1. Richard Beale Guy’s and St Thomas’ Hospital Trust London, UK
How and where should I measure arterial pressure in a shocked patient, and what does it mean?
Richard Beale
Guy’s and St Thomas’ Hospital Trust
London, UK

2. Disclosures
Dr Beale and the Adult Critical care Service at Guy’s and St Thomas’ NHS Foundation Trust have research collaborations with:
LiDCO Ltd
Philips Medical Systems
Edwards Lifesciences
Pulsion Medical Systems

3. Why is measuring arterial blood pressure important?
Although there are other, more sophisticated definitions of shock – hypoperfusion and hypotension are key aspects of the syndrome
Measurement and monitoring of arterial blood pressure are therefore intrinsic to the diagnosis and treatment of shock
In modern critical care practice, virtually all shocked patients have invasive arterial blood pressure monitoring

4. Current practice (1)
Continuous invasive ABP measurement is now absolutely standard
High quality, disposable measuring kits are routinely used
Blood pressure values are key components of definitions of shock and organ dysfunction, and of treatment guidelines

5. Current practice (2)
Modern haemodynamic monitoring now frequently includes:
continuous cardiac output monitoring based upon arterial pulse wave analysis
measurement of variation in ABP with controlled ventilation as a marker of volume responsiveness

6. Purpose of this review
To concentrate upon ABP measurement as currently performed in modern ICU practice
To consider the strengths and weaknesses of current practice
To make recommendations for practice based upon literature, experience and common sense!

7. Historical perspective
Attempts to interpret the pulse are as old as the practice of medicine
The “modern” era of invasive blood pressure measurement is generally held to have started with the Reverend Stephen Hales
Rapid developments in non-invasive and invasive blood pressure measurement in the last hundred years

10. Approaches to measuring ABP
Indirect (non-invasive) methods
Sphygmomanometry
Palpatory method
Auscultatory method
Oscillometric technique
Finger plethysmography
External tonometry

11. Approaches to measuring ABP
Direct (invasive) methods
Intra-arterial catheter
Widely used in modern intensive care
Radial artery the most common site
Femoral artery increasingly used
Brachial and axillary vessels sometimes used
Used with modern high-fidelity disposable transducer sets
Regarded as “Gold Standard”
Catheter tip transducers also available

12. A damped oscillator McDonald’s Blood Flow In Arteries 5th Ed
Hodder Arnold, London

13. A Wheatstone Bridge

14. Damping Four damping conditions Critically damped Overdamped
Mass does not oscillate and returns exponentially to equilibrium position
Overdamped
Rate of return is also non-oscillatory, but slower
Underdamped
Mass will oscillate, but will decay exponentially
Undamped
Mass will oscillate sinusoidally indefinitely

15. Different damping conditions
McDonald’s Blood Flow
In Arteries 5th Ed
Hodder Arnold, London

16. Artifacts in pressure recording (1)
End-pressure artifact
If a catheter tip faces the direction of flow, it will measure the sum of the lateral pressure and the kinetic energy pressure resulting from flow
Effect is usually small

17. Artifacts in pressure recording (2)
Catheter impact artifact
Transient pressures are created when a catheter is hit e.g. when in the heart
Any component that coincides with the resonant frequency of the system will cause a superimposed oscillation
May cause LV dp/dt to be as much as 100% too high, but will decay exponentially

18. Pressure patterns within the circulation
Arterial pressure waves vary considerably:
With site
With age
With drugs
With disease
Is this clinically relevant?

19. ABP and flow from centre to peripheries
McDonald’s Blood Flow
In Arteries 5th Ed
Hodder Arnold, London

20. IABP harmonics and distance
McDonald’s Blood Flow
In Arteries 5th Ed
Hodder Arnold, London

21. Contemporaneous central and peripheral ABP
McDonald’s Blood Flow
In Arteries 5th Ed
Hodder Arnold, London

22. Pressure and flow in the circulation
McDonald’s Blood Flow
In Arteries 5th Ed
Hodder Arnold, London

23. Changes in aortic and radial BP during exercise
McDonald’s Blood Flow In Arteries
5th Ed, Hodder Arnold, London

24. Effect of nitroglycerin
McDonald’s Blood Flow In Arteries 5th Ed
Hodder Arnold, London

25. Clinical implications of these differences
Important to understand
Not too problematic if peripheral SBP is higher than central SBP, DBP lower and MAP similar
Does the obverse occur?
What happens in shock?
How might treatment be altered?
What are the clinical implications?

26. Kanazara et al, Anesthesiology 2003;99:48-53
12 patients undergoing CPB
Wire tip transducer used to obtain pressure recordings from aorta to radial artery
7 patients developed a reduction in ABP towards the peripheries
This was explained by a reduction in elasticity

27. Kanazara et al, Anesthesiology 2003;99:48-53

28. Kanazara et al, Anesthesiology 2003;99:48-53

29. Arnal et al, Anaesthesia 2005;60:766-771

30. Arnal et al, Anaesthesia 2005;60:766-771

31. Dorman et al, CCM 1998; 26:
14 patients with septic shock, requiring norepinephrine >5 mcg/min
Simultaneous radial and femoral artery pressure measurements
Two patients also studied after resolution of shock

32. Dorman et al, CCM 1998; 26:

33. Dorman et al, CCM 1998; 26:

34. Mignini et al, Crit Care 2006

35. Mignini et al, Crit Care 2006

36. Mignini et al, Crit Care 2006
Authors conclude that two approaches are interchangeable

37. Brachial vs Femoral

38. Femoral Brachial One Beat

39. Radial vs Femoral

40. Radial vs Femoral (One Wave)

41. In Extremis- epinephrine bolus

42. Post Epinephrine

43. Pre Dobutamine and Bicarbonate

44. Post Bicarbonate (Dobutamine still running)

45. Pre Second Epinephrine

46. 2nd Epinephrine Bolus

47. Post 2nd Adrenaline Injection

48. Post 2nd Epinephrine Bolus

49. Low output state and vasopressors: effect on PPV etc

50. Our Clinical Observations
Dramatic FA – RA ABP gradients do occur in severe shock
This is especially so with high dose vasopressor and hypovolaemia
Peripheral perfusion is usually poor clinically
Peripheral ABP may lead to false assumptions about need for more vasopressor
Central ABP may allow vasopressor dose reduction and volume therapy
Phenomenon reverses as patient improves – perhaps a new therapeutic goal?

51. Conclusions: measuring IABP in shock
Clinicians should be aware of technical issues when measuring ABP
They should be aware of potential effect of site on amplitude and morphology
In shock, peripheral BP may substantially underestimate central ABP
If in doubt, measure central BP