Page 726 - Clinical Application of Mechanical Ventilation

692 Appendix 1

500
=
20 - 5
500
=
15
= 33.3 or 33 mL/cm H O
2

F. CORRECTED TIDAL VOLumE (V )
T
EQUATION
Corrected V 5 Expired V 2 Tubing Volume
T
T
Expired V : Expired tidal volume in mL
T
Tubing volume : Volume “lost” in tubing during inspiratory phase (Pressure change 3
tubing compression factor)
EXAMPLE
Expired V 5 650 mL
T
Peak inspiratory pressure 5 25 cm H O
2
Positive end-expiratory pressure (PEEP) 5 5 cm H O
2
Tubing compression factor 5 3 mL/cm H O
2
Calculate the corrected tidal volume.
Tubing volume 5 Pressure Change 3 3 mL/cm H O
2
5 (25 – 5) cm H O 3 3 mL/cm H O
2
2
5 20 3 3
5 60 mL
Corrected V 5 Expired V 2 Tubing Volume
T
T
5 650 2 60
5 590 mL

G. DEADSPACE TO TIDAL VOLumE RATIO (V /V )
T
D
EQUATION
V D = (P CO - P CO )
2
E
a
2
V T P CO 2
a
V /V : Deadspace to tidal volume ratio in %
T
D
P CO : Arterial carbon dioxide tension in mm Hg
2
a
P CO : Mixed expired carbon dioxide tension in mm Hg
E
2
NORMAL VALUE
20% to 40% in patients breathing spontaneously
40% to 60% in patients receiving mechanical ventilation
EXAMPLE
Given: P CO 5 40 mm Hg
2
a
P CO 5 30 mm Hg
E
2

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