MEDICAL GAS SYSTEMS
Table 1: Diversified flow calculations for oxygen. Clinical Area
Rooms or bays
Anaesthetic rooms Operating rooms
Post-anaesthesia recovery TOTAL
4 4 8
Design flow per
terminal unit (L/min) 100 100
Diversified flow formula
Q = no addition made Q = 100 + (nT–1)10 Q = 10 (n–1)6
Diversified flow (Case study)
0
130 52
182
that those anaesthetic workstations previously converted to be driven from medical air will be reverted back to being oxygen-driven. This will result in saving the 5-10 L/min of oxygen required to drive the ventilator bellows which is not used for the patients but vented to atmosphere.4
Volatile liquid anaesthetics Volatile liquid anaesthetic agents (inhalation agents) have been in clinical use since the mid- 1950s with the introduction of Halothane. With modern inhalation anaesthetic agents such as Sevoflurane and Isoflurane there has been a move to employing low-flow or even minimal flow techniques. Fresh Gas Flow (FGF) rates of carrier gases of typically 0.2 to 0.4 L/min are readily achievable for the majority of cases where the circle breathing system is employed to deliver closed circuit anaesthesia. This system offers the benefits of being very efficient with fresh gas flow, as any significant leaks from the system are eliminated. Even for procedures where the valveless Mapleson F breathing system is used for the anaesthesia of neonates, infants, and paediatrics of up to 25-30 kg body weight, the flow rates are only typically 2.5 to 3 times that of those using the circle circuit. Levels of awareness, indicating depth on anaesthesia, are now monitored independently using Bispectral Index (BIS) monitoring.
BIS monitoring
BIS monitoring is a technique that measures brain waves in the frontal lobe (Fig 2), allowing lower concentrations of anaesthetic agents. Prior to the adoption of low-flow anaesthesia, flow rates were typically several litres per minute, and even into double figures, with no tangible benefit to the patient. With high rates of fresh gas flow, as much as 80 per cent of the anaesthetic gases are wasted, and end up being extracted to the environment through the scavenging system. In addition, there are considerable financial benefits in adopting low- or minimal flow techniques, both in terms of the volume of medical gases consumed, and therefore the capacity required in the plant, but also the amount of volatile liquid anaesthetic agent, the cost of which is considerable.
60 Health Estate Journal January 2022
Nitrous Oxide Nitrous oxide (N2
O) is a weak anaesthetic
gas, and in the past had been favoured as the carrier gas for volatile anaesthetic agents. The HTM states: ‘Nitrous oxide is primarily for anaesthetic purposes, and occasionally for analgesic purposes’. (2006: Para 4.28).3
However, in spite of
the recognised benefits of rapid anaesthetic induction with the addition of N2
O, its use in the operating theatre is going out of favour. In modern anaesthesia practice the preference is a mix of oxygen and medical air as the carrier gases for the anaesthetic agent. This trend of less reliance on N2
waste as potent greenhouse gases is harmful to the environment, and contributes a significant burden globally. Sevoflurane is a hydrofluorocarbon, and the least harmful of the anaesthetic agents; that said it still has a global warming impact. One hour of typical use of Sevoflurane in the atmosphere has the same environmental impact as driving 30 miles in a modern car.6
O has also
been mirrored in the design of modern anaesthetic workstations. It is now commonplace to see anaesthetic workstations supplied with only oxygen and Air cylinder backup, with no provision for nitrous oxide if the pipeline fails. Furthermore, in the UK the ‘green agenda’ is also seeing a drive to minimise or even eliminate N2
O for anaesthesia.5
Anaesthetic gas scavenging The long-term health effects of exposure to waste anaesthetic gases are difficult to quantify. There are concerns, however, of an increased risk of spontaneous abortion (miscarriage) in female anaesthetists and theatre nurses. Both passive and active anaesthetic gas scavenging systems (AGSS) have been successfully employed to remove these waste gases and exhaust them to the atmosphere, yet anaesthetic
Low and minimal flow techniques The adoption of low and minimal flow techniques has gone some way towards mitigating the problem. The increasing use of Total Intravenous Anaesthesia (TIVA), using drugs such as Propofol, further reduces the risks, particularly for short procedures. However, inhalation anaesthetic agents are likely to be around for some time yet. This having been said, the environmental impact of waste anaesthetic gases is now being addressed, and the argument is compelling. Technology is now beginning to come on online to collect and capture the waste gases at the anaesthetic machine in canisters. The canisters can then be removed to a processing unit, where the gases are converted back into usable volatile anaesthetic agent. With this technology starting to find its way into the operating theatre environment, the future need for anaesthetic gas scavenging systems may well be in question. Of course, the added benefit of this sort of technology is that anaesthetics can be delivered (using cylinders) in different locations, where the medical gas pipeline has not been installed, or where access is difficult. A good example would be a Nuclear Medicine Department, where the need for anaesthetics is infrequent, and hence the economic case for installing permanent medical gases and scavenging would not stack up.
Figure 2: Levels of awareness, indicating depth on anaesthesia, are now monitored independently using Bispectral Index (BIS) monitoring – a technique that measures brain waves in the frontal lobe), allowing lower concentrations of anaesthetic agents.
Case study on medical gas design If we consider the technological advances outlined above, and the changes in clinical practice, then a case study relating these to the HTM may serve to illustrate the points further. Consider an operating theatre suite with four theatres, their associated anaesthetic/induction rooms, and eight recovery bays. If we first, consider O2
from HTM 02-01 Part A,
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