Airway pressure is the pressure that occurs due to the resistance exerted by the respiratory path on the flow of air and the elastic pressure exerted by the lungs and chest walls. Peak pressure and plateau pressure are the important types of airway pressure which we will be discussing along with lung compliance.
The respiratory path also includes instruments such as an endotracheal tube while using the ventilator.
Pressure applied by the ventilator
We have,
Ventilation pressure = Resistive pressure + Elastic pressure
1) Resistive Pressure
Resistive pressure is the pressure that occurs due to the resistance of the respiratory path to the airflow.
Causes of high Resistive Pressure
- Secretion
- Patient biting the tube
- Mucus plug
- Kinked Endotracheal tube (ETT)
2) Elastic Pressure
Elastic pressure is the pressure that causes the lungs and chest walls to inflate due to which gaseous exchange can take place.
Peak Pressure (Peak Inspiratory Pressure)
PIP is the highest level of ventilation pressure (airway pressure) which causes air to flow to the alveoli for gaseous exchange during inhalation. The value of PIP should never exceed 40 cm H2O in a normal condition. In the case of a ventilator, PIP also includes instruments for air supply such as an endotracheal tube.
Plateau Pressure
It is the airway pressure in the alveoli when the breathing is on hold for positive pressure ventilation. Holding air causes resistive pressure (V’ * R) to become 0 as airflow (V’) becomes 0. In that case, airway pressure becomes equal to the alveolar pressure (plateau pressure). This pressure is measured by the pressor sensor of the ventilator pausing the inspiration and expiration of the patient.
Normally, the hold time in the ventilator ranges from 0.5 seconds to 1 second. Also, the normal value for plateau pressure should not exceed 35 cm H2O. Else the patient may have to face problems such as barotrauma.
Mathematically,
P = V/C + PEEP (or Auto-PEEP)
Where P = plateau pressure
V= tidal volume
C= lung Compliance
PEEP = Positive end-expiratory Pressure ( normally, 5 cm H2O )
Lung Compliance
It is the change in volume of lungs per unit change in the trans-pulmonary pressure. Normally, it ranges from 50- 100 ml/ cm H2O.
Mathematically,
Lung Compliance (C ) = ΔV/ΔP where
ΔV = change in the volume of lungs
ΔP= change in the trans-pulmonary pressure
From the above equation, we can conclude that lung compliance is directly proportional to the change in volume and inversely proportional to the change in transpulmonary pressure. It is independent of airway resistance. If the lung compliance is low, more pressure is required to inflate the lungs and vice versa.
Reasons for Low Compliance
Some of the reasons for low compliances are as follows.
- Pneumonia:- Infection that inflames the air sac of one or both the lungs
- Pulmonary Edema:- Accumulation of fluid inside the tissues and the air sacs of the lungs.
- Pleural Effusion:- Accumulation of fluids between the pleural layers of the lungs.
- Tension pneumothorax:- Condition when the air gets trapped in the pleural cavity under the positive pressure ventilation.
Relationship between Plateau Pressure, PIP and Compliance
We have, airway pressure = flow * resistance + plateau pressure
= V’ * R + V/C + PEEP
When there is a decrease in compliance the plateau pressure increases. This causes a rise in PIP pressure. Thus the overall airway pressure also increases.
Equivalent Circuit Model for Peak and Plateau Pressure
We can consider that the positive airway pressure applied to the lungs is analogous to the closed-circuit shown in the figure.
The equivalent resistance (Req) ≈ resistance in the respiratory pathway (R )
or, V/ I ≈ ΔP/ V’
Where V= voltage
I= current
ΔP = change in pressure
V’ = airflow
Also,
The equivalent capacitance (Ceq) ≈ compliance of the lungs
or, Q/ V ≈ ΔV/ ΔP
Where Q= charge
ΔV = change in volume
In conclusion
Charge ≈ Change in Volume
Current ≈ Airflow
Capacitance ≈ Compliance
Voltage ≈ Change in Pressure
