Session 2: Body Fluids and IV Fluid Prescribing Flashcards
Describe the main fluid compartments of the body
List the normal volumes of each fluid compartment in adults
Describe the electrolyte composition of each fluid compartment
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Distribution of principle ions between ECF and ICF:
- Interstitial fluid: High Na+, low K+ and high Cl-
- Intracellular fluid: Low Na+, high K+ and low Cl-
Explain the processes by which electrolytes and water move between body fluid compartments
Forces and processes governing movement of solutes and solvents between the compartments
- Osmotic and oncotic pressures
- Hydrostatic pressure
- Diffusion
- Facilitated diffusion
- Active transport
- Vesicular transport e.g. pinocytosis
Permeability also determines movement between the compartments
- Cell membrane is freely permeable to water and urea but limited permeability to Na+ and K+ (ion channels and transporters are required).
- Capillary wall is freely permeable to water, urea, Na+ and K+ (they can all diffuse between intercellular clefts) but not really permeable to plasma proteins (although in burns/sepsis, capillaries become more leaky therefore more permeable).
What are the mechanisms involved in the regulation of ECF volume and composition?
Thirst (behavioural response)
ADH
RAAS
NB: the decrease in plasma volume is a minor, weaker stimulus (needs large drop in volume).
As people age, osmoreceptors become less responsive so elderly people need careful monitoring. Dehydration can lead to confusion quickly.
What stimulates secretion of ADH?
increased plasma osmolarity or significant (10%) decrease in plasma stimulates secretion of ADH.
What stimulates RAAS?
stimulated by low blood pressure and/or volumestimulated by low blood pressure and/or volume
Consider a patient with low BP, what will the physiological responses be?
- Venoconstriction
- Cardiac response
- Capillary fluid shift
What are the symptoms and signs of hypovolaemia? Consider mild and severe
Signs and symptoms of mild hypovolaemia
Decreased urine output
Dry mucous membranes, such as the mouth and nose
Loss of skin elasticity
Thirst
Signs and symptoms of severe hypovolaemia (hypovolemic shock)
Anxiety
Blue lips and fingernails
Low or nor urine output
Profuse sweating
Shallow breathing
Dizziness
Confusion
Loss of consciousness
Low blood pressure
Rapid heart rate
Weak pulse
Sign of external bleeding
Signs of internal bleeding: abdominal pain, blood in the stool, blood in the urine, vaginal bleeding, vomiting blood, chest pain and abdominal swelling
Compare the effects of fluid loss via diarrhoea to sweating
Effect of diarrhoea
- Loss of isotonic fluid
- ECF volume decreases with no change in osmolarity
- No shift of water between ECF and ICF
- Plasma protein concentration and haematocrit increase because of the loss of ECF
- RBF will not shrink or swell
- Arterial blood pressure decreases because ECF volume decreases
Effect of sweating (in a hot desert)
- Osmolarity of ECF increases because sweat is hyperosmotic, relatively more water than salt is lost during sweating (less salt is reabsorbed compared to sweating on a cool day)
- ECF volume decreases because of the loss of sweat
- ICF osmolarity increases until it is equal to ECF osmolarity – thus ICF volume decreases
- Protein concentration increases because of the loss of ICF volume
- Haematocrit may increase but it is unchanged because water shifts from erythrocytes which decrease their volume.
Compare the effects of fluid loss from vomiting to haemorrhage
Effect of vomiting
- Reduces ECF volume
- Gastric vomiting leads to loss of acid (protons) and chloride directly leading to hypochloremic metabolic alkalosis as [HCO3-] increases.
- Hypokalaemia is an indirect result of the kidney compensating for the loss of acid.
Effects of haemorrhage
- The haematocrit (packed cell volume) determines the percentage of red blood cells in the plasma.
- A haematocrit done immediately after a haemorrhage usually does not show the extent of RBC loss because at the time of haemorrhage, plasma and red blood cells are lost in equal proportions.
- However, within several hours after haemorrhage, plasma volume increases due to a shift of interstitial fluid into the vascular space.
- Red blood cells, however, cannot be replaced quickly, as the bone marrow takes approximately ten days to produce mature red blood cells so the haematocrit taken after several hours of haemorrhage will be decreased. Plasma volume has been compensated but the red blood cells cannot be replaced for days.
Where will the fluid go if you give 5% dextrose, 0.9% NaCl saline or colloid solution?
Infusion of 1L water (as 5% dextrose, osmotically correct) will first go into the plasma => interstitial fluid => intracellular fluid so all 3 compartments as there is no NaCl, dextrose is quickly taken up by the cells leaving water which both the cell membrane and capillary wall are freely permeable to.
Infusion of 1L 0.9% NaCl saline will first go into the plasma => interstitial fluid. NaCl is not able to cross the capillary membrane into the ICF so saline remains in the ECF.
Infusion of 1L colloid solution will only go into the plasma as colloids are too big to diffuse through the intercellular clefts of the endothelial capillary wall to go into the interstitial fluid,
Describe the electrolyte and water composition of the commonly used intravenous fluids
The percentage of body weight accounted for by water in adults is 60% in males and 50% in females. Why do women have a lower total fluid body weight?
women have a higher body fat percentage,
Why are you prescribing IV fluids? What do you need to consider?
Prescribing IV fluids for
- Resuscitation
- Routine maintenance
- Replacement
Need to consider
- Does patient need IV fluids? (especially if they’re able to take oral fluids already, do they need to be nil by mouth?)
- Why? (have you reviewed diuretics, other drugs, considered causes of ongoing losses – can you stop them)?
- What should be the composition of the fluid you prescribe
- Consider the goals of therapy
- Consider potential complications
- Write an IV fluid management plan
What are the effects of giving 1L 0.9% saline? Where will the fluid go?
155mmol Na+, 155mmol Cl-
Osmolarity = 310mOsm/kg
Intracellular: no change in volume or osmolarity
ECF: Na+ remains in the ECF + no change in osmolarity
Interstitial = ¾ x 1000ml = 750ml
Intravascular (plasma) = ¼ x 1000ml = 250ml.
What is meant by Resuscitation? What do you need to consider?
Expanding intravascular space => restore organ perfusion
Options
- Crystalloid: 0.9% sodium chloride or Hartman’s BUT risk of interstitial oedema
- Blood
- Colloids: most high in sodium and chloride, some contain potassium, rapid intravascular expansion BUT risk of volume overload
Need to monitor volume status, electrolyte balance and nutrition. Monitoring urine output is better for monitoring fluid base balance compared to thirst (as urine output is more quantifiable).
Remember: Hypotension could be due to hypovolemic shock, haemorrhagic shock, septic shock or cardiogenic shock.
Describe Maintenance IV Fluids
Are not sufficient to meet nutritional requirements
Amounts derived from daily requirements of people with normal physiology
Based on caloric expenditure
Overall hypotonic
- 25-30 ml/kg/day water
- 1mmol/kg/day sodium, potassium and chloride
One size does not fit all!
What do you need to remember about hospitalised patients? Consider ADH and RAAS especially
- Vasopressin (ADH)
- Generally do not sweat excessively
- RAAS
- Catecholamines
- Reduced caloric expenditure
- Reduced free water excretion – hyponatremia
- Increased water and salt retention – volume overload
ADH: secretion is stimulated by
- Osmotic stimuli: sodium ion effect osmoreceptor stimulus
Non osmotic stimuli:
- Drugs (morphine)
- Pain
- Nausea
- Low effective circulating volume
- Hypothyroidism
RAAS:
- Activated when effective circulating volume low
- Stress response perioperatively
There are many different types of fluid loss: consider Vomiting & NG tube loss, biliary drainage loss and diarrhoea/excess colostomy loss
Vomiting and nasogastric tube loss: gastric fluid contains
- 20-60 mmol Na+/L
- 14 mmol K+/L
- 140 mmol Cl-/L
- 60 – 80 mmol H+/L
- Excessive loss causes a hypochloraemic (hypokalaemic) metabolic alkalosis.
- Correction requires supplemental K+ and Cl-
Biliary drainage loss
- 145 mmol Na+/-
- 5 mmol K+/L
- 105 mmol Cl-/L
- 30 mmol HCO3-/L
Diarrhoea or excess colostomy loss
- 30-140 mmol Na+/L
- 30-70 mmol K+/L
- 20-80 mmol HCO3-/L
Consider ileal loss and pancreatic drainage loss
High volume ileal loss via new stoma, high stoma or fistula
- 100-140 mmol Na+/L
- 4-5 mmol K+/L
- 75-125 mmol Cl-/L
- 0-30 mmol HCO3-/L
Lower volume ileal loss via established stoma or low fistula
- 50-100 mmol Na+/L
- 4-5 mmol K+/L
- 25-75 mmol Cl-/L
- 0-30 mmol HCO3-/L
Pancreatic drain or fistula
- 125-138 mmol Na+/L
- 8 mmol K+/L
- 56 mmol Cl-/L
- 85 mmol HCO3-/L
What other types of fluid loss are there?
Jejunal loss via stoma or fistula
- 140 mmol Na+/L
- 5 mmol K+/L
- 135 mmol Cl-/L
- 8 mmol HCO3-/L
Inappropriate urinary loss (e.g. polyuria)
- Na+/L and K+/L very variable so monitor serum electrolytes closely.
- Match hourly urine output (minus 50ml) to avoid intravascular depletion.
Ongoing blood loss e.g. melaena
What do you need to consider when designing an individual prescription?
- Does the patient need fluids
- Does the patient still need fluid
- Assess volume status
- Define primary outcome
- Resuscitation, maintenance, replacement
- Correcting electrolyte disturbance
- Fluid management plan
