart) with 69% of those displaying symptoms of DCS for participants that did not pre-breathe oxygen (pre-oxygenate). A set period of de-nitrogenation with 100% oxygen prior to ascending through 16,000 ft has shown to have a considerable effect in reducing Nitrogen levels, however incidences are still common. A study by Webb et al (1998) found that more than 50% of subjects at 22,500 ft altitude developed DCS despite breathing oxygen (2) and an additional study by Pilmanis & Bisson (1992) reported that 73% of research subjects exposed to 28,000 - 30,000 ft reported DCI (3). The study provided evidence to suggest that another causal factor, strenuous exercise at altitude, could produce even higher numbers. Diving operations both military and civilian have definitive guidelines that direct the proximity (in hours) that they must be within access to hyperbaric oxygen, but no such guidelines currently exist in high altitude aviation. With a minimum risk DCS of 9.99% for every jump, this is an eventuality that must be planned for.

Reducing the incidence of DCS

Decompression Sickness presents a real risk when operating at altitude. Common risk reduction exercises include breathing 100% oxygen for up to one hour prior to exposure to a reduced pressure environment (pre-oxygenate or ‘pre-ox’) in order to displace nitrogen from the body and continue breathing supplemental oxygen until operations return to below 10,000 ft, a process known in the industry as ‘de-nitrogenation’.  According to the current NATO Standard, STANAG 7056, when an aircraft intends to ascend above an altitude of 18,000 ft, a period of de-nitrogenation is required to be completed prior to the aircraft reaching 16,000 ft, providing an extended period of reduced nitrogen content within the tissues prior to high altitude exposure. Whilst these exercises do not remove the possibility of a DCS incident altogether, they do reduce its prevalence. It remains of critical importance not only that prolonged exposure at altitude be minimized, but also that operational care plans are in place when (not if) a DCS incident occurs.  

Leading By Example

In efforts to address an inevitable incident, several of the world’s leading Special Forces Units are utilising portable hyperbaric chamber systems for on-scene treatment and emergency evacuation of symptomatic personnel. A case published by the US Air Force in Fairbanks, AK, USA demonstrated this capability (1). The USAF was conducting training at high altitude. Post-exposure - a member of the aircrew reported to the unit medical facility in Anchorage, AK, USA complaining of signs and symptoms of a physiological incident. All physiological precautions were taken pre-exposure and no further incidents occurred during the scheduled training, the member was physically fit with no pre-existing conditions. Upon examination the diagnosis was Neurologic DCS, treatment being hyperbaric oxygen. Typically the closest hyperbaric facility is Seattle, WA, USA at a flight duration of 3h 25m (at 30,000 ft) and a treatment cost of approx. $150 000 USD. Luckily a short distance away, at Eielson AFB; the unit had procured a portable hyperbaric stretcher (SOS Hyperlite 1 (4)) and were able to conduct the treatment on-site. “This patient’s care could not have been done without the procurement of a hyperbaric chamber. This case demonstrates the utility and necessity for these capabilities at more facilities that manage significant flying operations. Military bases should ensure that hyperbaric treatment capabilities are available within a close proximity”.

The use of such portable chambers for military activities is widespread amongst diving communities as a ‘hyperbaric stretcher’ for emergency evacuation to nearby medical facilities, however the application for altitude related DCS has, until more recently, been reserved for high-altitude pilot operations for sudden loss of cabin pressure. The move by several countries to enforce mandatory presence of portable hyperbaric facilities has led to calls for the update of the NATO Standard (STANAG 7056), which is expected to be ratified in 2021.

Modernization of Standards

The current minimum standards defined by North Atlantic Treaty Organization (NATO)/Air and Space Interoperability Council (ASIC) standards for altitude exposure considers a number of human factors such as trapped gases, hypoxia/hyperventilation, unaided/aided night vision, laser safety and more; but have failed to address physiological incidents involving pressure related injuries. Whilst precautionary measures and safety equipment are used to reduce the risk of an incidence, the regulatory of incidents (and the recognition that the numbers of unreported cases are likely to be significant), the ratification of a new set of minimum standards which incorporate the need for nearby hyperbaric chamber facilities are expected within the coming months.

The USAF case study details a new method to use legacy equipment to treat altitude related decompression sickness  without the need for extended and costly medical evacuation in pressurised aircraft,  or exposing the patient to the risk of increased severity of symptoms through casualty evacuation at altitude in an unpressurised aircraft. The USAF Eielson AFB study is an example to follow of how versatile modular portable hyperbaric chambers should (and are expected to) become mandatory for future high altitude air-drop missions.

Sources:

1. VM Lee and AE Hay (2000), Altitude Decompression Illness - The Operational Risk at Sustained Altitudes up to 35,000 ft. Research & Training Organisation - Medical Program 62
2. Webb JT, Pilmanis AA & O’Connor RB. (1998) An abrupt zero-preoxygenation altitude threshold for decompression sickness symptoms. Aviation Space & Environmental Medicine 69
3. Pilmanis AA & Bison RU (1992) Incidence of decompression sickness (DCS) in high altitude reconnaissance pilots. Aviation Space & Environmental Medicine 63
4. SOS Group - https://sosgroup.co/products/hyperlite-1

(U.S. Navy photo by Mass Communication Specialist 3rd Class Garrett LaBarge)