Page 298 - Clinical Application of Mechanical Ventilation

264 Chapter 9

P(a-et)CO Gradient
2

The correlation between PaCO and PetCO is excellent and the P(a-et)CO gra-
2
2
2
Under ideal conditions, dient (difference) between these two measurements is about 2 mm Hg in normal
the PaCO 2 is about 2 mm Hg
higher than the PetCO 2 result- individuals. For critical patients, a gradient of 5 mm Hg is considered acceptable
ing a P(a-et)CO 2 of 2 mm Hg. (Niehoff et al., 1988).
The P(a-et)CO gradient is primarily affected by alveolar deadspace ventilation
2
(Perel & Stock, 1992), old age, presence of pulmonary disease, and changes in me-
chanical volume and modality. Table 9-11 lists some specific conditions that in-
crease the P(a-et)CO gradient.
2
Disposable ETCO Detector. Capnography can also be estimated via a low-cost, dis-
2
posable, plastic, CO (pH)-sensitive device. With this device attached to the en-
2
dotracheal tube, one can quickly differentiate tracheal from esophageal intubation
(Hess, 1990). This occurs when the pH-sensitive device on the material senses the
changes in CO concentration.
2
Limitations of Capnography Monitoring

Capnography readings reflect only the changes in a patient’s ventilatory status, rather
Capnography readings than the improvement or deterioration of the patient (Whitaker, 2001). An example
reflect only the changes in a
patient’s ventilatory status, of this is deadspace ventilation as seen in pulmonary embolism (Figure 9-15). A
rather than the improvement decrease in PetCO due to physiologic deadspace ventilation does not mean that the
or deterioration of the patient. 2
patient’s ventilatory status has improved. Lowering the ventilator frequency in this
situation could lead to grave consequences.
Other conditions leading to an increase in deadspace ventilation (thus a de-
Decreased PetCO 2 may crease in PetCO ) are hypotension and high intrathoracic pressure secondary to
not be indicative of improve- 2
ment in gas exchange. mechanical ventilation (Whitaker, 2001). This could cause the practitioner to
incorrectly assume that the decreased PetCO indicates an improvement in gas
2
exchange.

TABLE 9-11 Factors That Increase the P(a-et)CO 2 Gradient

Clinical Condition Factors
Ventilation Increased deadspace ventilation
Positive pressure ventilation

Perfusion Decreased cardiac output
Decreased pulmonary perfusion
Cardiac arrest
Pulmonary embolic disease
Temperature Hyperthermia
Hypothermia
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