Albeit very new in the US, helmet based ventilation is a growing interest within the medical community, and its simplicity and modularity are allowing use configurations in resource limited settings that enable significant reductions in ventilator demands. We are now seeing helmet NIV driven with wall-gas titration, driven with BiPAP, driven with modified CPAP, and even other novel and emerging device developments.
As we’ve moved along in advancing the Subsalve Oxygen Treatment Hood, two distinct hood development paths have been observed – those that are themselves quite simple, and others that are feature rich. There are of course risks and benefits to each development path. Interestingly, we see this same development path divergence in diving technology. I tend to lean on the simplicity side, though that does not mean that there is any less functional performance, nor does it mean that risks [be them real or perceived] cannot be mitigated.
For discussion here, I am presenting a few respiratory circuits that can be used with any oxygen treatment hood. I will forewarn that I am not advocating any of these be used in a clinical setting without physician evaluation and direction, and only present these for sake of education and encouraging some critical thinking towards addressing perceived risks with helmet NIV.
In this case, I am presenting the Subsalve Oxygen Treatment Hood as a model. It is arguably the most simple design on the market being all one piece. This reduces any chance of assembly error, and further mitigates any risk of leakage do to improper assembly – this is as simple as it gets. In circuit one, I am presenting a very basic inspiratory and expiratory pathway. On both sides I incorporate a 100cc hose section for patient comfort. The inspiratory side includes two oxygen t’s and an open end for a high flow air source. On the expiratory side is a viral filter and PEEP valve. This is representative of the most common circuit configurations viewed in the literature to-date.
In this configuration, logic would have it that the patient should be continuously attended and monitored, similar to an ICU environment. This is because there are few automatic contingencies incorporated, and should there be any number of concerns, the clinician should immediately intervene to correct the concern, or remove the helmet. I will omit discussion of specific risks or any type of formal risk analysis here, as this topic is better reviewed case by case with a given manufacturer and with clinician input.
Now, let’s do a bit of a leap in capabilities to allow for 2 additional features. These may or may not be desirable or warranted depending on the nature of the treatment environment and degree of clinical monitoring and oversight. First, is the concept of CO2 rebreathing in the event of a primary gas supply failure. In most instances, the community consensus is that a minimum 60LPM constant gas flow is the recommendation to mitigate CO2 rebreathing risks – 60LPM comes in, and 60LPM goes out. This flushes the space within the helmet adequately, and life is good. Should flow be lost (a very minimal risk with well maintained and appropriately monitored equipment), the patient would be at rest within a confined space. The below inhalation circuit includes a one-way directional valve that fails ‘open’ meaning with lost flow, the hood is essentially open to the ambient environment. Full disclaimer is that this specific component device has not been tested in the context of relieving excess CO2 buildup within a helmet, however logic would tell you that having an open loop rather than a closed loop is an advantage during a gas failure. This warrants continued evaluation.
The second feature in the above circuit incorporates an over-pressure relief valve (OPRV). This was incorporated quite simply by utilizing a second PEEP valve. The OPRV need stems from concern of a viral filter clogging, thus preventing gas escape, and the hood potentially over-inflating. To configure this OPRV functionality, the expiratory limb is split – one leg passes through a filter and then to a PEEP valve (primary), and the other is PEEP valve only. To enable relief settings, close both PEEP valves and establish flow. Open the primary PEEP valve to desired pressure setting. Then, open the OPRV PEEP valve until it just starts to leak, then close 1/4-1/2 a turn. Functionally, if the filter became clogged, the OPRV would actuate to relieve excess inflation. Replace the clogged filter, reset the PEEP valves, and back in action, all while not having any major pressure changes for the patient.
A third circuit configuration takes protecting the viral filter in to account. A low positioned filter is in the worst possible position since any condensate or other fluids will make their way to this low point. In the above image, we again split the expiratory limb, and run the filtered limb ‘up’. This is still the primary PEEP valve, though is very well isolated from any condensate running down in to the filter. The OPRV PEEP valve is then low positioned. If condensate or fluid visibly accumulates, this PEEP valve can be opened slightly to purge out the fluid. This would of course be messy, so for your benchtop experimenting, I’d suggest placing a towel below the valve before expelling the fluid. Once expelled, simply re-set to 1/4-1/2 a turn past the primary PEEP setting.
In close, it’s all in the respiratory circuit. The oxygen hood itself can be a simple as a one-piece chamber that serves the fundamental purpose of creating a pressurized environment. By keeping it simple and limiting assembly requirements or any penetrating accessories in to the hood itself, the hood technology can remain very, very simple. The respiratory care community has done a great job in providing us with lots and lots of circuit components that can be configured to achieve all sorts of desireable features or capabilities. The above few are quite literally just scratching the surface.