Page 556 - Clinical Application of Mechanical Ventilation

522 Chapter 16

f/V . The f/V index reflects the degree of rapid shallow breathing. In patients who
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An f/V T index of
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100 breaths/min/L correlates are breathing rapidly with small tidal volumes, the V /V ratio and f/V index
with weaning success. would be increased. An f/V index of 100 breaths/min/L correlates with weaning
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success.
Minute Ventilation. The patient’s minute ventilation (either spontaneous or assisted)
For a successful weaning should be less than 10 L/min for a successful weaning outcome (assuming the cor-
outcome, the patient’s minute
ventilation should be less than responding PaCO is normal). A high minute ventilation requirement (.10 L) to
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10 L/min with acceptable normalize the PaCO implies that the work of spontaneous breathing is excessive.
blood gases. 2
The patient is unlikely to be able to sustain the increased work of breathing once
the weaning process begins.
An excessive minute ventilation requirement may result from increased carbon
dioxide production secondary to an increased metabolic rate, an increase in alveolar
deadspace, or metabolic acidosis. Causes for increased carbon dioxide production
include extensive burn injuries, an elevated body temperature, and sometimes over-
feeding, especially with excessive carbohydrate supplements. Alveolar deadspace will
be increased if the alveolar ventilation exceeds the alveolar perfusion (V/Q . 0.8).
This condition of deadspace ventilation may occur when (1) the alveoli are overven-
tilated as in hyperinflation of the lungs (e.g., emphysema) and (2) the pulmonary
circulation is underperfused (e.g., pulmonary embolism, decreased cardiac output).

Oxygenation Criteria

The oxygenation status of a patient may be used to evaluate the readiness and out-
come of weaning attempts. Weaning success will be more likely if the patient is
adequately oxygenated while receiving partial or no ventilatory support before or
during the weaning process. The generally accepted oxygenation weaning criteria
For a successful weaning include a PaO of greater than 60 mm Hg (or SaO . 90%) on an F O of 0.40
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outcome, the PaO 2 and SaO 2
should be .60 mm Hg and or less (Barnes, 1994), a PaO /F O index greater than 150 mm Hg, an intrapul-
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.90%, respectively on F I O 2 monary shunt (Q /Q ) of less than 20%, and an alveolar-arterial oxygen tension
, 40%. S T
gradient (P (A-a) O ) less than 350 mm Hg at an F O of 100% (Boles et al., 2007;
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Feeley et al., 1975; Girault et al., 1994).
PaO and SaO . A PaO of 60 mm Hg corresponds to an SaO of about 90%. It is
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essential to note that in patients with anemia or increased level of dysfunctional
hemoglobins (carboxyhemoglobin), the PaO and SpO (pulse oximetry) do not
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reflect the true oxygenation status of the patient. In those instances, the arterial
oxygen content (CaO ) and arterial oxygen saturation (SaO ) should be measured
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and used for clinical decisions.
If pulse oximetry is used to monitor a patient’s oxygenation status, the pulse
oximetry O saturation (SpO ) should be kept in the mid-90s for allowance of
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machine inaccuracies, because SpO readings in critical care are accurate to within
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2% to 4% of the SaO (Malley, 1990).
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For a successful weaning PaO /F O . The arterial oxygen tension to inspired oxygen concentration (PaO /
outcome, the PaO 2 /F I O 2 should 2 I 2 2
be $150 mm Hg. F O ) or P/F index is a simplified method for estimating the degree of intrapulmo-
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nary shunt. A PaO /F O of $150 mm Hg suggests acceptable physiologic shunt
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