Читать книгу Emergency Medical Services - Группа авторов - Страница 160

True decrease in blood PaCO₂:

 Hyperventilation (primary or secondary)

 Shock/cardiac arrest (with constant ventilation)

 Hypothermia /decreased metabolism

True increase in blood PaCO₂:

 Hypoventilation

 Return of circulation after cardiac arrest

 Improved perfusion after severe shock

 Tourniquet release

 Administration of sodium bicarbonate

 Fever/increased metabolism

 Thyroid storm

Increased gap between blood PaCO₂ and EtCO₂:

 Severe hypoventilation

 Increased alveolar dead space

 Decreased perfusion

 Disconnected or clogged tubing

Figure 6.2 Capnography waveforms. (a) Normal waveform. Point A is beginning of expiration. A‐B is expiration of dead space air. B‐C shows rapid rise in level of CO₂ as air from lungs is exhaled. C‐D is the plateau phase representing primarily alveolar air. D represents the value used for determination of EtCO₂. D‐A represents inspiration. (b) Effect of bronchospasm. Note the slower rise in the CO₂ level leading to the so‐called shark fin waveform. (c) Hypoventilation. (d) Hyperventilation

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.