Abstract
Background
High flow nasal oxygen (HFNO) therapy delivers pure oxygen at rates over 40 L/min during preoxygenation and 60 L/min during apnoea, preventing hypoxia and extending safe apnoeic time. HFNO is beneficial in certain clinical scenarios. However, cost and limited availability of HFNO devices restrict routine use. Auxiliary oxygen outlets on anaesthesia workstations present a potential alternative HFNO source.
Aim
This study aimed to assess the maximum flow rates from auxiliary oxygen outlets on various anaesthesia workstations and wall flowmeters, to explore their viability as a source for HFNO.
Methods
After calibration in Switzerland, a CITREX H4 gas flow analyser was used to measure flow rates across multiple anaesthesia workstations and wall-mounted flowmeters at five hospitals affiliated with one academic anaesthesia department.
Results
Only two of nine tested workstations could reliably support flows sufficient for high flow apnoeic oxygenation. The General Electric (GE) CS 650 workstation had the highest average flow rate (120 L/min) while the Dräger Fabius GS Premium had the lowest (13 L/min). Wall flowmeters had flow rates ranging from 108 to 28 L/min, with most supporting flow rates greater than 60L/min.
Conclusion
The auxiliary oxygen outlet on anaesthetic workstations and general oxygen flowmeters is underutilised. Sufficient flow rates to function as a source of HFNO are attainable with many types, but should be individually tested. In a resource-limited environment, this could provide a cost-effective alternative in providing apnoeic oxygenation for a time-limited period, although safety has not yet been established.
Graphical abstract
Highlights
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Auxiliary oxygen outlets have the potential to deliver HFNO, broadening their use in clinical settings.
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The flow rates generated for HFNO vary widely among anaesthesia workstation models.
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Some anaesthesia workstations and most wall flowmeters tested generated sufficient oxygen flow for effective HFNO delivery.
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Wall flowmeters offer a low-cost HFNO alternative in resource-limited settings.
1
Introduction
Maintaining adequate oxygenation during elective and emergency intubation is critical for patient safety. Preoxygenation prolongs the time before desaturation occurs [ ]. Safe apnoeic time is the duration of time until critical arterial desaturation (SaO 2 88 %–90 %) occurs following the cessation of breathing or ventilation [ ]. Apnoeic oxygenation is used to extend the ‘safe apnoeic time’ beyond that which can be achieved by preoxygenation alone.
In the apnoeic patient, extraction of oxygen from the alveolus into the blood causes alveolar pressure to become sub-atmospheric, generating a pressure gradient which facilitates diffusion of oxygen from the upper airway into the alveoli. Apnoeic oxygenation is aided by ‘de-nitrogenation’ of the patient’s lungs, by inspiration of a high fraction of oxygen for a suitable duration before the onset of apnoea. ‘Re-nitrogenation’ is prevented by the delivery of pure oxygen during the apnoeic period [ ].
Apnoeic oxygenation can be attained by any device that has oxygen flow into the respiratory tract. Nasal canulae have the added benefit that they can be kept on the patient during preoxygenation with a face mask [ ].
High-risk populations like the parturient, morbidly obese, paediatric or elderly, patients with pulmonary disease, and patients at high altitudes are more likely to desaturate rapidly. They are thus also more likely to require additional techniques, such as apnoeic oxygenation.
High flow nasal oxygen therapy (HFNO) is one method of providing apnoeic oxygenation. It has been shown to prolong apnoeic times in high-risk populations [ ]. HFNO has utility in settings ranging from procedural sedation, anticipated difficult airway and bariatric surgery, to routine use during rapid sequence induction. Typically, HFNO techniques require the use of special equipment to create flows high enough to aid in apnoeic oxygenation. HFNO therapy in adults weighing >20 kg is defined as a flow of pure oxygen at > 40 L/min during preoxygenation, and >60 L/min during apnoea [ ]. The difference in flow requirements is due to breathing during preoxygenation compared to apnoeic oxygenation with no spontaneous ventilation. Higher flows during apnoea promote bulk flow of oxygen, enabling gas exchange and higher mean airway pressures.
There is a linear relationship between HFNO flow rate and positive airway pressure generated in the nasopharynx of awake patients. Each 10-litre increase in flow rate achieves an additional positive airway pressure of approximately 0.5 cm H 2 0 with an open mouth, and 1 cm H 2 0 with a closed mouth [ ]. Common practice when using HFNO is to use flows of at least 30–50 L/min, resulting in low levels of positive airway pressure, which augments the mean airway pressure and functional residual capacity (FRC), improving oxygenation [ ].
Dedicated HFNO devices provide warmed, humidified oxygen through large-bore nasal cannulae, which prevents desiccation of mucus membranes, and improves patient comfort. Gaseous exchange, including oxygenation and carbon dioxide elimination, occurs by bulk flow of gases at high flows during apnoea [ ].
Specific HFNO devices are not readily available in all theatres, which, in combination with the cost of dedicated consumables, is a barrier to their use in daily practice.
Most anaesthesia workstations have an auxiliary oxygen outlet. Although the flowmeter on this outlet typically only measures up to 15 L/min, it is often possible to increase the flow to much higher rates. However, the respective workstation technical manuals do not specify a maximum flow rate for the auxiliary oxygen outlet [ ].
While it is traditional to provide warming and humidification during prolonged periods of HFNO in order to prevent discomfort, evaporative cooling, and desiccation of the mucous membranes [ ], this may not be necessary for short periods used to provide apnoeic oxygenation during intubation, as recommended by the British Thoracic Society [ ]. Brainard et al. demonstrated that the use of high-flow nasal cannula at 15 L/min for 10 min was not associated with any documented adverse events [ ]. Use of cold, dry HFNO may risk desiccation of or damage to mucous membranes, potentially leading to complications such as epistaxis. These risks may be further compounded in the high-risk environments where this technique is most likely to be employed. Safety must be tested before this approach can be recommended.
With this proof-of-concept study, we aimed to measure the maximum flow rates of auxiliary oxygen outlets on different anaesthesia workstations to determine if they would be adequate as a source for HFNO. Additionally, we looked at the maximum flow rates of wall flowmeters. If the flow rates generated are adequate for HFNO, these auxiliary outlets or wall flowmeters would provide a cheaper alternative to expensive HFNO machines, especially in low-resource settings.
2
Materials and methods
The CITREX H4 is a mobile gas flow analyser that is intended for testing and calibration of medical devices or systems that generate gas flows or pressures. This includes ventilators and anaesthetic equipment. It measures flow rates up to 200 L/min, and is specifically designed for testing oxygen. Our test device was calibrated by the manufacturer in Switzerland with validity for 12 months, which spanned our study period.
For each test, the CITREX H4 was connected to the auxiliary oxygen outlet of the anaesthesia workstation or to the wall flowmeter with a universal adapter. Turbulent flow was noted at very high flow rates. Video recordings captured pipeline pressures, flow rates at 2 and 10 L/min, and the flow rates when the outlet was turned open to maximum. Readings were recorded every second for 10 s. All anaesthetic workstations currently in use at University of Cape Town (UCT)-affiliated theatres, including Groote Schuur Hospital (GSH), New Somerset Hospital (NSH), University of Cape Town Private Academic Hospital (UCTPAH), Red Cross War Memorial Children’s Hospital (RCWMCH) and Mowbray Maternity Hospital (MMH) were included in this study. All anaesthetic workstations tested were in clinical use at the time of the study. These are maintained by our clinical engineering department with up-to-date service records. Additionally, all wall oxygen flowmeters in induction rooms and recovery wards at these hospitals were included. Data were collected over a period of two months.
2.1
Statistical analysis
The distribution of flow rate was assessed using a histogram. Flow rates were summarised using the mean (average) and standard deviation (SD). The coefficient of variation (CV) was used to compare flow rates across different anaesthetic workstations or flowmeters. One-sided t-tests were used to determine if the anaesthetic workstations or flowmeters would be able to support HFNO therapy of greater than 40 L/min during preoxygenation or greater than 60 L/min during apnoea. Pearson’s correlation coefficient was used to assess the relationship between pipeline pressure and flow rate. Statistical significance was defined as α < 0.05. Data analysis was performed using R Statistical Software (version 4.3.0) [ ].
3
Results
A total of 42 anaesthetic workstations and 39 wall flowmeters were evaluated across all UCT-affiliated theatres.
3.1
Anaesthetic workstations
The General Electric (GE) CareStation (CS) 650 had the highest average flow rate (120 L/min), while the Dräger Fabius GS Premium had the lowest average (13 L/min) ( Table 1 ). Only two of the nine anaesthesia workstation types tested could reliably support flows sufficient for high flow apnoeic oxygenation (Fig. 1) .
| Anaesthetic workstations | Average (SD) flow rate across 10 intervals | Coefficient of variation (CV) | Greater than 40 L/min p-value | Greater than 60 L/min p-value |
|---|---|---|---|---|
| GE CS 650 (n = 14) | 120.09 (0.72) | 0.006 | <0.001 | <0.001 |
| GE Aespire View (n = 10) | 44.37 (0.84) | 0.019 | 0.024 | 0.999 |
| GE Aespire (n = 1) | 104.95 (0.78) | 0.007 | <0.001 | <0.001 |
| GE AISYS CS2 (n = 10) | 34.01 (0.80) | 0.023 | 0.999 | 0.999 |
| Dräger Perseus A500 (n = 3) | 26.05 (0.14) | 0.005 | 0.999 | 0.999 |
| Dräger Atlan A350 XL (n = 1) | 26.06 (0.12) | 0.005 | 0.999 | 0.999 |
| Dräger Primus (n = 1) | 41.00 (0.16) | 0.004 | <0.001 | 0.999 |
| Dräger Fabius GS Premium (n = 1) | 13.00 (0.00) | 0.000 | 0.999 | 0.999 |
| Mindray WATO EX-30 (n = 1) | 39.02 (0.11) | 0.003 | 0.999 | 0.999 |
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