Читать книгу Emergency Medical Services - Группа авторов - Страница 161

While oxygenation and ventilation are distinct parameters, their assessment and management are often interdependent. Thus, we discuss them together.

The initial and most basic treatment for inadequate oxygenation is the administration of supplemental oxygen to increase the relative amount, or fraction, of oxygen in inspired gases (i.e., FiO₂). Oxygen should be provided to all patients with respiratory distress, with any clinical markers of respiratory compromise (e.g., altered mental status), or with measured inadequate oxygenation or ventilation. There is an increasing trend toward more selective application of oxygen with the growing recognition of oxygen toxicity. Most current guidelines and protocols endorse administering supplemental oxygen only if the oxygen saturation is less than 94%. Unnecessarily elevating the SpO₂ above normal levels may in fact be harmful to patients experiencing neurological or cardiac insults associated with ischemic damage [7].

Patients with underlying pulmonary disease, such as COPD and interstitial fibrosis, may have oxygen saturations below 94% on a chronic basis. A subset of these patients will also have chronically high PaCO₂ levels (hypercapnia), which lead to dependence on a hypoxic drive for ventilatory control and stimulation. Providing supplemental oxygen, especially at high flow rates, may contribute to respiratory depression and potentially produce apnea [8]. EMS clinicians must carefully assess and monitor these patients, administer oxygen if needed, and be prepared to assist ventilation. Oxygen should not be withheld from a hypoxic patient because of concern for their dependency on a hypoxic drive for breathing.

Supplemental oxygen can be administered through various devices that deliver different ranges of oxygen concentration (Table 6.2). Most EMS systems carry nasal cannulas and non‐rebreather facemasks, allowing clinicians to choose either a lower or a higher FiO₂. When supplemental oxygen itself does not lead to adequate oxygenation of blood, noninvasive positive‐pressure ventilation (NIPPV) can be beneficial to supplement ventilatory function in addition to providing increased FiO₂. This modality is most effective in patients with pulmonary edema, who have poor oxygen diffusion between alveolar air and the pulmonary capillary blood. Further, it is useful for patients with other conditions, including asthma, COPD, and pulmonary hypertension. NIPPV is described in more detail below.

Table 6.2 Devices for delivery of supplemental oxygen

Device name	O2 flow rate (L/min)	FiO2 (approximate %)
Nasal cannula	1–6	24–44
Simple face mask	5–12	35–55
Partial rebreather mask	8–15	35–60
Non‐rebreather mask	8–15	60–95
Venturi mask	4–15	24–50
Tracheostomy mask	10–15	35–60

While hypoxemia in the setting of adequate ventilation can be treated with supplemental oxygen and augmentation of ventilatory function, inadequate ventilation requires immediate intervention. The EMS clinician should rapidly determine the likely cause (Box 6.1) of the patient’s ventilatory insufficiency and determine if it can be quickly corrected. Examples of this are removal of upper airway obstruction, administration of bronchodilators for bronchospasm, sealing of sucking chest wounds, administration of naloxone for opioid overdose, and needle decompression of tension pneumothorax. Some conditions cannot be immediately alleviated, particularly in the prehospital setting, such as muscle weakness from Guillain‐Barré syndrome or severe physical fatigue, vital capacity reduction from a large pleural effusion, and non‐reversible drug toxicity. In other cases, medical interventions may not be sufficiently effective immediately, such as for acute pulmonary edema or severe asthma. Whenever ventilation is compromised and cannot be immediately alleviated, mechanical ventilatory support must be provided.