OACCM Advisor's Blog #4 - January 2012 - William T. McGee, M.D., M.H.A.

An Algorithmic Approach to Functional Hemodynamics

The clinical question that must always initiate entry into this algorithm asks specifically about the adequacy of blood flow (CO).  The fundamental question being; are the organs adequately perfused or not?  Assessment always includes incorporation of SV/CO along with DO2; but we will focus on the flow parameter SV/CO as the discussion of adequate O2 content, Hb and SaO2 generally involves beliefs and possibly cults and only sometimes science or physiology and is better left to a different forum or requires more space! 

Hypotension and oliguria are two of the most common examples involving questions about the adequacy of blood flow encountered in the critically ill.  The outcome of the clinical question is either resolution of the problem, improvement in flow (SV/CO), or potentially that the therapy necessary to achieve the improvement in flow either does not work or involves greater risk than benefit.  Tachydysrthythmias with catecholamines or incipient volume overload and worsening of the A-a gradient with fluid loading are examples of greater risk than benefit.

Our algorithm is proposed solely as a supplement to clinical acumen and skill as we recognize the inherent variability of the clinical dilemmas that we all face daily.  Although strongly rooted in accepted physiologic principles this management algorithm cannot and should not be substituted for clinical skill.

 

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a Pathway A provides an approach using functional hemodynamics toward optimizing stroke volume (SV) using stroke volume variation (SVV) as a measure of volume responsiveness.  The adequacy of SV is determined by clinical findings.  Assessment of O2 extraction may clarify SV adequacy.  If SV is adequate the other components of oxygen delivery (DO2) should be assessed and O2 flow (SV x arterial O2 content (CaO2) ) re-evaluated prn. For patients with adequate SV and DO2 who remain persistently hypotensive or with acute lung injury/adult respiratory distress syndrome (ALI/ARDS).  Pathway B provides a physiologic and evidence-based approach to management and is likely to improve outcome. 1,2  Further intravenous (IV) fluid therapy may be stopped when SV is > normal.

b Any no answer; proceed to C Pathway.  For patients with tidal volume (Vt) <8cc/kg. Vt can be increased temporarily to assess volume responsiveness; if SVV increases (with increase in Vt) these patients are typically volume responsive, SVV may be low in volume responsive patients when Vt is <8cc/kg as the pleural pressure change may not be great enough to meaningfully impact SV.  Spontaneously breathing patients which also include many patients or pressure support ventilation (PSV) should undergo the passive leg raising (PLR).

c 10-15% represents a range of SVV that allows discrimination of clinical volume responsiveness in multiple studies. The higher the variation the greater the  expected response to a volume challenge.  A meaningful improvement in SV is usually defined as greater than 10%.  Since linearity between the response in SV and SVV has been shown experimentally in several studies, this is chosen for the lower cut off value.  Higher levels of SVV  improve specificity of this measure as lower SVV cut offs diminish sensitivity. Simply the higher the variability the greater the response to volume and a clinically significant response is generally defined as a >10-15% change in SV.

d ScVO2 allows determination of perfusion adequacy on an individual basis
delivery is generally adequate if extraction is not elevated.  Echo is useful especially if inotropes, mechanical cardiac support,  or vasodilators are indicated, or outcome will not be influenced by DO2 manipulation.
(Copyright 2012 by William McGee, MD. Used with permission)

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mcgee

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mcgee

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1.Rivers E et al:  Early goal-directed therapy in the treatment of severe sepsis and septic shock.  N Engl J Med 2001;345:1368.
2.The NHLBI ARDS Clinical Trials Network:  Comparison of two fluid-management strategies in acute lung injury.  N Engl J Med 2006;354:2564-2575.
3.Monnet, Pinsky et al:  Passive leg raising predicts fluid responsiveness in the critically ill.  Crit Care Med 2006;34(5):1402-7.
4.Vincent JL: Let’s give some fluid and see what happens” versus the “mini-fluid challenge” Anesthesiology 2011;115(3):541-7.

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September , 2011 - Blog # 3 - Dr. McGee on Hemodynamics

June , 2011 - Blog # 2 - Dr. McGee on Hemodynamics

May , 2011 - Blog # 1 - Dr. McGee on Hemodynamics

William T. McGee, M.D., MHA, FCCP is Associate Professor of Medicine and Surgery at Tufts University School of Medicine, Boston, Massachusetts.  His interests are in ARDS, vascular access (pulmonary artery catheterization), sepsis, nutrition, and nosocomial pneumonia.  He has published > 67 papers, chapters and abstracts.  He is the principal investigator for clinical trials studying the efficacy and safety of rfPAF-AH for the prevention of ARDS in patients with severe sepsis.  He is a three-time recipient of Excellence in Teaching Award from Tufts University School of Medicine, the Society of Critical Care Medicine Internal Medicine Specialty Award for “Influence of Insurance Status on Pulmonary Artery Catheter Use” and The Presidential Citation Award from the Society of Critical Care Medicine for outstanding contribution to the Society.  He is a Director for the Fundamentals of Critical Care Support Course for the Society of Critical Care Medicine.  Dr. McGee is also a reviewer for several critical care journals. 

McGee