The term, Positive pressure ventilation, consists of the positive pressure, which means that the applied pressure is higher 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 higher pressure than the normal atmospheric pressure. It is the replacement of negative pressure ventilation. Similarly, it also refers to the higher level of air pressure within a room in comparison to that of the surrounding. Here, we will discuss the concept of the positive pressure ventilation room and mechanical ventilation.
Positive Pressure Ventilation Room
A positive pressure ventilation room is a system, which maintains a positive pressure inside the room. In other words, the pressure inside the room will be greater than the outer pressure. This will allow the one-directional flow of air from inside to outside.
In hospitals, a positive pressure ventilation room is important for the safety of the patients (eg. cancer patients) who are vulnerable to the diseases. The contaminations and diseases will not be able to get inside the room. Thus, the patients will be protected.
Design of Positive Pressure Ventilation Room
You need to maintain the positive pressure gradient between an isolated room to that of the environment by the exhaust system that removes the quantity of air less 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 positive 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 room to the anteroom, After that, air should pass through the corridor.
You need to supply the air from the ceiling right above the bed of the patient. Similarly, the exhaust air should be taken away from the ceiling near the door of the room. But remember that before the air gets inside the room, the air should be pre-filtered. For this, you can use a HEPA filter at the main-air handling unit or at the supply terminal in the room.
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 for safety purposes. Also, install an air space diffuser to limit the air velocity within the room.
When not in use, this room can also be used for the general patient, just like the other rooms.
Mechanical Ventilation as a Positive Pressure Ventilation
Types of Positive Pressure Ventilation
- Non-invasive Positive Pressure Ventilation ( e.g. CPAP and BiPAP)
- Invasive Positive Pressure Ventilation (e.g. Endotracheal Tube)
- Variation in the value of FiO2 from 24% to 100%
- Highly effective for providing higher pressure gradient
- Treatment can depend upon the patient to patient
Breathing Cycle under Positive Pressure Ventilation
Positive pressure, greater than the atmospheric pressure (say 10 cm H2O), is applied to the respiratory tract. This creates a pressure gradient between the ventilator output and the intra-alveolar sac. So, air rushes to the lungs until the applied pressure and the intra-alveolar pressure come to equilibrium.
The positive pressure cuts off. So, the outside pressure will be equal to the atmospheric pressure (0 cm H2O). This will create a pressure gradient between high intra-alveolar pressure and low atmospheric pressure. So, the air is released out of the lungs until the pressure gradient becomes 0.
Graph of a Breathing Cycle
The above graph shows that initially, the lung is in equilibrium with the atmospheric pressure (0 cm H2O). When the lungs expand for inspiration, intra-alveolar pressure suddenly decreases. On sensing this drop in the pressure by the pressure sensor, the machine provides an external high pressure. The pressure gradient between high applied pressure (say +10 cm H2O) and low intra-alveolar pressure (0 cm H2O) causes a rush of air into the lungs. Air exerts pressure onto the walls of the lungs increasing the intra-alveolar pressure. After mid inspiration flow rate decreases. Thus the rate of rising pressure decreases. At the end of inspiration, intra-alveolar pressure becomes equal to the applied external pressure.
During expiration, applied pressure cuts off. So, the pressure gradient causes the flow of air from intra-alveolar space to the atmosphere. Hence, the pressure decreases sharply within a short period of time. Then the internal pressure decreases slowly, due to scarcity of air and comes to equilibrium with the atmospheric pressure.
Positive Pressure Ventilation Fan
Positive Pressure Ventilation (PPV) fan is used by firefighters. This fan is used for blowing high-velocity air which helps to drive away the smoke after the fire. It can also divert the flow of smoke to the desired direction. Thus, this will help the firefighters to rescue the victims.