Sun28November0440PM 19
A 65-year-old male is admitted to the critical care unit following a laparotomy for faecal peritonitis.
He is intubated and is receiving ventilatory support for a provisional diagnosis of adult respiratory distress syndrome. His expired tidal volumes are 800 ml with a plateau pressure of 50 cmH2O and a PEEP of 10 cmH2O.
Which one of the following options approximates to the static pulmonary compliance of this patient?
(Please select 1 option)
2 ml/cmH2O
20 ml/cmH2O Correct
200 ml/cmH2O
0.2 ml/cmH2O
400 ml/cmH2O
Explanation
Static lung compliance is the change in volume for any given change in pressure. Static compliance is measured at a period when there is no gas flow, for example during an inspiratory or expiratory pause.
Compliance =
The units are ml/cmH2O or L/cmH2O.
Intrapleural pressure can be measured indirectly using an oesophageal manometer. Respiratory volumes can be measured at the mouth using a pneumotachograph or spirometer.
The normal compliance (Cl) of a normal lung is 200 ml/cmH2O.
For example, if a patient inhales 600 mL of air from a spirometer with an intrapleural pressure before inspiration of -6 cm H2O and -12 cm H2O at the end of inspiration.
Cl = 600mL/-6 -(-12)cmH2O = 600/6 = 100ml/cmH2O.
In this example with a ventilated patient the static compliance represents pulmonary compliance during periods without gas flow, such as during an inspiratory pause.
It can be calculated with the formula:
Cstat = Vt/Pplateau-PEEP
where:
Vt = tidal volume
Pplateau = plateau pressure
PEEP = peak end-expiratory pressure.
So:
Cstat = 800/50-10 = 20 ml/cmH2O.
Answer Statistics
1
3%
2
84%
3
12%
4
1%
5
2%
Times answered: 251