Pressure = Force / Area
Therefore the same force will generate more pressure if applied to a smaller area (syringe).
Force – that which changes or tends to change the state of rest or motion (acceleration or change in direction) of an object.
1 N of force accelerates a frictionless mass of 1kg 1m/s/s.
1 Pa = 1 N applied to 1 m2.
1 Bar = 100 kPa = 1000 cmH2O = 1 atmosphere (101 kPa).
7.5mmHg = 10cm H2O
Absolute pressure = gauge pressure + atmospheric pressure.


Liquid manometers
Pressure measured is independent of the volume supported or the cross sectional area of the tube or tilting of the tube.
Bourdon gauge
Coiled tube of oval cross-section uncoils as pressure causes it to become circular in X-section
Used for high pressures (>100 kPa).
Pressure transducers
Pressure changes cause movement in a flexible diaphragm which are transduced by wire strain gauges whose resistance changes in response to pressure.
NIV arterial BP measurement

Clinical uses

Arterial BP

  • Blood volume – hypoxia and venous drainage
  • Formation and drainage of aqueous humour
  • External pressure

Surface tension

The force acting perpendicularly across a line of unit length.
Molecules in the bulk of the liquid are attracted in all directions whereas those on the surface are attracted inwards and in a direction parallel to the surface only.
Pressure gradient across the wall of a tube (transmural pressure gradient) = wall tension / radius.

P = T / R

For a sphere

P = 2T / R
(Laplace’s law)

So a reservoir bag maintains safe pressures as the radius goes up as well as the wall tension.
Surfactant lowers surface tension so improving compliance.

Invasive BP measurement

Parallel walled intra-arterial cannula.
Column of saline in continuity with blood.
Constant flush through constriction so cannot exceed 4mls/h at pressure > systolic (usually 300mmHg).

  • Mirror coated moving diaphragm which reflects light from an optical fibre or
  • Wire resistor in a Wheatstone bridge circuit (4 resistors, 1 a strain gauge, 1 variable. Variable resistance altered so no current flow - R1/R2 = R3/R4).
  • Frequency response must be x10 the fundamental frequency (pulse rate) for accuracy ie 0.5-30 Hz.

Natural frequency is the frequency at which any system will resonate.
If this frequency overlaps with the measured frequency then signal distorted.
Resonance proportional to diameter of cannula and inversely proportional to compliance, length of tubing and density of liquid.
So short, wide cannula with stiff tubing lifts the resonant frequency out of the measured response range.

Progressive decrease in amplitude of oscillations in a resonant system caused by dissipation of stored energy.
With no damping the system oscillates at its natural frequency D=0.
Critical damping is when the recorded signal falls slowly to baseline with no overshoot D=1.
If D>1 then trace flattened.
Best compromise for speed and accuracy is optimal damping D=0.64.
Underdamped waveform has
systolic and diastolic; overdamped both .
MAP unchanged in both.

Information from the arterial waveform
Arterial BP
  • Rate of pressure change per unit time (dP / dT) – contractility
Area under the curve (systolic part of waveform - up to dicrotic notch)
  • SV (and CO when factor in HR)
Dicrotic notch
  • Low - SVR or hypovolaemia
  • High - SVR
Slope of diastolic decay
  • Outflow resistance – shallow if SVR, steep in SVR
Respiratory variations
  • Systolic pressure variation
  • Pulse pressure variation
  • SV variation
  • >10% = very high positive predictive value for fluid responsiveness.