Pressure transduction, the arterial line, damping, and temperature measurement

Model answer plan (20 marks, 20 minutes)

A high-scoring answer is structured in five blocks that mirror the stem: (1) principles and units; (2) the transducer and the arterial line; (3) zeroing, levelling and the dynamic response; (4) temperature principles; (5) monitoring sites. Allocate roughly 4 marks to each block. ^[1]

1. Principles and units (4 marks)

• Pressure is force per unit area (P = F/A); SI unit the pascal (Pa = N/m2).
• Conversions (state at least three): 1 kPa = 7.5 mmHg = 10.2 cmH2O = 1000 Pa; 1 mmHg = 1.36 cmH2O = 0.133 kPa = 133.3 Pa; 1 atm = 101.325 kPa = 760 mmHg = 1.013 bar; 1 bar = 100 kPa = 750 mmHg. ^[1]
• Distinguish absolute (referenced to vacuum), gauge (referenced to atmosphere, the usual clinical and cylinder reading) and differential pressure.
• Mechanical devices: Bourdon gauge (curved flattened tube, high-pressure cylinders), aneroid gauge (sealed capsule, anaesthetic-machine pressures), liquid manometer (mercury for BP, water for venous/airway, the calibration reference).

2. The transducer and the arterial line (4 marks)

• The invasive transducer converts hydraulic pressure into an electrical signal. Commonest design: a piezoresistive strain-gauge — fluid pressure deforms a thin diaphragm; four strain gauges bonded to it change resistance with deformation; arranged in a Wheatstone bridge they convert the change to a voltage proportional to pressure. (Mention the alternative capacitive design: diaphragm as one plate of a capacitor.)
• Arterial line components: arterial cannula (smaller diameters damp more; 20-gauge preferred for radial access); non-compliant pressure tubing; a continuous-flush device (pressurised bag of heparinised saline delivering a slow constant flush, the high driving head keeping flow independent of arterial pressure); the transducer; the monitor.
• Complications (briefly): temporary occlusion is commonest (1.5 to 35 per cent, mean about 20 per cent), major complications under 1 per cent and similar across radial, femoral and axillary sites; rarer serious entries are permanent ischaemic damage, sepsis, local infection, pseudoaneurysm, haematoma and bleeding; occlusion risk rises with catheter-to-vessel ratio, haematoma, duration of cannulation, and female sex ^[5].