Thanks Bob and all for the informative discussion.
I believe the assumption of ventilator 02 consumption used above is significantly off, making the estimated bulk 02 consumption calculations above approximately 10 times higher than actual.
Let’s review the science – calculations are rounded. The assumption of 318 SCFH per ventilator translates to 150 SLPM flow, using the standard conversion factor of 28.3 liters per SCF. The 150 LPM is close to the peak inspiratory flow (volume of gas moved per unit of time) of a ventilator, but is not at all related to the volume (amount of space a substance occupies or enclosed) of gas used by the ventilator. From my discussion of ventilator flowrate on the forum yesterday, we know a “typical” adult male patient minute volume is approximately 8 LPM, meaning if the patient is receiving 100% 02 (more on this below) the 02 consumed by the ventilator is about the same, 8 LPM. This would compare to approximately 17 SCFH. So, let’s use a 100% safety factor, just for the heck of it, and we get each vent consumption over an hour of 34 SCFH (16 liters consumed per min. ÷ 28.3 x 60 mins per hr.), or slightly less than 10 times the assumed consumption in the above bulk 02 calculations. This makes a huge difference when calculating sizing for bulk 02 systems (including vaporizers) and central med air compressors.
The ventilator peak inspiratory flowrate duration is usually a fraction of the 1 to 2 seconds of typical inspiratory cycle. Peak flow is an extremely important value used when calculating med gas pipe distribution sizing, but not for actual consumption of gas for sizing central 02 or med air systems.
Now let’s talk a bit about % oxygen patient usage. Clinicians try to use the lowest inspired oxygen concentration, referred to as factional concentration of inspired oxygen (Fi02), needed to attain an adequate 02 level in the patient’s arterial blood – determined by arterial blood gas analysis, oximeter, and clinical observation. Most patients will not need 100% 02 administration – there are a few exceptions to very high inspired oxygen concentrations and one is advanced pneumonia, which is the way many COVID-19 patients are presenting at emergency departments. Patients in acute care hospitals very rarely receive 21% 02 (all air outlet consumption), so in my view central air compressor sizing should not assume vents use 100% piped med air. Patients intubated or with a tracheostomy, but not on a ventilator, can consume 12 LPM 02 (typical) when receiving 40% 02 by an oxygen nebulizer (aerosol therapy) due to the function and design of this medical device – yea crazy. Additional detail upon request. So, some ambient pressure respiratory care devices can use as much 02 as a ventilator.
No doubt hospitals will be using more, and in some cases substantially more, bulk 02 for the duration of this COVID-19 pandemic.
Stay Well
