As the term suggests, ‘airway pressure‘, is the pressure that occurs due to the resistance exerted by the airway. In the case of human beings, the airway refers to the respiratory tract (such as the trachea, bronchus, etc) and the alveoli of the lungs. In the case of the ventilators, the airway tract also includes the parts of the machine such as the endotracheal tube and the breathing circuit. Similarly, the pressure is due to the resistance exerted by the respiratory tract and the pressure exerted for the inflation of the lungs and the chest walls. The airway pressure is further classified into two types.
They are the peak pressure and the plateau pressure. Here we will be discussing these types of pressures in depth. Besides that, we will also discuss lung compliance and compare the airway pressure with the closed circuit.
Table of Contents
Pressure applied by the ventilator
We know that, in the case of the ventilators
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
Some of the causes of high resistive pressure are as follows.
- Secretion of the mucus
- Patient biting the tube
- Mucus plug
- Formation of kink in the endotracheal tubes
(Note:- For the ventilation breathing, the airway also includes parts of the machines such as the breathing circuit. Thus, in comparison to normal breathing, the airway pressure also increases. Hence, the ventilation pressure will be greater than the normal breathing pressure.)
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 and the breathing 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. The normal value of the PEEP pressure is around 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
There can be a number of reasons for the low compliance. Some of the reasons are as follows.
- Pneumonia:- It is a type of infection that inflames the air sac of one or both lungs.
- Pulmonary Edema:- It refers to the accumulation of fluid inside the tissues and the air sacs of the lungs.
- Pleural Effusion:- It refers to the accumulation of fluids between the pleural layers of the lungs.
- Tension pneumothorax:- It is defined as the condition when the air gets trapped in the pleural cavity under 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. So we can explain the properties of the positive airway pressure with the help of this circuit.
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, we have got,
The equivalent capacitance (Ceq) ≈ compliance of the lungs
or, Q/ V ≈ ΔV/ ΔP
Where Q= charge
ΔV = change in volume
Thus from the above equations, we can conclude the following things.
Charge ≈ Change in Volume
Current ≈ Airflow
Capacitance ≈ Compliance
Voltage ≈ Change in Pressure
