Michard 28/7/08 2:09 pm Page 99
Haemodynamic Management
Volume Management in Critically Ill Patients
an interview with
Frédéric Michard
Department of Anaesthesia and Critical Care, Beclere Hospital, University Paris XI
Volume expansion is one of the most common therapeutic procedures increase in SV and cardiac output according to the Frank-Starling
in intensive care units (ICUs). There is no doubt that in some cases (e.g. mechanism) occurs first. If not, fluid administration is useless or even
haemorrhage or severe diarrhoea) care-givers can reasonably rely on potentially harmful, e.g. leading to a worsening in pulmonary oedema.
clinical examination to identify patients who will benefit from fluid
loading. However, in more complex – but not uncommon – situations How useful are standard pre-load indices such as central
(e.g. septic shock), both clinical examination and indicators of cardiac venous pressure and pulmonary capillary wedge pressure
pre-load have been shown to be of minimal value in answering the in predicting cardiac response to fluid therapy?
question: ‘Can we improve cardiac output and hence haemodynamics Many clinical studies have demonstrated that central venous pressure (CVP)
by administering fluid?’
1
and pulmonary capillary wedge pressure (PCWP) are not always useful for
predicting cardiac response to fluid therapy.
7,8
For example, some patients
Over the last decade, many clinical studies have demonstrated the can respond positively to fluid administration while the CVP and PCWP are
value of stroke volume variation (SVV) induced by mechanical elevated. Indeed, CVP and PCWP are often overestimated in patients
ventilation to predict fluid responsiveness, i.e. an increase in cardiac whose lungs are being mechanically ventilated with positive end-expiratory
output as a result of fluid infusion.
2–6
SVV is now automatically pressure (PEEP).
9
CVP and PCWP are also overestimated in patients with
calculated and displayed on minimally invasive cardiac output dynamic hyperinflation; that is, in patients with auto-PEEP, e.g. in patients
monitors. This should greatly facilitate the volume management of with chronic obstructive pulmonary disease (COPD). Overestimation of CVP
critically ill patients. and PCWP can also occur in patients with abdominal hypertension. In fact,
all of these situations are quite common in critically ill patients, and that is
In the following interview, Frédéric Michard, MD, PhD, discusses one of the reasons why cardiac filling pressures are not always accurate in
some of the important matters relating to the volume management predicting cardiac response to fluid therapy.
of critically ill patients using SVV and new cardiac output
monitoring technologies. We also must keep in mind that the physiological relationship between
ventricular end-diastolic pressure and volume is not linear but curvilinear,
Fluid therapy is often used to increase cardiac pre-load and highly dependent on cardiac compliance. This means that a given
and improve the haemodynamic status of patients with CVP or PCWP value can be associated with a different cardiac filling
circulatory shock. However, cardiac output increases volume in two patients in whom ventricular compliance is different.
after a fluid challenge in only approximately 50% of
such patients. What is the clinical significance of Assessment of stroke volume variation has been proposed
this observation? to guide fluid therapy in patients receiving mechanical
Only 50% of patients with shock experience a significant increase in ventilation. Could you comment on the rationale for using
cardiac output in response to fluid administration when the decision to stroke volume variation?
give fluid is based on the clinical examination or on the measurement By increasing pleural pressure, mechanical inspiration induces cyclic
of cardiac filling pressures.
1
This observation means that, until recently, variations in cardiac pre-load that may be turned into cyclic changes in left
clinicians were unable to accurately identify patients who could benefit ventricular SV.
6
SVV is neither an indicator of volume status nor a marker
from fluid expansion – in other words, fluid-responsive patients. of cardiac pre-load, but rather is an indicator of the position on the Frank-
Over the last few years, the concept of fluid responsiveness has become
popular in Europe and South America, likely because it is a pragmatic
Frédéric Michard works in the Department of Anaesthesia
approach to fluid therapy. Indeed, we have a clear idea of the normal total
and Critical Care, Beclere Hospital, University Paris XI. A
blood volume (800–1,000ml/m
2
), and of normal right and left ventricular former Chef de Clinique of Assistance Publique-Hopitaux
end-diastolic volumes (90–110ml/m
2
and 60–80ml/m
2
, respectively) in
de Paris, he has conducted several clinical studies in
critically ill and surgical patients and published numerous
healthy subjects. However, it is much more difficult to determine which
articles on haemodynamic monitoring and heart–lung
level of pre-load is optimal in an ‘abnormal’ situation, e.g. vasodilation
interactions. Dr Michard trained at the University of Paris V
and at Harvard Medical School, Boston, and is certified in
induced by sepsis. Therefore, a practical approach to determine fluid
respiratory medicine and critical care medicine. He earned
therapy consists of detecting patients who will be able to turn fluid loading the Habilitation to Lead Research (the highest diploma of
into a significant increase in SV and cardiac output. Of course, clinical end-
French universities) in 2005.
points of fluid therapy are usually different, e.g. increasing blood pressure E:
michard.frederic@free.fr
or urine output, but will be achieved only if the physiological effect (an
© TOUCH BRIEFINGS 2008 99
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