Obstructive respiratory physiology is the most often described diagnosis made upon EtCO2 waveform analysis. Both chronic obstructive pulmonary disease (COPD) and asthma fall into this category, and the waveform produced will be similar. The classic description of this waveform is the “shark fin” morphology, consisting of a shallower upward sloping of the initial rise of the EtCO2 wave (Figure 6.2b). This represents a slower rate of exhalation. It may be considered analogous to the forced expiratory volume in one second measurement of the pulmonary function test. This slower exhalation is precipitated by collapse or partial occlusion of bronchioles in emphysema and chronic bronchitis and spasm in acute asthma attacks. As the condition improves following bronchodilation, the initial upward segment will become more vertical. However, in more severe cases, the numeric value or amount of EtCO2 will also rise, heralding respiratory insufficiency, and should lead the clinician to consider ventilatory support measures.
Although less commonly employed, EtCO2 and waveform analysis may also be useful in assessment of metabolic derangements such as diabetic ketoacidosis and aspirin overdose. These conditions cause respiratory compensation of metabolic acidosis and will present with hyperventilation, typically with a decreasing level of EtCO2.
Assisting Oxygenation and Ventilation
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.
Noninvasive Positive‐Pressure Support
Patients who are awake, protecting their airways, have respiratory drive, and can cooperate may be given ventilatory support with noninvasive modalities that provide positive airway pressure. These include continuous positive airway pressure (CPAP), which delivers constant pressure above that of the atmosphere throughout the ventilation cycle, and bi‐level positive airway pressure (BiPAP), which delivers different pressures during the inspiratory and expiratory phases. Portable devices for the delivery of CPAP in the prehospital setting are generally of three types. Two of these require a high‐pressure (50 psi) oxygen source. One continuously delivers oxygen under pressure to a mask with a pop‐off valve that opens when the desired pressure is reached. The other uses a controller that essentially acts as a demand valve, adjusting flow to maintain the desired pressure. The third type of device uses oxygen flow from a standard regulator and a Venturi valve to create a virtual valve resulting in elevated pressure. Usually, treatment is started at 5–10 cm H2O and increased as needed to a maximum of 20 cm H2O. Prehospital devices generally deliver near 100% FiO2, while more advanced devices allow the FiO2 to be titrated. Until recently, BiPAP has been only available to EMS personnel who carry full‐function mechanical ventilators, but newer devices can now provide this modality for the field.
NIPPV has been shown to improve oxygen delivery. This is thought to be the result of the hydrostatic pressure effects of increasing the gas diffusion gradient, promoting displacement of fluid in the alveoli back into the capillary bed, and stenting open small bronchioles (which do not have cartilaginous walls), thereby increasing both the volume of air exchanged and the number of alveoli ventilated. NIPPV also decreases the work of breathing. The ultimate clinical effects are that patients will often have
