Pressure and temperature measurement — physics viva

A structured oral covering: pressure as force per unit area with the unit conversions (1 kPa = 7.5 mmHg = 10.2 cmH2O; 1 atm = 101.325 kPa = 760 mmHg); the strain-gauge piezoresistive transducer (diaphragm, four strain gauges, Wheatstone bridge, voltage proportional to pressure); the arterial line components (cannula, non-compliant tubing, pressurised heparinised-saline flush, transducer, monitor); zeroing to atmosphere and levelling to the mid-axillary line with the hydrostatic error of 0.75 to 1 mmHg per cm; the natural frequency and damping (under/over/optimal) and the fast-flush square-wave test with mean preserved in all states; the CVP a/c/v waves and x/y descents with ECG timing and the pathological patterns; the PA catheter pressures at 30/40/50/60 cm and the wedge approximating left atrial pressure; the four temperature principles (thermistor negative coefficient, thermocouple Seebeck voltage, RTD positive coefficient, infrared); and the monitoring sites. The under-damping data probe tests the recognition that a ringing waveform gives a spurious high systolic and that the mean is the trustworthy value.

Prompt

A physics viva on the measurement of pressure and temperature in anaesthesia. The examiner asks the candidate to define pressure and state the clinical units and conversions, to describe the invasive arterial pressure transducer and the requirements for zeroing and levelling, to explain damping and the fast-flush test, to describe the CVP and pulmonary artery catheter waveforms, and to state the principles of temperature measurement and the monitoring sites. A short data probe presents an arterial line reading a systolic of 235 mmHg with a ringing waveform, testing the candidate's recognition of under-damping.

Structure of the viva and expected answers

Opening (2 minutes) — pressure, units and the transducer

The examiner opens with: "Define pressure and tell me how invasive blood pressure is measured." A model opening:

"Pressure is force per unit area; the SI unit is the pascal. Clinically we use mmHg, cmH2O, kPa, bar and atmosphere, with standard interconversions — see a reference table for the exact factors. Most clinical pressures are GAUGE pressures, referenced to atmosphere."^[1]

"Invasive blood pressure is measured by a pressure transducer — a device that converts the hydraulic pressure of a fluid column into an electrical signal."^[1]

The arterial line and zeroing and levelling (3 minutes)

The examiner probes: "What is in the line, and what two setup steps must you do?" The candidate answers:

"The line runs from the arterial cannula through short, stiff, non-compliant pressure tubing, a continuous-flush device, the transducer, to the monitor. The flush bag is pressurised well above systolic pressure and delivers a slow constant heparinised-saline flush.^[3]"

"The two mandatory steps are zeroing (open to atmosphere, defining zero) and levelling (align the transducer with the right atrium). Both are systematic errors — re-level whenever you move the table or the patient.^[5]"

Probe: "Why is the flush bag pressurised?" Answer: "So the flow through the restrictor stays slow and constant regardless of the patient's arterial pressure, keeping the column moving and opposing clotting at the tip."

^[3]

Damping and the fast-flush test (4 minutes)

References6Show

[1]Gardner RM Direct blood pressure measurement--dynamic response requirements. Anesthesiology, 1981.PMID 7469106
[2]Kleinman B, Powell S, Kumar P The fast flush test measures the dynamic response of the entire blood pressure monitoring system. Anesthesiology, 1992.PMID 1466471
[3]Scheer B, Perel A, Pfeiffer UJ Clinical review: complications and risk factors of peripheral arterial catheters used for haemodynamic monitoring in anaesthesia and intensive care medicine. Critical Care, 2002.PMID 12133178
[4]Sessler DI Temperature monitoring and perioperative thermoregulation. Anesthesiology, 2008.PMID 18648241
[5]Pickering TG, Hall JE, Appel LJ, et al Recommendations for blood pressure measurement in humans: an AHA scientific statement from the Council on High Blood Pressure Research Professional and Public Education Subcommittee. J Clin Hypertens (Greenwich), 2005.PMID 15722655
[6]Harvey S, Harrison DA, Singer M, et al Assessment of the clinical effectiveness of pulmonary artery catheters in management of patients in intensive care (PAC-Man): a randomised controlled trial. Lancet, 2005.PMID 16084255