Page 557 - Clinical Application of Mechanical Ventilation

Weaning from Mechanical Ventilation 523

and compatible to successful weaning trial (Girault et al., 1994; Grap et al., 2003;
MacIntyre et al., 2001).

QS/QT. The physiologic shunt to total perfusion (Q /Q ) ratio is used to estimate
S
T
how much pulmonary perfusion is wasted. Shunted pulmonary perfusion cannot
take part in gas exchange due to mismatch of ventilation (e.g., atelectasis). The
Q /Q ratio can be calculated using the classic physiologic shunt equation:
S
T
Q S (CcO - CaO )
2
2
See Appendix 1 Q = (CcO - CvO )
for example. T 2 2
Q /Q : Shunt percent in %
T
S
CcO : End-capillary oxygen content in vol%
2
CaO : Arterial oxygen content in vol%
2
CvO : Mixed venous oxygen content in vol%
2
In clinical settings, a calculated physiologic shunt of 10% or less is considered
For a successful weaning normal. Shunt of 10% to 20% indicates mild physiologic shunt, and shunt of 20%
outcome, the Q S /Q T should be
,20%. to 30% shows significant physiologic shunt. Greater than 30% shunt reflects critical
and severe shunt (Malley, 1990; Shapiro et al., 1994).
Since physiologic shunt in mechanical ventilation is usually intrapulmonary in origin
(inadequate ventilation in relation to pulmonary perfusion), weaning failure becomes
likely when spontaneous ventilation cannot keep up with the pulmonary perfusion.
For this reason, significant and severe intrapulmonary shunt (Q /Q . 20%) should
S
T
be corrected before any weaning attempt.
P (A-a) 2 (A-a) O ) is used to estimate
O . The alveolar-arterial oxygen tension gradient (P
2
the degree of hypoxemia and the degree of physiologic shunt. The P (A-a) O gradient
2
may be obtained by subtracting the measured PaO from the calculated P O value.
2
2
A
This gradient is directly related to the degree of hypoxemia or shunt (a larger gradi-
ent reflects more severe hypoxemia or shunt).
The alveolar-arterial oxygen tension gradient (P O ) can be calculated as
See Appendix 1 for (A-a) 2
example of P(A-a)O 2 . follows:
P (A@a) O = P O - PaO
2
A
2
2
P (A-a) O : Alveolar-arterial oxygen tension gradient in mm Hg
2
On 100% oxygen, every
50 mm Hg difference in P O : Alveolar oxygen tension in mm Hg
2
A
P (A-a) O 2 approximates
2% physiologic shunt. PaO : Arterial oxygen tension in mm Hg
2
On room air, the P (A-a) O should be less than 4 mm Hg for every 10 years in age.
2
For example, the P (A-a) O should be less than 24 mm Hg for a 60-year-old patient.
2
On 100% oxygen, every 50 mm Hg difference in P (A-a) O approximates 2% physi-
2
For a successful weaning ologic shunt (Barnes, 1994; Burton et al., 1997; Shapiro et al., 1994).
outcome, the P (A-a) O 2 should In mechanical ventilation, P O of less than 350 mm Hg while on 100% oxy-
be ,350 mm Hg while on (A-a) 2
100% O 2 . gen suggests a likelihood of weaning success. P (A-a) O of 350 mm Hg while on
2
100% oxygen approximates 14% shunt and values of greater than 350 mm Hg may

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