A discussion of invasive arterial blood pressure measurement is a slippery slope into terrible gibberish. Instead, one should focus on what is expected from a viva answer. The specific things which could be asked are about the method of arterial cannulation, indications for it, sources of inaccuracy, and possibly Complications of arterial cannulation. One may wish to expand into such themes as the zero level of the arterial transducer, what relevance this has to the accuracy of measurement, and how it influences the utility of cerebral perfusion pressure as a therapeutic target. Question 6.2 from the second paper of 2009 briefly touches on these issues.
- Method of invasive blood pressure monitoring
- Arterial cannula
- No more than 1.2m of fluid-filled compression-resistant tubing
- Fluid column is exposed to the Wheatstone bridge strain-gauge
- Wave form built up by Fourier analysis from sinusoids
- Different waveform morphology is to be expected depending on the site of measurement
- Indication for invasive haemodynamic monitoring:
- Rapidly or dangerously fluctuating blood pressure
- A prolonged course of blood pressure monitoring (avoids the pressure-related complications of repeated NIBPs)
- Titration of vasoactive agents
- Frequent blood sampling
- Inaccuracy of non-invasive monitoring (eg. in massive obesity)
- Sources of inaccuracy in the invasive measurement of arterial blood pressure
- Cannula is in a damaged artery; spasm or partial obstruction
- Cannula or tubing is kinked
- Air bubbles or clots in the tubing
- Inadequate frequency response of the transducer (should be at least 30Hz, and more like 100Hz)
- Inappropriately zeroed
- Inappropriately levelled
- Factory-calibrated transducer came with faulty calibration.
Zeroing and levelling of the transducer
- "Zeroing" the transducer sets the atmospheric pressure as the "0 mmHg" reference point.
- "Levelling" the transducer sets a particular height along a fluid column as the "0 mmHg" reference point.
- The arterial line pressure transducer should be "levelled" at the phlebostatic axis, which is a reference level we have used since probably 1945.
- The phlebostatic axis corresponds roughly with the position of the right atrium
- This level has generally been accepted as the ideal reference level for measure the pressure of the blood returning to the heart, and was therefore adopted as the reference level for CVP measurement.
- For arterial pressure measurements, at least since 2001 or so we have been also levelling the arterial lines at the phlebostatic axis. Prior to that, some units levelled their arterial lines at the level of the catheter insertion site.
- The specific reference point for the arterial transducer is actually the aortic root, but because it is very close to the right atrium the two reference levels are essentially the same.
- For measurement of cerebral perfusion pressure, the reference level should be at the middle cranial fossa, but whether this matters or not is a subject of some debate.
Advantages and disadvantages of different sites have come up in Question 11.1 from the first paper of 2021. Here, the table is self-plagiarised from the chapter on the factors involved in site selection for arterial line insertion.
- Easily accessible even during surgery
- Generous collateral circulation
- Easily compressed
- No collateral damage (not surrounded by delicate structures)
- Does not restrict patient movement
- Large and proximal: more accurately reflective of central arterial pressure
- Much larger than the radial, and therefore easier to access and palpate
- Easily compressible
- Largest and most proximal
- Most accurate reading of central arterial pressure (least affected by pulse wave amplification and peripheral vascular disease)
- Easily palpated
- Distal enough to be accessible even during CPR
- Distal; pulse amplification makes the systolic and diastolic less accurate
- A highly mobile site; can easily kink or dislodge
- Small; may be difficult to access
- Subject to considerable anatomical variation
- Difficult to access during surgery or CPR
- End-artery; i.e. no collateral circulation (theoretical increase in the risk of limb ischaemia)
- Right next to the median nerve
- More likely to kink and occlude with patient arm movement
- Potential for retroperitoneal haematoma, which is not compressible
- Potential for through-and-through puncture of the femoral vein, resulting in an AV fistula
- Theoretically, a higher risk of infection
- Makes it challenging to mobilise patients
- Cerebral embolization
- Peripheral neuropathy
- High risk of thrombotic complications
- Median nerve damage
- Cerebral embolization
- Retroperitoneal haematoma
- Abdominal visceral injury
- Arteriovenous fistula
General complications for all sites
- Pain and swelling
- Accidental dislodgement
- Limb ischemia
- Catheter-related infection including bacteremia
- Iatrogenic blood loss from frequent sampling
- Heparin-induced thrombocytopenia (if heparin is used in the flush bag)