Anesthesiology is where physics meets physiology. Every breath delivered through a ventilator and every arterial waveform on a monitor represents the interplay between static pressure (the pressure exerted at rest) and dynamic pressure (the pressure exerted when fluid or gas is in motion). Misinterpretation can lead to inappropriate ventilator settings, flawed hemodynamic management, and increased perioperative risk.
Static pressure represents potential energy in a fluid system.
Dynamic pressure represents kinetic energy due to fluid motion.
The relationship is described by Bernoulli’s principle:
Where:
PtotalPtotal = total pressure
PstaticPstatic = static pressure (potential energy)
ρρ = fluid density
vv = velocity of flow
➡️ In anesthesia, this equation explains why a high-flow jet can entrain air (Venturi principle), why peak inspiratory pressure differs from plateau pressure, and why arterial line fidelity depends on distinguishing static vs dynamic components.
Pressure exerted by a fluid at rest.
Equal in all directions.
Examples in anesthesia:
Airway static pressure = plateau pressure during inspiratory hold.
Vascular static pressure = mean intravascular pressure when no flow artifact exists.
Pressure due to motion (kinetic energy).
Examples in anesthesia:
Airway: resistance component in peak inspiratory pressure.
Circulation: pulsatile pressure waves from cardiac output.
In ventilation: PIP = static + dynamic.
In circulation: arterial trace = static (MAP) + dynamic (pulse pressure).
Case vignette – Bronchospasm vs ARDS
Imagine you are ventilating a 45-year-old asthmatic patient under general anesthesia. Suddenly, you notice the peak inspiratory pressure (PIP) rising on the ventilator. What does this mean?
If you perform an inspiratory hold (pausing airflow for 2 seconds) and the plateau pressure stays normal, the problem is increased airway resistance (e.g., bronchospasm, kinked tube, secretions).
If both PIP and plateau are elevated, then the problem is reduced lung compliance (e.g., ARDS, pulmonary edema, pneumoperitoneum).
Key principles to remember:
PIP – Plateau = Airway resistance (the difference tells you how hard it is to push gas through narrowed tubes or bronchi).
Plateau pressure = Compliance (how stiff or elastic the lungs and chest wall are).
Ventilator graphics – Pressure-Volume loop:
In reduced compliance (e.g., ARDS): the loop is steeper, showing that a large pressure is needed for even a small volume.
In increased resistance (e.g., asthma): the loop widens, reflecting wasted pressure just to overcome airway narrowing.
Clinical Relevance:
Always check both PIP and Plateau together before reacting to high pressures.
A plateau >30 cmH₂O significantly increases the...
