Combined tests

Combining measurements may improve one's ability to predict weaning outcome. Sassoon and Mahutte9 repeated the analysis of rapid shallow breathing but combined it with the occlusion pressure in the first 100 ms (P01), an index of central drive. At this early phase of respiration, where little length has changed, the pressure generated is related to the degree of stimulation to the respiratory muscles. Although the combination of f/Vt and P01 provided the most sensitive and specific predictor, receiver operator curve (ROC) analysis showed only a modest gain with the addition of P0.1.

Table 11.1

Commonly quoted predictive

variables of weaning8

Threshold

Variable

value

Sensitivity Specificity

Minute

15 l/min

0.78

0.18

ventilation

Maximum

-15 cm H2O

0.97

0.36

inspiratory

pressure

Tidal volume

4 ml/kg

1.00

0.11

Threshold value=

=value beyond which weaning is

predicted to fail

Table 11.2 The three determinants of ventilation and common pathophysiological conditions associated with failure to wean

Central drive

• Sedation, analgesia or anaesthesia

• Raised intracranial pressure

• Hypercapnia

Respiratory muscle strength

• Hypophosphataemia

• Disuse atrophy

• Polyneuropathy/myopathy

Load applied to the muscles

• Hyperinflation

• Left ventricular failure

• Bronchospasm

• Lung fibrosis

CNS drive

Respiratory muscle strength

Applied load

Failure to generate force (fatigue)

Figure 11.1 Key components of spontaneous breathing. Drive from the central nervous system acts on the peripheral respiratory muscles. The balance between the components can be disordered, leading to fatigue of the respiratory muscles, failure to generate force, and a decrease in alveolar ventilation.

Yang and Tobin8 devised the CROP index ((Cdyn x Pimax x [Pao2/PAo2])/rate) which consisted of dynamic compliance (Cdyn), maximum mouth pressure (Pimax), oxygenation (Pao2/PAo2), and respiratory rate. This was no better than f/Vt alone when assessed prospectively. Measurements integrating ventilatory endurance and the efficiency of gas exchange yield the most successful results but are complex and difficult to use.10

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