This chapter is vaguely relevant to Section G7(iii) of the 2017 CICM Primary Syllabus, which asks the exam candidate to "describe the invasive and non-invasive measurement of blood pressure, including limitations and potential sources of error". It trespasses slightly into the territory of non-examinable content, insofar as the CICM Part I is concerned. Historically, no SAQ or viva has asked about this topic. This digression is justified by the author's suspicion that some minority of CICM trainees will at some stage need to know this material. Reading between the lines of the syllabus handbook, one develops the sense that that the college probably expects junior intensive care trainees to be able to competently use arterial lines beyond the brutally technical aspects of inserting and troubleshooting them.
Broadly, indications for establishing arterial access are as follows:
1. Haemodynamic monitoring:
- Where blood pressure is labile
- Where haemodynamic instability is anticipated (eg. major surgery)
- Where haemodynamic therapy is being titrated (eg. vasoactive drugs such as noradrenaline)
- Where non-invasive blood pressure monitoring would be inaccurate or unreliable, for example in the context of arrhythmia or morbid obesity
2. Blood sampling:
- Where respiratory therapy such as mechanical ventilation is being titrated
- Where frequent biochemistry testing is required and venous sampling is difficult
3. Diagnostic or interventional procedure:
- Intra-arterial drugs
- Endovascular procedures
- Intra-aortic balloon counterpulsation
- Arterial embolisation
4. Continuous cardiac output monitoring
This list was adapted from Irwin and Rippe (Chapter 3 by Lee-Llacer and Seneff). Not all of these indications make sense in the context of the ICU, but it is meant to encompass the full range of reasons as to why one might want to puncture an artery and get up into it. The ICU will generally see only points 1 2 and 3, as we rarely perform any (intentional) arterial embolisation, and our "endovascular procedures" are limited to tickling the atria with guidewires.
Of these indications, a couple are sufficiently interesting to merit a more indepth exploration.
Morbid obesity as indication for invasive arterial pressure monitoring
Morbidly obese patients tend to have conical arms. Blood pressure cuff manufacturers clearly have in their minds some idealised image of the obese person which does not correspond to this reality, because their “large” cuffs seem to be designed to fit around a cylinder. This is a problem, mainly in terms of accuracy. Specifically, the inappropriately sized cuff will usually underestimate the blood pressure of the morbidly obese person. For example, Araghi et al (2006) enrolled a group whose mean BMI was about 32, and found that both oscillometric and auscultatory methods were unreliable. Non-invasive methods consistently underestimated systolic blood pressure and overestimated diastolic blood pressure. The range of agreement was very wide – the discrepancy was often in the realm of 50mmHg. In short, if one is truly interested in the blood pressure of a morbidly obese person, one should seriously consider measuring it invasively.
Atrial fibrillation as a weak indication for invasive arterial pressure monitoring
Automated oscillometric measurement of blood pressure tends to rely on the regularity of heart rate, and the devices which measure NIBP are calibrated to sinus rhythm. In rapid AF, blood pressure variability might give rise to algorithm confusion. The oscillometer may not recognise some of the shallow rapid beats as oscillations and measure only the “good” beats, for example. Or such is the usual thinking.
This idea does not seem to stand up to close scrutiny. Lakhal et al (2015) tested oscillometric cuffs in ICU patients and found that their measurements were generally fairly accurate, insofar as it satisfied the ISO standards for accuracy. Pagonas et al (2013) also found the agreement between NIBP and invasive pressure to be satisfactory, provided one takes three repeated measurements. There was increase intra-patient variability of oscillometric measurements, but after several measurements the mean values were close enough to the invasively measured pressure.
In neither of the quoted studies were there many haemodynamically unstable patients. However, with haemodynamic instability the point becomes moot. You’ve got a labile blood pressure? Perhaps that’s enough of an indication for measuring it invasively, irrespective of what the rhythm is doing. Conversely, if the patient is in AF but the non-invasive blood pressure measurements are fine and the patient looks awesome (i.e. not dying of shock), then how is this different to – for example – the community setting? Again, one would not even consider invasive arterial pressure monitoring in such a scenario. In summary, invasive arterial pressure monitoring in patients with arrhythmia is mainly indicated for haemodynamic instability rather than because of the arrhythmia.
Contraindications to arterial line insertion
Contraindications to arterial line insertion are largely factors which increase the risk of infectious or ischaemic complications to the point where it exceeds the benefit. These are generally site-specific; i.e. an absence of collateral circulation in the hand is usually a unilateral phenomenon and if 30% of the patient's body is covered with full-thickness burns you'd be reasonably confident that the remaining 70% has an accessible artery in it somewhere. There being no convenient NEJM review titled "Contraindications to arterial line insertion", the list offered here has been remixed from several sources, including UpToDate, Medscape, OpenAnaesthesia and LITFL.
- Absence of collateral circulation (eg. abnormal modified Allen's test)
- Local infection
- Distorted anatomy (eg, previous surgical interventions, congenital malformations)
- Active Raynauds Disease
- Thromboangitis obliterans (Buerger disease)
- Stent or synthetic vascular graft
- Arteriovenous malformation or AV fistula
- Severe peripheral vascular disease
- Severe coagulopathy (INR > 3.0, APTT > 100sec as suggested by UpToDate)
- Severe thrombocytopenia (platelet count 50 x 109/L, as suggested by UpToDate)
- Recent use of thrombolytic agents
Pretty much everywhere you read, Raynauds and Buerger diseases are mentioned as absolute contraindications, which is interesting. Why not other vasculitic conditions? For example, how does giant cell arteritis or Takayasu syndrome affect the risk profile of arterial cannulation? Surely, Behçet syndrome should also be listed?
On one hand, these conditions are sufficiently rare that one could reasonably expect there to be virtually no data about how they complicate arterial line insertion. On the other hand, these conditions are usually sufficiently severe to end up in the ICU, where people will try to put arterial lines is. Ergo, somebody somewhere should have published on this. A short literature search confirmed this suspicion.
In summary, vasculitis does not appear to be an absolute contraindication, but it certainly increases the risk of complications. One of the reasons for this is the tendency of the ulnar artery to be diseased, for instance in the context of scleroderma or CREST syndrome (Taylor et al, 2002). For instance, Rose (1993) reports a case where a woman with CREST syndrome (which nobody knew she had) developed severe digital ischaemia after radial arterial cannulation (interestingly even while complaining of ischaemia-related pain her Allen's test was normal). It is possible to access the arterial circulation safely in these people, but it requires steely nerves and a damn strong indication for arterial monitoring. For instance in 1987 Yacoub et al inserted a left axillary arterial line to monitor the blood pressure of a 45 year old man with severe Buerger's disease, who was undergoing aneurysm clipping after several stroke-like episodes. The authors cheated by performing a pharmacological sympathectomy - a left brachial plexus block was used to keep the vessels patent.