Tuesday April 25, 2006
Esophageal Pressure Measurements and compliance
At bedside compliance is measured as
Cs = Vt / Ppl - (PEEP + autoPEEP)
Where Cs = compliance of static thorax, Vt = tidal volume, Ppl = plateau pressure and PEEP is postive-end-expiratory pressure
or in more precise terms
C stat = Vt / (Pao end'inhalation - Pao end'exhalation)
Where Pao = pressure at the airway opening. Pao end'inhalation is same as Ppl. and Pao end'exhalation is same as TotalPEEP.
This compliance measures the whole thorax including chest wall and lungs. Normal Cs is ideally 100 ml/cm H2O or practically 50 to 80 ml/cm H2O is acceptable.
Placement of esophageal catheter can give lung compliance (CL) and chest wall compliance (Ccw) separately. Formulae are
CL = Vt / (Pao - Pes) end'inhalation - (Pao - Pes) end'exhalation
Ccw = Vt / (Pes - Patm) end'inhalation - (Pes - Patm) end'exhalation
or practically done simply as Ccw = Vt / Pes end'inhalation
Where Pes = Esophageal pressure and P atm = Atmospheric pressure
Normal CL and Ccw is 200 ml/cm H2O.
Read article Esophageal and Gastric Pressure Measurements , including all the basics of how to insert and measure the esophageal catheter pressures by Dr. Joshua O Benditt (ref: Resp. Care, Jan. 2005, vol 50, no. 1)