Page 731 - Clinical Application of Mechanical Ventilation

Appendix 1 697

EXAMPLE
Given: C O 5 20.4 vol %
c
2
C O 5 19.8 vol %
2
a
C O 5 13.4 vol %
v
2
C O - C O
c
a
2
Q /Q 5 C O - C O 2 2
sp
T
c
2
v
20.4 - 19.8
=
20.4 - 13.4
0.6
=
7
5 0.086 or 8.6%
N. SHuNT EQuATION (Q /Q ): ESTImATED
T
sp
EQUATION 1
For non-critically ill individuals:
Q SP = C O - C O 2
c
a
2
Q T 5 + (C O - C O )
a
2
2
c
EQUATION 2
For critically ill (e.g., mechanical ventilation) patients:
Q SP = C O - C O 2
c
a
2
Q T 3.5 + (C O - C O )
2
2
c
a
Q /Q : Physiologic shunt to total perfusion ratio in %
T
sp
C O : End-capillary oxygen content in vol %
2
c
C O : Arterial oxygen content in vol %
a
2
NORMAL VALUE
Less than 10%
EXAMPLE 1
Given : Normal patient
C O 5 20.4 vol %
c
2
C O 5 19.8 vol %
2
a
Use 5 vol % as the estimated C(a - v)O and calculate Q /Q .
2
sp
T
Q SP = C O - C O 2
2
c
a
Q T 5 + (C O - C O )
2
a
c
2
20.4 - 19.8
=
5 + (20.4 - 19.8)
0.6
=
5 + 0.6
0.6
=
5.6
= 0.107 or 10.7%
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