Healthcare and Medicine Reference
In-Depth Information
Clinical examples of data interpretation
Box 19.4 Interpretation of mixed venous oxygen saturation
Example 1: The septic, hypotensive patient
A 25-year-old man was admitted with an area of spreading cellulitis
to his leg which developed into necrotising fasciitis.
On assessment by the intensive care team his observations
were: heart rate 140 bpm, blood pressure 75/45 mmHg and CVP 2
cmH 2 O. On examination he appeared fl ushed and was noted to have
a bounding pulse. A diagnosis of sepsis causing hypotension and
vasodilatation was made. The hypotension was due to a decrease in
systemic vascular resistance (SVR) due to septic mediators.
He was given several fl uid boluses (250-500 mL of crystalloid)
which provided only a transient increase in CVP and BP. His treat-
ment would require further fl uid boluses and a vasoconstrictor
infusion via the central line to increase his SVR and perfusion pres-
sure to his organs.
Decreased mixed venous oxygen saturation (<65%)
Low cardiac output states (e.g. hypovoalemia, myocardial
infarction, heart failure)
Hypoxia, respiratory distress syndromes
Increased oxygen consumption (e.g. fever, exercise, thyrotoxicosis)
Low Hb (e.g. bleeding, haemolysis)
Increased mixed venous oxygen saturation (>80%)
High cardiac output (e.g. sepsis, burns, inotrope excess, hepatitis,
pancreatitis and left-to-right shunts)
Low oxygen consumption (e.g. cyanide toxicity, carbon monoxide
poisoning, sepsis and hypothermia)
Box 19.5 Hagen-Poiseuille equation
The rate of fl ow of fl uid through a catheter is described by the
Hagen-Poiseuille equation. This states that the fl ow is directly
proportional to the radius of the catheter to its fourth power
and the pressure gradient along the infusion tubing. Flow is also
inversely proportional to the length of the catheter and the viscosity
of the fl uid being infused.
Example 2: The trauma patient
A 40-year-old woman was admitted following a road traffi c
accident. Her injuries included a fractured femur and bilateral
chest trauma. The results of the primary survey were that her
airway was safe, she had a right-sided pneumothorax which was
decompressed using an intercostal drain, and she was hypotensive
(BP 86/55 mmHg) and tachycardic (HR 130).
After several fl uid boluses she remained hypotensive but stable. It
was decided to insert a central venous catheter to aid further fl uid
resuscitation. The initial CVP reading was 20 mmHg and it was
then noted that her neck veins were distended whilst at the same
time she started to display increasing respiratory distress.
Further examination revealed absent breath sounds on the left
and a hyperresonant chest. A left tension pneumothorax was suc-
cessfully decompressed which immediately resulted in a reduction
of CVP and an increase in blood pressure.
4
p
h
Pr
l
Flow
=
8
P = pressure difference along the catheter
r = radius of catheter
l = length of catheter
η
= viscosity of liquid
= constant
π
oxygen saturation of mixed venous blood will fall from the normal
range of 68-77%.
Blood from a central venous catheter taken with an arterial blood
gas syringe can be used to estimate mixed venous saturations. (The
saturation measured in this way is slightly higher than true mixed
venous blood as it does not include the deoxygenated blood from
the cardiac and pulmonary circulations; Box 19.4.)
Further reading
Pinsky MR, Payen D. (2005) Update in Intensive Care and Emergency
Medicine 42 : Functional Haemodynamic Monitoring . Springer Verlag,
Berlin, Heidleberg.
Rivers E, Nguyen B, Havstad S, Ressler J, Mussin A, Knoblich B. (2001) Early
goal-directed therapy in the treatment of sepsis and septic shock. N Engl J
Med 345: 1371-7.
Fluid resuscitation
Unless peripheral venous access is problematic, central venous cath-
eters should not routinely be used for fl uid resuscitation especially
where the patient is shocked due to haemorrhage. The fl ow of fl uid
through a central line is too slow to allow rapid administration of
fl uids or blood products (Box 19.5).
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