Up to date, expert answers to frequently asked questions (FAQ) about oxygen supply systems, respiratory care and pulse oximetry written by OCC & collaborators.
Ventilator connections vary but NIST connectors are common. Most ventilators are supplied with a high pressure hose with a NIST fitting on one or both sides and the other end of the hose with a fitting suitable to match the oxygen source (e.g. Shrader quick release)
- Without access to an oxygen analyser it can be difficult to determine the gas contents in a cylinder. International standards for color of cylinders are not straightforward.
- Most cylinders are filled using pressure swing adsorption (PSA) oxygen plants and thus have a maximum FiO2 of 95% and often considerably less.
- Older oxygen cylinders may be black with a white top. Outside of these standards many variations also exist. All the cylinders in the images below contained oxygen.
- Oxygen analysers should be used to determine gases before use. Cylinders require routine testing and cleaning by certified technicians. Outlet fittings should be examined for damage or corrosion.
Most oxygen regulators will have a gauge attached, a full J size oxygen cylinder full will have a weight of 78kg (172lb) the oxygen contained in the cylinder weighs approximately 9kg (20)lb, Therefore gauging the volume of oxygen in a cylinder is difficult.
How do I know how much oxygen my facility is using?
There are multiple strategies to quantify oxygen consumption and need for a facility. The optimal strategy may vary by facility size, oxygen source and intended use of the data. Below are several potential strategies for quantifying oxygen consumption broken down by supply side and demand side approaches.
Of note, if using data to forecast oxygen needs, plan for surges, or expand facility capacity, then current oxygen consumption may grossly underestimate optimal consumption (i.e. you must also account for patients and potential patients who currently do not have access to oxygen due to inadequate current supply).
Supply side indicators:
Measuring facility oxygen consumption may (in some settings) be most accurately done by measuring production of oxygen at the facility source. This may not always be feasible depending on the availability of special equipment and/or human resources to log such data.
- Cylinders – logs (including number, pressure and size) of cylinders delivered to wards and/or delivered to the facility
- Portable oxygen concentrators – logs of use (hours of operation can be accessed from built in hour meter found on most units, though flow rates must be known or can be estimated)
- LOX – logs of cryogenic cylinders filled and emptied based on onsite biomed logs or LOX provider delivery logs; Log of changes in mass of LOX cryogenic storage tank contents may also be available
- Oxygen PSA Plants – monitoring output of PSA plants may depend on how the plant is used – i.e. filling cylinders (in which a cylinder log may be most useful) vs directly supplying a reservoir and/or the facility via piping. Some plants may continuously display hourly flow rate of output which may be logged onsite. An alternative method for PSA plants that directly supply a facility via piping is to record flow with a mass flow meter (Example of mass flow meter) which can be configured to display and save/send such data automatically or detect flow problems.
Quantifiable demand (patient) side indicators:
Quantifying oxygen consumption from the demand side can be challenging and time consuming. This may be the only option when supply side estimates are not available or when you sek to estimate consumption for specific locations within the hospital (e.g. the intensive care unit) or specific patient populations (e.g. COVID19 patients).
When measuring or estimating oxygen consumption, one must attempt to account for leak in the O2 infrastructure as a common source of underestimating oxygen demand. This may occur for many reasons including: human error (e.g. inadvertently leaving an unused flow meter on), suboptimal practice patterns (e.g. titrating to too high of an SpO2 target) or inadequate equipment or maintenance (e.g. poorly fitted regulators, cracked tubing or piping). (Top 10 ways to conserve oxygen)
- Crude modeling of oxygen consumption by patients can be done by using oxygen consumption modeling tools but yields imprecise results.
- Pragmatic primary data collection can be done by recording (at the bedside or via electronic medical record) current consumption per patient in a facility. This includes oxygen delivery device type and settings. While total consumption will vary throughout each day for each patient and thus for the facility as a whole, it may be practical to sample such data once per day or less to obtain snapshots of facility oxygen consumption (Oxygen quantification tools compilation)
- Comprehensive primary data collection can be done daily or multiple times per day to provide the most accurate demand side estimates of oxygen consumption though require considerable resources or comprehensive EMRs.
Common pitfalls in estimating O2 consumption
- Not accounting for leak on the O2 infrastructure
- Inaccurately estimating device consumption (e.g. BIPAP/CPAP leak, ventilator bias flow etc)
- Not accounting for increased flow demand due to decreased oxygen concentration (i.e. a cylinder filled by a low quality supply or a PSA being outstripped of capacity or that is poorly maintained may produced FiO2 that is much lower than expected and thus users may attempt to improve persistent hypoxemia in a patient by increasing flow. Thus, flow consumption may overestimate true demand)
- Estimating only current consumption and not accounting for future increases and decreases in need
Long story short: If you have an oxygen plant that can fill and produce 32 cubic meters per hour, can expect 100-120 cylinders (42L) to be filled in a 24-hour period. Manufacturer gives you rate of capacity if operating every minute, every hour. Assume 80% in real life. Plants are also rated at 1000ft above sea level or lower – much of Sub Saharan Africa is above this so have to derate the plants significantly.
Also, need to consider what size of cylinder you are using – too small, too large cylinders not good – middle is where you get best economy and life span.
Additional details for understanding these conversions:
Cylinder sizes are commonly referred to by volume of liquid (water) such as 42 Liters. Then you must covert the liquid liter size to gas volume. For example, a cylinder that is 42 Liters (liquid) actually contains 6.287 (or round to 6.3) CM of oxygen in a gas state, when the cylinder is filled under pressure to 152 Bar (or 2200 psi).
PSA Plants are built to a specification that identifies a “rated” capacity of production. For example, the plant is rated to produce 16 Cubic Meters per hour. Therefore, you may calculate that 24 hour in a day X 16CM Per hr. = 384CM per day. Once the total CM per day is understood you can divide by the common size of cylinder to know how many cylinders of a given size can be produced per hr or per day. However, it is not likely that the plant will operate every minute of every hour, therefore it is reasonable to assume 80%-90% of the rated capacity will be the actual working capacity. When a PSA plant is ordered, the manufacturer will need to know the location where the plant is to be installed to design the plant with consideration to altitude above sea level, and environmental common temperature range.
FAQ by Assist International
Ventilators without a turbine or compressor generally require both high pressure oxygen (green) and high pressure air (yellow) input to function appropriately, and cannot function at all without at least one of these
Ventilators with a turbine or compressor have the ability to entrain room air directly without a compressed air source; and have variable gas inputs depending on the manufacturer, including the ability to have some combination of:
- low pressure oxygen (e.g. from a portable concentrator), via common smooth bore oxygen tubing (this may require a reservoir to augment FiO2 and an adapter to connect to the device)
- high pressure (55psi/4bar) oxygen (from central pipes or a cylinder)
- high pressure air (usually not, as the turbine or compressor provides this)
First recall the weight of atomic and molecular oxygen: O = 15.99 g/mol and O2 = 32 g/mol
Use the ideal gas law: PV = nRT
P = pressure (in atm); V = volume (in Liters); n = amount of substance (in moles); R = ideal gas constant = 0.0821 (units are (Liters*atm)/(Kelvin*n) ); T = absolute temperature (in Kelvin)
To get the gaseous volume of 1 mole of gas: V=nRT/P
(1 mol)(0.0821 L/atm/K/mol)(273K)/1atm = 22.4 L gas
1 mole gas (STP) = 22.4L = 32g of O2
(32g/mol)(1L/22.4L/mol)= 1.428 g O2 per gaseous liter at STP
Standard temperature and pressure is 1 atm and 273K or O C
If we assume a ‘room temperature’ of 22 degrees C, then:
(1 mol)(0.0821 L/atm/K/mol)(295K)/1atm = 24.2 L gas
(32g/mol)(1L/24.2 L/mol) = 1.322 g of O2 per gaseous liter at 22 degrees C
So if you have 1000 L of gaseous oxygen: 1.322 g/L (1000L) = 1.32 kg
High pressure oxygen sources are capable of delivering oxygen at ~50psi/4bar to a device. These include oxygen cylinders (via regulator), oxygen plants via a compressor, liquid oxygen (via vacuum insulated evaporators) and very few portable oxygen concentrators (via additional compressor). High pressure oxygen is required for most ventilators, high flow nasal cannula and non-invasive positive pressure ventilators when taking care of critically ill patients.
Low pressure oxygen sources deliver oxygen at far less than <50psi/4bar. These include oxygen from a low flow flowmeter or a portable oxygen concentrator. Generally, these cannot deliver high enough oxygen concentrations to take care of severely hypoxemic patients.
***Some ventilators (e.g. LTV2200, Zoll 731) can operate with either high pressure or low pressure oxygen input. Other ventilators (e.g. PB560) may only be capable of utilizing a low pressure oxygen input. Of note – when utilizing low pressure oxygen input, some ventilators may not be able to provide high enough concentrations of oxygen to care for critically ill patients. Check manufacturers’ reports for maximum oxygen concentration delivery.