The Helmet or Helmet is a non-invasive method of treatment consisting of a transparent hood that fits over the patient’s entire head without a point of contact, anchored to a soft, extensible collar that fits comfortably over the neck. This method, used since 2000, particularly in Italy, is frequently used in various respiratory diseases, whether acute or chronic, as a treatment for hypoxemic respiratory failure.
The call type Helmet, most used in Brazil, has two (or more) connectors where the gases enter, one enriched with oxygen which can be supplied by a Venturi system, or a flow generator such as a mechanical ventilator.
In the context of Covid-19, this method has been widely used in treatment, as it is a method with good tolerability, cost-effectiveness, and good sealing ability with little risk of soft tissue injury.
The review published in September 2021 in Critical Care is divided into two sections. The first concerns using these helmets to deliver continuous positive airway pressure (H-CPAP), which depends on a system with continuous free flow and a PEEP valve. During CPAP, the patient inhales or exhales freely, while the pressure remains constant inside the Helmet, with no interaction with the ventilator and no “active” inspiratory support.
The second section explains non-invasive positive pressure ventilation (NPPV), which offers active support for inspiration (usually by pressure support) provided by a mechanical ventilator. Both methods, CPAP and NPPV, are grouped as non-invasive methods.
Learn more: What is the impact of non-invasive ventilation with Helmet on severe Covid-19?
When used with a helmet, this method has better pneumatic performance than a face mask and is better tolerated by the patient. The main disadvantage is the risk of rebreathing carbon dioxide.
The simplest configuration of the H-CPAP is by the constant gas flow (in variable FiO2) through the Helmet connected to an oxygen-only valve or a blender with associated compressed air. The expiratory valve is connected to a device that regulates PEEP and disperses the expired gas to the environment.
Adequate gas flow is required to maintain positive pressure through the expiratory valve and pre-exhaling carbon dioxide. It is important to emphasize the proper adjustment of the gas flow. As demonstrated by Patroniti, a flow rate of less than 40L/min leads to significant rebreathing of CO2 during inspiration. It has also been demonstrated that using a mechanical ventilator, set to CPAP mode, should be avoided as the circulation of gas flowing through the system is very similar to the patient’s minute ventilation, and CO2 washing is inadequate.
The most commonly used method with a face mask connected to the ventilator. It consists of providing a positive airway pressure that can be defined by PEEP. The use of a helmet was proposed because replacing face masks brings fewer complications, can stay in use for longer, and has greater patient comfort and better coupling. In addition, end-expiratory lung volume is greater during an NPPV helmet than during an NPPV face mask, possibly due to reduced activation of the expiratory muscles.
For the method to be effective in the use of Helmet, greater pressurization is necessary to keep the Helmet more rigid and therefore more effective. Therefore a higher PEEP is necessary to harden the Helmet, increase the PSV level, longer pressurization time (i.e., low rise time), and cyclical flow limit.
Rebreathing CO2 is a key issue during NPPV due to the greater amount of dead space created than in the face mask; however, the effective dead space may be less than expected. The Helmet’s average CO2 concentration depends primarily on the Helmet’s CO2 production and total ventilation (monitored by the fan as “minute volume” ):. Higher pressure support levels lead to an increase in minute volume, thus resulting in better washing of CO2.
Contextualizing Covid-19’s Current Moment
Patients with Covid-19 can develop severe hypoxemia and depend on PEEP. Several authors reported using H-CPAP inside and outside ICUs, and Bellani, for example, had a success rate of over 60%. Coppadoro 69% success rate. It was shown that positive pressure improved oxygenation and allowed for better stratification of patient severity through the calculation of PO2/FiO2.
Only one study in this review was found on NPPV. Grieco compared helmet NPPV with high nasal oxygen flow (HFNO), showing that 48 h after randomization, patients treated with Helmet NPPV had better oxygenation, a lower respiratory rate, and lesser hypocapnia.
The Covid-19 experience has led to more widespread use of helmets. Different technical solutions can be applied (free flow CPAP vs. mechanical ventilator NPPV). The therapy can be used safely and effectively to deliver NIV during hypoxemic respiratory failure, improving oxygenation and leading to better patient-centered outcomes than other NIV interfaces.