**2.2 Cardiovascular changes**

*Special Considerations in Human Airway Management*

techniques during air transport and aviation conditions.

**2. Physiological changes in microgravity**

junction changes.

**2.1 Pressure related effects**

underlying medical conditions. Although medical services in aviation has evolved over years based on our understanding of physiology, advancement in monitoring technology but airway management was only recently studied with a focus on space environment. Airway management and other hemodynamic goals parameters, especially during medical air transport and aviation put the patient and medical team under unfamiliar and extreme physiological conditions, with detrimental clinical sequalae. In this chapter will cover the airway management in aviation with high emphasis on physiological changes and he preferred airway management

The changes occur to human body during aviation can affect the anesthesia delivery if surgery is needed. Almost all the body organs will be affected, but what is more relevant to anesthesia administration is: cardiac systolic and diastolic changes, gastric motility, reduction in blood volume as well as neuromuscular

At sea level, barometric pressure is 760 mmHg with a partial pressure of oxygen of 160 mmHg. The barometric pressure of ambient air declines as altitude increases, while the volume of air in a confined space will increase according to Boyle law, and this is why oxygen concentration remains at a constant 21% [1]. The intracranial air volume could be increased by 30% at the usual maximum cabin altitude of 8000 feet. These volume and pressure effects are sometimes associated with hemodynamic compromise (tension pneumothorax), barotrauma (sinuses), equipment malfunction (blood pressure cuffs), and possible injury or compromised monitoring as inflated gas bubble in the arterial line. Certain conditions such as pneumopericardium, subcutaneous emphysema, gas gangrene, systemic air emboli, decompression sickness, and gastric distension may be worsened at altitude [2]. Altitude sensitive equipment includes endotracheal and tracheostomy cuffs, pneumatic antishock garments (eg, medical antishock trousers), air splints, colostomy bags, foley catheters, orogastric and nasogastric tubes, ventilators, invasive monitors, and intra-aortic balloon pumps. Most aircraft cabins are usually pressurized to a pressure equivalent to 5000 to 8000 feet, giving an atmospheric partial pressure of oxygen of 118 mm Hg [3]. Thus, the oxygen requirement (Fio2) of a patient on mechanical ventilation may increase at altitude. ARDS in animal models were more

responsive to increased PEEP, yet resistant to increases of (Fio2) [4].

tion injury, anxiety and prolonged access to healthcare.

Hypoxemia, even at low altitudes (3281–9843 feet), which is the usual flight range for the medical helicopter transport, could lead to global hypoxic pulmonary vasoconstriction and pulmonary edema. Hypoxemia is detrimental to patients with coronary ischemia, pulmonary compromise acute respiratory distress syndrome [ARDS], or neurologic injury. Besides, the hypoxia associated-tachycardia and hypertension increases the cardiac mechanical load and myocardial oxygen

Flying at low altitude is commonly known as "Altitude restrictions" which mainly for pressure sensitive conditions as in eye trauma, pneumothorax, intracerebral air, and sinusitis [5]. Those low altitude flying restrictions come with the cost of more turbulence and longer transport times, another risk factor for more vibra-

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consumption.

The loss of gravity effect of the distribution of blood volume in different body compartments is notable in a microgravity environment. In normal environment at the earth surface there is a pressure gradient created by the gravity and the loss of this gradient during aviation result in more diuresis and by so reduction in blood volume [6–8]. The blood volume is one of the determinants of cardiac output, the reduction in the blood volume in microgravity will result in a 20% reduction in COP [6]. The reduction in these parameters will definitely reduce the body compensation capacity for hemorrhage, while the redistribution of the blood can affect intubation by causing facial edema [6, 9–11].

#### **2.3 Musculoskeletal changes**

Two main issues regarding musculoskeletal system changes in microgravity are the reduced bone mass [12] and the muscle atrophy which may lead to increase expression of extra junctional acetylcholine receptors [13, 14]. The abnormally expressed receptors can explain the risk of severe hyperkalemia after succinylcholine in space men [6, 10, 11].

Along with muscle atrophy changes in fat distribution affect the pharmacokinetics of anesthetic medications.

#### **2.4 Gastrointestinal changes**

Some studies suggest that there is a decrease gastric emptying during aviation in the first three days [6, 15, 16]. Some studies used paracetamol absorption as an indicator of gastric emptying [17, 18]. In anesthesia, gastric emptying time is very important factor in the assessment of aspiration risk following anesthesia induction.
