The term, Negative pressure ventilation, consists of the negative pressure, which means that the applied pressure is lower than that of the environmental pressure. This kind of ventilation is defined differently, in different sectors. For example, in health care, it refers to the supply of fresh gas to the patient at a lower pressure than the normal atmospheric pressure. It is the replacement of positive pressure ventilation. Similarly, it also refers to the lower level of air pressure within a room in comparison to that of the surrounding. Here, we will discuss the concept of the negative pressure ventilation room and the mechanical ventilation.
Negative Pressure Ventilation Room
A negative pressure ventilation room is a system, which maintains a negative pressure inside the room, in comparison to that of the atmosphere.
A negative pressure room is used for a patient who is suffering from an airborne disease. In other words, a negative pressure room helps to reduce the transmission of diseases (eg. coronavirus) via the airborne route. It is also known as ‘Airborne Infection Isolation (AII)’ or Infectious Isolation Units (IIU)’.
Design of Negative Pressure Ventilation Room
You need to maintain the negative pressure gradient between an isolated room to that of the environment by the exhaust system that removes the quantity of air more than that of the supply. For this, you need to maintain the air change rate (ACH) of the supply air of 12 air changes per hour or 145 liters per second per patient, whichever gives the greatest air quantity result. Similarly, at least 0.01 inch WC of differential negative pressure should be maintained.
For the above ACH value, the Cubic Feet per Minute (CFM), generally ranges from 150 to 200, which depends on the volume of the room. To understand ACH and CFM I advise you to see the following video.
If you use an anteroom, the flow of air should be from the corridor to the anteroom, After that, air should flow towards the isolated room.
There should be a constant volume system between the air supply and exhaust unit. The supply air unit should be on the ceiling right above the foot of the patient bed. It should be independent of the common building supply air system. Similarly, the exhaust can be from the exhaust grilles located right above the patient bed on the ceiling or can be low on the wall near the head of the bed. If you use exhaust grilles below 7 feet on the wall, NFPA 90A requires that it should be protected by a grille or the screen.
The exhaust from the isolated room, anteroom, and the associated toilet should all be discharged to the out room. But remember that, there should not be any contact with the exhaust from other non- AII rooms.
The room should be air-tight. For this, you can have a plasterboard ceiling, a tight-fitting door and windows, and a door griller. This allows for the better maintenance of the pressure gradients reducing the load for the air handling units.
Built an ensuite bathroom within the room. Fit an audible alarm outside the room for safety purposes. Also, install an air space diffuser to limit the air velocity within the room.
All the mechanical and electrical plants should be outside of the room. This will make the system easily accessible for maintenance.
When not in use, this room can also be used for the general patient, just like the other rooms.
Mechanical Ventilation as a Negative Pressure Ventilation
Negative pressure ventilation applies negative pressure to the patient during inspiration. With the advancement in technologies, positive pressure ventilation has replaced negative pressure ventilation.
- The patient can talk and eat
- Cannot provide much pressure gradient
- It may not be useful for patients who cannot undergo spontaneous respiration
- A lower value of FiO2
Types of Negative Pressure Ventilator
Some of the negative pressure ventilators are as follows.
Breathing Cycle under Negative Pressure Ventilation
The negative pressure of about -10 cm H2O is applied inside the chamber. The low pressure inside the chamber causes the expansion of the chest walls. This will cause an increase in the volume of the alveolar sac. When volume increases pressure decreases and vice versa. So, intra-alveolar pressure decreases from 0 cm H2O to -7 cm H2O at mid inspiration. Due to the pressure gradient between the atmosphere and the alveoli, the air moves into the lungs. After mid-inspiration, the intra-alveolar pressure increases due to the pressure exerted by the air onto the walls of the lungs. Thus, at the end of inspiration intra-alveolar pressure comes back to 0 cm H2O.
Atmospheric pressure (0 cm H2O) is applied to the thoracic cavity. The elastic recoil causes the lungs to contract. Intra-alveolar pressure increases from 0 cm H2O to 7 cm H2O at the mid- expiration. Due to the pressure gradient between the lungs and the atmosphere, the air moves out of the lungs. After the mid- expiration, the intra-alveolar pressure decreases due to the lack of air in the lungs. Finally, at the end of the expiration, the intra-alveolar pressure becomes equal to the atmospheric pressure.