Physiological effects of winching
Sami Ollila, a rescue swimmer and paramedic with the Finnish Border Guard’s Air Patrol Squadron, shared his thesis with AirMed&Rescue, offering valuable insights into the physiological effects of winching equipment
at the Metropolia University of Applied Sciences and graduated as a Bachelor of Healthcare with a degree in Emergency Care in 2018. Finalising my studies with a thesis, I was permitted to work on a topic relating to my current profession and one that would benefit my employer. At the time, I discovered an inspiring article by Dr Alan Garner from Careflight (Australia): Physiology in the Winch. The article referred to related research and to a fatal accident that occurred in Australia while winching with the single-sling method. The accident in Australia raised some personal concerns in my own working environment and the current standard operating procedure regarding the use of a rescue sling.
The Air Patrol Squadron has over 35 years of experience in operating with SAR helicopters and winch rescue operations, and during that time, operational procedures have evolved to make the current procedures and equipment as practical and safe as possible. In this article, I want to view the physiological effects of winching equipment according to related research so that one can understand the physiological effects of the winching equipment most commonly used in helicopter rescue.
I predicted the collection of sources to be challenging, but I soon realised that there was more than enough for a reliable thesis. Many of the research papers are relatively old, going back to the late 80s and 90s, but most can still be seen as scientifically relevant. The latest research relating directly to the physiological effects of winching equipment in helicopter rescue was conducted in 2011 (Australia). No related researches of Finnish origin could be found on this topic.
The equipment used in winch rescues can roughly be divided into four types of devices: the rescue sling, the harness or the vest, winch stretcher and the rescue basket. Patients or survivors are affected physiologically in different ways depending on the rescue equipment. These physiological mechanisms and effects should be understood by the rescue personnel.
An increase in heart rate is one of the initial compensation responses when predisposed to vertical suspension
The use of a rescue sling with the single-sling method is one of the most traditional ways to lift a survivor from water in helicopter rescues. The sudden vertical suspension, without support to the legs, challenges the blood circulation where natural compensation mechanisms aim to uphold efficient arterial blood pressure.
An increase in heart rate is one of the initial compensation responses when predisposed to vertical suspension. How these normal compensation mechanisms are able to work efficiently enough is dependent on the overall rescue scenario and on the health of the individual. Static vertical suspension reduces the central blood volume, and can lead to inefficient circulatory compensation despite the body’s best efforts, and in this case, the circulatory system is unable to maintain arterial pressure at a level where the brain gets enough oxygenated blood flow. The compensatory process is further complicated in patients suffering from chronic or underlying heart problems. The normal compensatory mechanisms occurring in vertical suspension such as elevated heartrate can have devastating effects for a patient with an already-weakened heart.
The physiological effects are more complex when rescuing from water. The Circum Rescue Collapse hypothesis is widely quoted and is also one of the sources in my thesis. It describes the physiological responses that lead to collapse pre-rescue, during rescue or post-initial rescue from water. Hydrostatic pressure and cold water are the main factors affecting the physiological responses when lifted to vertical suspension. After prolonged immersion, especially in vertical position, the hydrostatic pressure causes an increase in the central blood circulation and a decreased blood flow in the extremities.
The theory describes the physiology behind pre-rescue as being caused by decreased sympathetic activity. Normally, when a person is under stress or, for example, in survival mode, the sympathetic nerve system is active due to a high production of hormones that activate the sympathetic tone. The arteries squeeze into a smaller diameter and heart rate increases to uphold the arterial pressure. According to the hypothesis, when a rescue is perceived as imminent, the sympathetic activity may decrease, causing the arterial pressure to drop dramatically and the collapse is possible due to inefficient or failed compensation mechanisms.
During rescue, when lifted to vertical suspension, the effects of hydrostatic pressure changes as per the effects of gravity. Blood starts pooling back to the extremities, generating a decreased central circulation, which reduces the preload of the heart leading to decreased arterial pressure. A collapse during rescue is a possibility because the compensation mechanisms may fail or be inefficient to uphold the arterial blood pressure at the required level. After collapsing in vertical suspension, victims may wake up when placed horizontally.
Post-rescue problems relate more to hypothermia, where central blood volume increases to uphold the core temperature and to protect the vital organs from hypothermia. During rescue, when predisposed to physical activity, the heart of a hypothermic victim calls for more cardiac work and therefore more oxygen supply to the myocardium. Elevated heart rate decreases the time for coronary filling and this may not be tolerated by a cold weakened heart. Increased blood viscosity caused by severe hypothermia further increases the workload of the heart, which reduces the coronary perfusion. A sudden vertical suspension triggers the compensation response by raising the heart rate, which may result in cardiac arrest. It’s also notable that defibrillation of a severely hypothermic heart turned into ventrical fibrillation (VF) is likely not to be successful. Effective and continuous CPR en route to a hospital with ECMO (Extracorporeal Membrane Oxygenation) treatment available are the main elements affecting the survivability in this case.
The single-sling method also affects the respiratory system more than any other device used in winch rescues. Suspension in a single sling causes a thoracic squeeze, which induces a mechanical respiratory strain. The squeeze also increases intrathoracic pressure, which may induce a decreased preload of the heart. Vertical suspension without support to the legs causes pooling of blood in the legs, which may further compromise the preload phase. Decreased preload, on the other hand, may cause a vasovagal response, leading to a slow heart rate and lowering of the arterial pressure, further compromising efficient blood flow to the brain.
The most prominent indication of failed or inefficient compensation mechanisms during winch rescue is a collapsing survivor. Losing consciousness during vertical suspension in a rescue sling causes the loss of muscle tone and can result in slipping out of the sling and eventually to a fall, even if the victim is escorted on the hook. Such an occurrence was demonstrated in a fatal winching accident in Australia, 2013.
Winching equipment that is used to lift a patient in a horizontal position can pose a risk to the respiratory functions. Lying supine is known to increase the workload of the respiratory system. The pressure against the upper abdomen increases, inducing a decreased residual capacity in the lungs. A patient locked in with straps is unable to move or change position and sometimes even turning the head is limited. Vomiting caused by nausea during stretcher winch poses a risk of aspiration where the airways are at risk of becoming blocked.
Compared to vertical suspension in a single sling, the probability of losing consciousness in suspension with elevated legs is greatly decreased. Elevated legs prevent the pooling of blood to legs. Pooling of blood is one factor that causes the reduced central blood volume and decreased preload in the heart. All sources referred to come to the conclusion of favouring the double-sling method instead of single sling. The physiological benefits of using the double-sling method instead of a single sling may not be related only to rescuing a hypothermic victim, but to all victims winched from water. However, according to Tolerance to Head-up Tilt and Suspension with elevated legs (Madsen et al. 1998), being suspended with elevated legs does not totally eliminate the possibility of losing consciousness.
Rescue basket, ARV and AVED
Respiratory Function in Hoist Rescue (Murphy, Garner, Bishop, 2011) describes the physiological effects of rescue baskets as minimal, especially to the respiratory functions. The downside of the rescue basket relates to its functionality, mostly due to size. The basket takes up a lot of space in the cabin, and with many helicopters operating as a multirole platform, it can be difficult, if not impossible, to equip the aircraft with a rescue basket. Putting an unconscious victim in a rescue basket poses a risk to airway management due to the sitting position and the victim being unattended during the winch operation. The victim should be cooperative when using a rescue basket. Operators with heavy SAR helicopters with enough cabin space should consider the option of having the rescue basket in the toolbox due to the advantages offered from a physiological point of view.
Harness-type lifting devices represent the newest generation in helicopter winching devices. The sitting position in the so called AVED (Ambulatory Vertical Extrication Device) or ARV (Airlift Rescue Vest) is similar to the position in the double-sling method. AVED/ARV can be seen as safer compared to the double-sling method because the victim is strapped in in such a way that it’s practically impossible to slip through the device. This device is designed for dry operations and may be too complex to work with in water rescues. Potentially, the development of a water rescue version of an AVED/ARV could be worth consideration by the industry.
Much of the research concludes favouring rescue equipment and methods that allow the survivors or patients to be lifted horizontally rather than vertically with a single sling. As the vertical lift with a single-sling method poses the highest risk for safety, it should be considered as the last resort when choosing the right rescue equipment for the job.
However, categorising the winching equipment according to their physiological effects should not exclude any method available. Helicopter rescue scenarios can be challenging and complex, and sometimes the rescue method posing the highest physiological risk may be the only practical option. It´s equally easy to justify the use of a single sling in time-critical multi-casualty scenarios due to the simplicity and effectiveness of the rescue device.
Rescue swimmers, winchmen and flight paramedics have primary responsibility for the safety of their patients or survivors during the winch operation. It is vital, therefore, that they are prepared for any known outcome possible during winch rescues. It is as such equally important to understand the physiological strains the victims get exposed to, especially those with some form of pre-existing medical condition. Experienced rear crew members should also be granted with a major role in the development processes of rescue equipment in their organisation.
Images © Sami Ollila / Mikko Ketonen