Page 560 - Clinical Application of Mechanical Ventilation

526 Chapter 16

air divided by the arterial blood carbon dioxide tension. For a successful weaning
For a successful weaning outcome, the V /V ratio should be 60% or less (Fitzgerald et al., 1976).
outcome, the V D /V T ratio D T
should be ,60%. The deadspace to tidal volume (V /V ) ratio can be calculated as follows:
T
D
V D (PaCO - P CO )
2
E
2
V T = PaCO 2
V /V : Deadspace to tidal volume ratio in %
D
T
See Appendix 1
for example. PaCO : Arterial carbon dioxide tension in mm Hg
2
P CO : Mixed expired carbon dioxide tension in mm Hg
2
E
RAPID SHALLOW BREATHING INDEx (RSBI)

Failure of weaning may be related to the development of a spontaneous breathing
pattern that is rapid (high frequency) and shallow (low tidal volume). The rapid
rapid shallow breathing
index (RSBI): The RSBI (f/V T shallow breathing index (RSBI) or f/V index has been used to evaluate the effec-
T
index) is calculated by dividing tively of the spontaneous breathing pattern (Jacob et al., 1997; Tobin et al., 1986;
the spontaneous breathing
frequency (breaths/min) by Vassilakopoulos et al., 1998; Yang et al., 1991).
the average spontaneous V T Rapid shallow breathing is quantified as the f (number of breaths per minute)
(L). Absence of rapid shallow
breathing, as defined by an f/V T divided by the V in liters, and this breathing pattern induces inefficient, dead-
T
ratio of less than 100* breaths/ space ventilation. When the RSBI or f/V index is greater than 100 breaths/min/L
min/L, is an accurate predictor of T
weaning success. (*rounded from (rounded from 105 breaths/min/L), it correlates with weaning failure. On the other
105 breaths/min/L.)
hand, absence of rapid shallow breathing (f/V ratio ,100 breaths/min/L), is an
T
accurate predictor of weaning success (Yang et al., 1991).
Rapid shallow breathing To measure the f/V index, the patient is taken off the ventilator and allowed to
T
induces inefficient, deadspace breathe spontaneously for 3 min or until a stable breathing pattern has been estab-
ventilation.
lished. The minute expired volume (V ) and spontaneous frequency (f) are mea-
E
sured. The average V is calculated by dividing the V by f. The f/V is calculated
T
E
T
by dividing the f (breaths/min) by the average V (L). The procedure for measuring
T
and calculating the f/V is outlined in Table 16-3.
T
TABLE 16-3 Procedures to Obtain the f/V T Ratio
Procedure

1. Allow the patient at least 3 min to stabilize the spontaneous breathing pattern (ventilator
frequency must be off and PSV should not be used if tolerated by patient)
2. Measure expired volume and respiratory frequency for one min

3. Divide minute volume by frequency (f) to obtain an average tidal volume (V ) in liter
T
4. Divide f by V to obtain f/V index (breaths/min/L)
T
T
Note: See Appendix 1 for example.
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