How and where should I measure arterial pressure in a shocked patient, and what does it mean

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How and where should I measure arterial pressure
in a shocked patient, and what does it mean?

• Richard Beale
• Guys and St Thomas Hospital Trust
• London, UK

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Disclosures

• Dr Beale and the Adult Critical care Service at
  Guys and St Thomas NHS Foundation Trust have
  research collaborations with
• LiDCO Ltd
• Philips Medical Systems
• Edwards Lifesciences
• Pulsion Medical Systems

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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

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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

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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

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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!

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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

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Approaches to measuring ABP

• Indirect (non-invasive) methods
• Sphygmomanometry
• Palpatory method
• Auscultatory method
• Oscillometric technique
• Finger plethysmography
• External tonometry

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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

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A damped oscillator
McDonalds Blood Flow In Arteries 5th Ed Hodder
Arnold, London
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A Wheatstone Bridge
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Damping

• Four damping conditions
• Critically damped
• 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

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Different damping conditions
McDonalds Blood Flow In Arteries 5th Ed Hodder
Arnold, London
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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

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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

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Pressure patterns within the circulation

• Arterial pressure waves vary considerably
• With site
• With age
• With drugs
• With disease
• Is this clinically relevant?

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ABP and flow from centre to peripheries
McDonalds Blood Flow In Arteries 5th Ed Hodder
Arnold, London
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IABP harmonics and distance
McDonalds Blood Flow In Arteries 5th Ed Hodder
Arnold, London
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Contemporaneous central and peripheral ABP
McDonalds Blood Flow In Arteries 5th Ed Hodder
Arnold, London
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Pressure and flow in the circulation
McDonalds Blood Flow In Arteries 5th Ed Hodder
Arnold, London
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Changes in aortic and radial BP during exercise
McDonalds Blood Flow In Arteries 5th Ed, Hodder
Arnold, London
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Effect of nitroglycerin
McDonalds Blood Flow In Arteries 5th Ed Hodder
Arnold, London
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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?

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Kanazara et al, Anesthesiology 20039948-53

• 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

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Kanazara et al, Anesthesiology 20039948-53
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Kanazara et al, Anesthesiology 20039948-53
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Arnal et al, Anaesthesia 200560766-771
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Arnal et al, Anaesthesia 200560766-771
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Dorman et al, CCM 1998 26 1646-1649

• 14 patients with septic shock, requiring
  norepinephrine gt5 mcg/min
• Simultaneous radial and femoral artery pressure
  measurements
• Two patients also studied after resolution of
  shock

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Dorman et al, CCM 1998 26 1646-1649
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Dorman et al, CCM 1998 26 1646-1649
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Mignini et al, Crit Care 2006
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Mignini et al, Crit Care 2006
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Mignini et al, Crit Care 2006
Authors conclude that two approaches are
interchangeable
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Brachial vs Femoral
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Femoral Brachial One Beat
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Radial vs Femoral
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Radial vs Femoral (One Wave)
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In Extremis- epinephrine bolus
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Post Epinephrine
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Pre Dobutamine and Bicarbonate
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Post Bicarbonate (Dobutamine still running)
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Pre Second Epinephrine
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2nd Epinephrine Bolus
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Post 2nd Adrenaline Injection
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Post 2nd Epinephrine Bolus
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Low output state and vasopressors effect on PPV
etc
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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?

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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