Читать книгу Emergency Medical Services - Группа авторов - Страница 159

 Airway obstruction:UpperLower (asthma, chronic obstructive pulmonary disease)

 Muscle weakness (may be neurological)

 Pleural effusion (large)

 Pneumothorax

 Sucking chest wound

 Diaphragmatic malfunction (e.g., rupture, paralysis)

 Pleuritic pain

 Medications and recreational substances:OpioidsSedativesOxygen (in patients with hypoxic drive)

Ventilatory function can be determined directly by measuring the volume of air inhaled or exhaled per minute, or indirectly by measuring the CO₂ level in blood or exhaled air. The partial pressure of carbon dioxide (PaCO₂, may be measured in either arterial or venous blood samples using portable devices, as both provide similar results). However, just as oxygen content in the blood is usually assessed by noninvasive modalities in out‐of‐hospital settings, so too is CO₂. Three types of devices are currently in use to detect and measure the presence and level of CO₂ in exhaled air, which serves as a surrogate for the level of CO₂ in blood. The simplest, but least useful, are semiquantitative colorimetric devices that use litmus paper to detect the acid generated by absorption of CO₂ from exhaled air. These devices are compromised by prolonged exposure to air and by contamination from acidic gastric secretions. They may not be able to detect the extremely low levels of CO₂ generated by patients in cardiac arrest. For these reasons, and due to the increasing availability of devices that can measure and continuously monitor exhaled CO₂, colorimetric devices are being used less often than quantitative devices. Capnometry uses light absorption to measure the level of CO₂ in exhaled air. Clinically, the level at the end of exhalation is the most useful value and is referred to as end‐tidal CO₂ (EtCO₂). This measurement reflects the CO₂ content in alveolar gas and, therefore, in the pulmonary venous blood returning to the left heart.

The EtCO₂ level is typically 32–42 mmHg. It is 3–5 mmHg lower than the PaCO2 level in arterial blood, due to physiological alveolar dead space. Various clinical conditions such as poor pulmonary perfusion, greater than normal dead space (i.e., shunting and V/Q mismatch), and cuff or sampling device leak can widen this gap to 10–20 mmHg [4]. EtCO₂ cannot be higher than the PaCO₂, but may be significantly lower, and this should be considered when modifying manual or mechanical ventilation. Whenever possible, in critical care transport scenarios, for example, baseline PaCO₂ may be obtained and compared to the EtCO₂ level at the time of blood sampling. Venous PaCO₂ is typically about 4 mmHg higher than arterial PaCO₂ (range +10 to –2) [5]. Continuous waveform capnography provides additional information on the frequency of respirations and the flow rate of inhalation and exhalation by displaying a graphic depiction of measured expired CO₂ over time. Flow rate is affected by airway mechanics, including obstruction and bronchospasm, and is reflected in changes in the shape of the involved phase of ventilation. EMS clinicians should have a good understanding of the interpretation of EtCO₂ values as well as waveform morphology, as they both are altered by a variety of clinical conditions and may provide diagnostic information (Box 6.2) [6].

As a monitor of respiratory function, capnography is superior to pulse oximetry because it changes nearly immediately with changes in ventilation. On the other hand, hypoxia may be delayed by the body’s reserve and the physiology of hemoglobin oxygen dissociation, as discussed above. When capnography waveform analysis is included, a near real‐time assessment is possible and EMS clinicians may identify inadequacy of ventilation or the presence of various respiratory disease states, and they may glean information about circulatory and metabolic function as well.

Impairment of ventilation is associated with rising EtCO₂ values. When combined with waveform analysis, respiratory effort may also be monitored as to rate and depth of breathing. When respiratory rate or respiratory depth has become inadequate and EtCO₂ values rise, clinicians can initiate or augment respiratory support prior to the development of hypoxia. In the prehospital environment, this application of waveform capnography is especially useful in monitoring respiratory status following the administration of opioid analgesics, benzodiazepines, and other medications capable of producing respiratory depression (Figure 6.2).