Consequence of hypercarbia
The relationship between PCO2 levels and the state of ventilation is represented by the following equation:
PaCO2 = PACO2 = VCO2 x 0.115/VA
VA = VE – VD
alveolar ventilation (VA); minute ventilation (VE); dead space ventilation (VD)
The constant 0.115 is necessary to equate dissimilar units for VCO2 (ml/min) and VA (L/min) to PACO2 pressure units (kPa).
Question 1: What does the equation show to be the only physiologic reason for raised PaCO2?
Insufficient alveolar ventilation. Since VA is the difference between VE and VD, hypercarbia may occur due to insufficient VE and/or increased VD.
Question 2: Consider a patient with respiratory rate 25 bpm; tidal volume 280ml dead space volume 150 ml, CO2 production 300ml/min. The patient shows some evidence of respiratory distress. What is the PaCO2?
Using the alveolar ventilation equation:
VA = (25 x 280) – (25 x 150) = 25 x (280 – 150) = 25 x 130 = 3250 ml/min or 3.25 L/min.
PaCO2 = (300 x 0.115)/3.25 = 34.5/3.25 = 10.6 kPa
This example illustrates what may be obtained in a patient with COPD who increases his CO2 production, say during exercise or physiologic stress, but is unable to match this with a sufficient increase in ventilation.
What must not be inferred is that one can determine the adequacy of alveolar ventilation and thus PaCO2 – at the bedside.
Though VE can be readily determined with a spirometer (tidal volume x respiratory rate), one cannot know VD or the patient's rate of CO2 production. One would err in presuming that a patient breathing fast, hard and/or deep is hyperventilating.
The overnight physician was bleeped to the bedside of a man in his seventies, who had been admitted two days earlier on the Friday, for investigation of bilateral lower limb swelling. He was noted to be anxious and dysponeic with a clear chest, normotensive, heart rate of 102 bpm and respiratory rate of 32 bpm. The nurse informed the physician that this was common for him at night and that he may benefit from some night sedation, which the doctor duly prescribed for 'hyperventilation and anxiety'. One hour subsequently he stopped breathing and could not be resuscitated.