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.
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.
• 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
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.
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 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.
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.
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 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 loss may be controlled or uncontrolled, internal or external. Severe dehydration may cause similar problems.
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).
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.
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.