Shock And Intravenous Fluids
Intravenous
fluid therapy is a common medical treatment, but recently there has been a
reassessment of the role of intravenous fluids as some of the hazards have
become better understood.
Intravenous access
Poiseulle’s Law governing fluid flow through a tube
(assuming laminar flow):
Flow = Pr4/ηL
where P pressure difference, r = radius, h =
viscosity, L = length.
Therefore the ideal resuscitation fluid should be
non-viscous, driven by pressure through a short, wide cannula. For resuscitation,
or when giving blood (viscous), a 16 G or larger cannula is preferred. The
large veins in the antecubital fossa and the femoral vein are good for
resuscitation but are prone to infection and uncomfortable for patients in the
long term. A pressure bag inflated to 300 mmHg doubles fluid flow.
Before cannulation the skin should be thoroughly cleaned
using chlorhexidine in alcohol. A cannula in the forearm is (relatively)
comfortable for the patient and less likely to become infected compared to
other sites.
Special cases
•
Central lines are very useful in very sick patients, patients with poor access or
patients whose fluid balance is particularly difficult to regulate. Their
length means that they are not ideal for delivering resuscitation fluids.
Introducer sheaths offer a large bore central access.
•
Intraosseus needles have to be drilled into adult bone to give fluid, but can be
life-saving. Bone marrow aspirate may be used for blood cross-matching.
•
Venous cut-down involves cutting the skin to be able to cannulate a vein under
direct vision. The long saphenous vein 1 cm above and anterior to the medial
malleolus, or the basilic vein in the antecubital fossa, are the most common
sites for this.
Types of intravenous fluids
Crystalloids
Normal saline, Hartmann’s solution and Ringer’s lactate
are solutions that match plasma osmolality. All can be used to resuscitate
patients, and despite vigorous debate, no one variety has proven superior
clinical outcomes.
Dextrose
50% dextrose is used to resuscitate hypoglycaemic
patients. 10% dextrose is used to maintain a patient’s blood sugar and prevent
hypoglycaemia, and 5% is used to give ‘free water’ to avoid over- loading with
sodium or chloride.
Colloids
Colloids contain large molecules that help retain fluid
within the intravascular space, which improves blood pressure in the short
term. Unfortunately these molecules leak out of damaged capillaries, which may
cause resistant oedema in the brain and lungs, which increases mortality in
head-injured patients. Colloids may be helpful in sepsis, but should only be
used by senior doctors.
Blood
A full cross-match takes 30 minutes but type-specific
blood should be available within minutes. If blood is needed before the blood
type is known, Group O Rhesus negative blood is used.
Whole blood as donated is the best substitute in trauma,
but has a short shelf life (days). Separating blood cells into ‘packed cells’
extends storage time to 3 months, but deterioration may mean that the cells are
not fully functional for 24 hours. The citrate used to stabilise blood binds
calcium ions, which can cause problems in massive transfusions ( 50% blood
volume).
Fresh frozen plasma (FFP) or synthetic clotting factors
can be used to correct clotting problems. Tranexamic acid, platelets and FFP
are given as part of massive transfusion protocol.
Temperature
Evolution has given humans enzymes that function best at
37°C and pH = 7.4. Blood clotting is impaired in a cold acidotic patient, e.g. trauma
patient. Temperature < 34°C and pH < 7.20 reduce clotting to 1% of
normal. Laboratory measurements at 37°C will not accurately reflect the
clinical picture. For this reason, clotting factors are given early in trauma
resuscitation. Cold fluids (4°C) may be given after cardiac resuscitation as
part of an active cooling strategy to preserve brain function.
Shock
Shock is defined as inadequate tissue perfusion, i.e.
not meeting the metabolic demands of tissue. Pulse and blood pressure are
bedside measures of tissue perfusion, but are insensitive. pH, PCO2,
lactate and mixed venous blood oxygen levels, measured from a central venous
pressure (CVP) line, are better indicators.
Types of shock
The body pumps a limited amount of fluid around a series
of closed loops. Problems occur when the fluid disappears, the pump fails or
the fluid goes to the wrong loops.
Blood failure
Blood loss may be controlled or uncontrolled, internal
or external. Severe dehydration may cause similar problems.
Pump failure
The heart may fail due to internal pump problems, e.g.
myocardial infarction or heart failure, which impair the ability to pump.
Alternatively the pump may fail because there is inflow obstruction (cardiac
tamponade, tension pneumothorax) or outflow obstruction (pulmonary embolus,
aortic dissection).
Distribution failure
Blood may be distributed to the wrong organs.
Inappropriate vasodilation occurs in septic shock, anaphylaxis and spinal shock
(due to loss of sympathetic tone below the injury) diverting blood away from
vital organs.
Grades of shock
Compensated shock BP ® HR
Young adults are able to compensate for loss of blood
volume by vasoconstriction and increased cardiac output, maintaining a good BP
and perfusion of vital organs.
Decompensated shock BP ¯ HR
The body’s compensation mechanisms are overwhelmed, and
the blood pressure falls rapidly.
Fluid resuscitation
Traditional teaching: ‘Fill ‘em up’
•
Good blood pressure = good outcome.
•
Poor blood pressure = poor outcome.
• Therefore give fluid/blood to
achieve good blood pressure. Unfortunately this is an oversimplification. Short-term
poor perfusion is well tolerated and if blood loss has not been
controlled:
•
blood pressure = blood
loss.
Increased blood loss is due to loss of vasospasm,
dilution of clot- ting factors and dislodgement of clot.
Current teaching: ‘minimal volume fluid
resuscitation’
If there is uncontrolled bleeding (e.g.
penetrating trauma, ruptured AAA), large-bore intravenous access is obtained.
The minimum volume of fluid necessary to maintain cerebral perfusion or a
systolic BP of 60–80 mmHg is used (‘permissive hypotension’). The priority is
urgent control of bleeding in the operating theatre.
Exception: if there is brain injury, the need to
maintain cerebral perfusion pressure overrides hypotensive resuscitation.
