Arterial lines

Question 1

What is considered ‘optimal damping’?

Answer 1

Damping is the process of the system absorbing the energy (amplitude) of oscillations

Optimal damping where real-time accuracy is greatest:
Maximises frequency response
Minimises overshoot of oscillations
Minimises phase and amplitude distortion
Occurs when the damping co-efficient is 0.64

Fast flush test - Flushing the arterial line will produce a square waveform followed by a number of oscillation before the normal waveform is produced. In optimal damping 1-2 oscillation will be seen - less is overdamped, more is underdamped

Question 2

What haemodynamic variables can be shown on arterial line pressure-time curve?

Answer 2

Myocardial contractility - slope of the upstroke
Stroke volume - area under the curve
Systolic pressure - peak of the curve
Diastolic pressure - trough of the curve
Duration of systole - width of the curve

The position of the dicrotic notich reflects fluid status and aortic compliance

Question 3

Wheatstone bridge

Answer 3

• An electrical circuit that is used to measure an unknown resistance
• Consists of four resistors, three of which have a known resistance - two are fixed, one is variable. The last resistor is unknown, and the resistance being measured
• The resistors are arranged into two parallel circuits. A galvanometer reads the potential difference between the two circuits
• When the galvanometer reads 0, the two circuits must be balanced. Therefore R1 / R2 = Rvar / Runknown
• Rearranging this, R2 / R1 = Runknown / Rvar, then Runknown = (R2 / R1) x Rvar
• The invasive measurement if arterial blood pressure is based around the movement of a diaphragm within a transducer, which is in contact with the arterial blood via a column of fluid. A strain gauge is attached to the diaphragm, and its resistance changes as the diaphragm is stretched with the pulsatile movement of arterial blood. The strain gauge forms the unknown resistance in the Wheatstone bridge. Once the bridge is zeroed, any change in strain gauge resistance will cause a deflection of the galvanometer, which is amplified and converted to a blood pressure reading