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Hidden dangers: Prone restraint and positional asphyxia in custody deaths

In custody deaths of subjects restrained in the prone position are coming under increased scrutiny. Unlike the well-publicized George Floyd case where breathing was restricted due to pressure applied to the neck (described by Dr. Martin Tobin at the Chauvin trial), deaths have resulted when subjects are simply placed in handcuffs in the prone position with weight applied to the back. In the past, such deaths have often been ascribed to “excited delirium”, a now-debunked classification of death.¹ Rather, it has been recognized that these deaths have resulted from restricted breathing and are, more correctly, designated as positional asphyxia. While the duty to restrain a combative assailant may remain a priority, law enforcement must recognize the signs of positional asphyxia to take timely countermeasures to ensure the safety of detainees.

Although some studies² have suggested that subjects handcuffed in the prone position can breathe adequately—even when a significant load of up to 225 lb is placed on the back—these studies have been conducted on healthy, fit subjects and fail to account for the effect of the subject’s physique, physical exertion, emotional state or substance use, on respiration.

Briefly, respiration consists of two phases: inspiration, during which atmospheric air enters the lungs, and expiration, during which air from the lungs is returned to the atmosphere (Figure 1A). Gas exchange in the lungs allows O₂ to enter the blood and CO₂ to be removed from the blood (Figure 1B). The removal of CO₂ is equally, if not more, important to the entry of O₂. Too little O₂ in the blood can lead to a state of hypoxia and loss of consciousness, whereas too much CO₂ in the blood can lead to a state of hypercapnia, metabolic acidosis and cardiac arrest.

The prone position by itself, without restraint, significantly increases the work that muscles must perform during inspiration. For example, inspiration required 70 per cent more work while resting prone in water compared to sitting in air.³ Breathing, while prone on a hard surface, requires that the inspiratory muscles raise the mass of the torso, as well as any load on the back, in order to expand the chest (Figure 2). The greater the body weight and the greater the weight pushing down on the back, the more work that they must perform and the more work, the more CO₂ the muscles produce and the higher the concentration of CO₂ in the blood.

Photo: Geoffrey Desmoulin

Furthermore, in the prone position, the amount of air that can move in and out of the lungs is reduced because body weight and weight on the back compress the chest, reducing the volume of the thoracic cavity.⁴

Obesity can contribute to increased accumulation of CO₂ in the blood. The abdomen is compressed in the prone position which means that the diaphragm must do even more work during inspiration to overcome increased intra-abdominal pressure than in a standing or seated position. Because the increase in intra-abdominal pressure is greater for obese subjects than non-overweight subjects, the diaphragm of obese subjects must work harder to produce the same increase in thoracic volume and will, therefore, produce more CO₂. The more obese the subject, the greater the risk of hypercapnia in the prone position.

Emotional distress and physical struggle will increase heart rate and blood pressure, both of which increase metabolism, requiring more O₂ and producing more CO₂. Furthermore, struggling increases O₂ demand and CO₂ production in the same way as exercise. What may not be obvious is that struggling is frequently a sign that the subject is trying to move the body into a position where breathing is less difficult as CO₂ concentration in the blood increases to the point of metabolic acidosis as opposed to an attempt to escape custody. This increase in struggle may be interpreted by police as increased resistance to being restrained and in turn police may increase restraining effort, exacerbating the issue.

Substance use can also contribute to the accumulation of CO₂ in the blood. Opioids, such as oxycodone and fentanyl can depress respiration, reducing the volume of air moving in and out of the lungs.⁵ Methamphetamine, on the other hand, increases heart rate and reduces the capacity of the cardiovascular system to appropriately respond to increased levels of CO₂ in the blood.⁶ Therefore, substance use can both increase CO₂ production and reduce the ability to remove CO₂ from the blood, placing the subject at greater risk of hypercapnia and metabolic acidosis.

Photo: Geoffrey Desmoulin

Even two to three minutes in the prone position, while restrained, can compromise breathing to the point of positional asphyxia. The only sure way to reduce the risk of positional asphyxia is to move the subject from the prone position to a position where the chest is no longer compressed as quickly as possible after restraints have been successfully applied.

References

1. Weedn V, Steinberg A & Speth P. (2022) Prone restraint cardiac arrest in in‐custody and arrest‐related deaths. Journal of Forensic Science, 67: 1899–1914.
2. Michalewicz BA, Chan TC, Vilke GM, Levy SS, Neuman TS & Kolkhorst FW (2007) Ventilatory and metabolic demands during aggressive physical restraint in healthy adults. Journal of Forensic Sciences, 52: 171–175.
3. Leahy MG, Summers MN, Peters CM, Molgat-Seon Y, Geary CM & Sheel WA (2019) The Mechanics of breathing during swimming. Medicine & Science in Sports & Exercise, 51: 1467–1476.
4. Cary NRB, Roberts CA, Cummin ARC & Adams L. (2000) The effect of simulated restraint in the prone position on cardiorespiratory function following exercise in humans. Journal of Physiology, 525: 30P–31P.
5. Palkovic B, Marchenko V, Zuperku EJ, Stuth EAE, Stucke AG. (2020) Multi-level regulation of opioid-Induced respiratory depression. Physiology, 35: 391‒404.
6. Hassan SF, Wearne TA, Cornish JL & Goodchild AK. (2016) Effects of acute and chronic systemic methamphetamine on respiratory, cardiovascular and metabolic function, and cardiorespiratory reflexes. Journal of Physiology, 594.3: 763–780.

Geoffrey T. Desmoulin, PhD., RKin., PLEng., is the Principal of GTD Scientific Inc. in North Vancouver. He can be reached at gtdesmoulin@gtdscientific.com.