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Adequate oxygen delivery to body tissues is a necessity for life and is dependent on both the transfer of oxygen from the alveolar airspace to the blood and sufficient tissue perfusion with oxygenated blood. Oxygenation of the blood is dependent on a number of distinct factors, each of which can be impaired by various pathological processes (Table 6.1). Normal hemoglobin oxygen saturation in peripheral arterial blood is 96–99%. It is important to understand the relationship between oxygen saturation and the partial pressure of oxygen. This is depicted by the oxyhemoglobin dissociation curve (Figure 6.1). This curve demonstrates that above 90% the saturation percentage is very insensitive to changes in partial pressure of oxygen (PaO₂) between 150 mmHg and 760 mmHg. This means that, especially in patients receiving supplemental oxygen, severe impairment of oxygen transfer into the blood can occur without major changes in the saturation level and EMS clinicians can miss the physiological deterioration. Considered another way, if the partial pressure of oxygen in blood is at least 60 mmHg, hemoglobin is able to transport oxygen efficiently to the periphery.

Several tools have been developed that can reliably measure oxygenation of blood in the prehospital environment. Portable devices are available that can measure oxygen content in arterial and venous blood samples (i.e., PaO₂). However, because of cost and the need to perform vascular puncture, these devices are typically only used at selected special event venues and by critical care teams. Most commonly, oxygen levels in the field are determined by pulse oximetry (i.e., oxygen saturation, SpO₂). This simple, noninvasive method reports the percentage of hemoglobin in arteriolar blood that is in a saturated state. It is important for prehospital clinicians to understand that standard pulse oximetry does not discriminate between hemoglobin saturated with oxygen and hemoglobin saturated with carbon monoxide (i.e., oxyhemoglobin versus carboxyhemoglobin). In cases of carbon monoxide exposure, pulse oximetry will be misleading to the unsuspecting clinician [1]. Newer‐generation devices are available that can measure carboxyhemoglobin levels distinct from oxyhemoglobin [2].

Pulse oximetry may be unreliable in states of low tissue perfusion, such as with shock or local vasoconstriction due to cold temperature. Additionally, as this technology relies on transmission and absorption of light waves, barriers such as fingernail polish or skin disease can interfere with accuracy.

Measurement of tissue oxygenation saturation (StO₂) uses near‐infrared light resorption to measure oxygen saturation of blood in the skin and underlying soft tissue. This enables assessment of oxygen delivery and consumption in local tissue rather than simply the amount of oxygen circulating in the arterial system, which is measured by pulse oximetry. While there are increasing reports of the utility of this technology, it is not yet in widespread clinical use due to cost, technical limitations, and lack of large clinical studies [3].

Table 6.1 Conditions that impair oxygen transfer in the lungs

Physiological process	Pathological conditions
Partial pressure of oxygen in inhaled air	Displacement by other gases
Minute ventilation (volume of air inhaled per minute)	External compression of chest
Muscle weakness (chest wall and/or diaphragm)
Central nervous system control malfunction
Decreased lung compliance
Pneumothorax
Hemothorax and pleural effusion
Diffusion of oxygen across the alveolar membrane	Pneumonitis
Alveolar and/or interstitial edema
Perfusion of the alveoli	Decreased cardiac output
Hypotension
Shunting

Figure 6.1 Oxygen‐hemoglobin dissociation curve