Key assumptions and
conventions are:
· The gas in the hood is assumed to be “perfectly mixed”. Transient affects are ignored and there is assumed to be no preferential exhausting of the patient’s expired gases and no pooling of expired gases.
· The patient’s inspiratory and expiratory flows, which are highly pulsatile, are converted into their steady-state equivalents. This assumption has been found to be reasonably accurate for this type of device2, and it enormously simplifies the calculations.
· At the activity levels of interest, Respiratory Minute Ventilation (RMV) in actual liters per minute (alpm) is taken as 20 times oxygen consumption in standard liters per minute (oxygen consumption equals 5% of RMV)1 . At higher activity levels RMV increases to 25 times oxygen consumption. (oxygen consumption = 4% of RMV)1
· Carbon dioxide production is taken as 84% of oxygen consumption (respiratory quotient = 0.84).
· The patient’s expired gas is taken as saturated at body temperature. This produces a water vapor content of 6% surface equivalent in the patient’s expired gas.
· The supply gas is assumed to be dry, 100% oxygen.
· Concentrations are expressed in terms of % surface equivalent. Metabolic flow rates (oxygen consumption and CO2 production) are expressed in terms of standard liters per minute (slpm). Hood flow rate is expressed in terms of actual liters per minute (alpm). This combination of units removes chamber pressure from the calculations.
Flow through the hood expressed in terms of true volume at chamber conditions is the “carrier” parameter to which all other parameters are related. CO2 production expressed in terms of standard liters per minute (which assumes a pressure of 1 ATA) when divided by true volumetric flow rate (alpm) converts directly to CO2 concentration per unit volume, which is a definition of partial pressure. When this concentration is expressed in terms of percent surface equivalent (% S.E.), no conversion constants are required and the equations simplify. For example, a CO2 flow component of 2 slpm in an overall flow of 50 alpm produces a CO2 concentration of 4% S.E. regardless of the value of the local total pressure.
Patient activity levels were looked at from two viewpoints. The first was air conditioning technology3 which focuses more on the total heat produced by a person. The second viewpoint was physiology4 which focuses more on tissue oxygen consumption. When both approaches are compared for a 160 lb (72.6 Kg) male patient and normalized on total oxygen consumption at the Seated Quietly condition, the results are as shown below.
.
Activity Level Units Oxygen
Meta METSb Consumption (slpm)
Sleeping 0.7 0.88 0.22.
Reclining 0.8 1.0 0.25
Seated quietly 1.0 1.25 0.32
Standing quietly 1.2 1.51 0.38