Positive pressure ventilation

Positive pressure ventilation is the replacement of negative pressure ventilation. It is widely used in ICU and emergency.  There are two types of positive pressure ventilation. They are as follows:

• Non-invasive positive pressure ventilation ( e.g. CPAP and BiPAP)
• Invasive positive pressure ventilation (e.g. Endotracheal tube)

Advantages

• 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

Normal breathing Cycle

Breathing cycle for positive pressure ventilation

1. Inspiration

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 comes to equilibrium.

2. Expiration

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.