Fluid Balance Flashcards
What body compartments do different infusions of fluid move into?
5% dextrose (isotonic)
- 1/3 in ECF (1/4 in plasma, 3/4 in interstitial fluid)
- 2/3 in intracellular compartment
0.9% NaCl saline
- plasma
- interstitial fluid
(not intracellular compartment - capillary walls impermeable to NaCl)
Colloid (proteins, starch)
- plasma
(not interstitial fluid - cell membrane usually impermeable to large molecules)
Compare the concentration of electrolytes in the ECF and ICF.
ECF
- high Na+
- low K+
- high Cl-
ICF
- low Na+
- high K+
- low Cl-
Outline the thirst response.
Increased plasma osmolality:
- –> reduced saliva —> dry mouth —> hypothalamic thirst centre
- –> osmoreceptors in hypothalamus —> hypothalamic thirst centre
Reduced plasma volume —> reduced BP —> stimulates juxtaglomerular cells —> activates RAAS —> increases angiotensin II —> hypothalamic thirst centre
Hypothalamic thirst centre —> stimulates sensation of thirst
Drink taken:
- –> moistens mouth & throat —> reduces sensation of thirst
- –> stretches stomach & intestine —> reduces sensation of thirst
- –> water absorbed from GI tract —> reduces plasma osmolality —–> reduces sensation of thirst
note: sensation of thirst diminishes with age (confusion due to dehydration occurs quickly)
Outline the actions of ADH.
Increased plasma osmolality OR increased [Na+]plasma
—> stimulates osmoreceptors in hypothalamus —> pos. pituitary gland
Reduction in plasma volume OR reduction in BP (10-15%)
—> reduced stimulation of baroreceptors in atrium & large vessels —-> pos. pituitary gland
+ sensation of water in stomach stimulates increase in ADH secretion
Pos. pituitary gland —> release of ADH —> increases water reabsorption in collecting ducts & kidneys
- –> concentrated urine
- –> reduced osmolality & increased plasma volume —> inhibits osmoreceptors in hypothalamus
Outline the effect of aldosterone on the kidneys.
Reduction in [Na+] OR increase in [K+] in plasma OR RAAS activation —> stimulate adrenal cortex to release aldosterone
- –> increase [Na+] reabsorption
- –> increase [K+] secretion
Contrast some of the causes of hypotension.
Hypovolaemic shock
Haemorrhagic shock
Cardiogenic shock (IV fluid will make things worse - increases preload)
Septic shock (IV fluid will not work - fluid will leak into interstitial spaces, causing peripheral oedema and reducing BP)
What needs to be considered when prescribing resuscitation fluid?
- Do they need IV fluids?
- oral intake?
- nil by mouth? - Composition of fluid prescribed?
- Goals of therapy?
- Possible complications?
- review other drugs e.g. BP drugs, lithium - Causes of ongoing losses?
- e.g. bleeding, vomiting
- able to stop?
What needs to be considered when prescribing replacement fluid?
- Consider input & output of fluids
- note: cannulation difficulties may prevent IV fluids being delivered (central line may be required) - Review electrolytes & serum bicarbonate
- Consider insensible losses
- pure water loss (fever, dehydration, hyperventilation)
- vomiting & nasogastric tube loss
- biliary drainage loss
- pancreatic drain/fistula
- jejunal loss via stoma/fistula
- diarrhoea or excess colostomy loss
- ileal loss
- ongoing blood loss e.g. melaena
- inappropriate urinary loss e.g. polyuria - What are you replacing?
- ongoing losses hypotonic or hypertonic?
- note: ensure to correct hypokalaemia (activates RAAS, impairs ADH response, and effects muscle contractility and cardiac function)
What needs to be considered when prescribing maintenance fluid?
Meet nutritional requirements & caloric expenditure
- Consider oral intake
- need to be nil by mouth? - Review other drugs e.g. diuretics
- Consider causes of ongoing losses
- can you stop them?
Why do hospitalised patients (esp. the elderly) often require maintenance fluid?
- ADH affected by drugs (e.g. morphine), pain, nausea, and hypothyroidism
- do not sweat excessively
- RAAS activated by stress (peri-operative)
- catecholamine release
- reduced caloric expenditure
- reduced free water excretion —> hyponatraemia
- increased water & salt retention —> fluid overload
Estimate the volumes of fluid in the different compartments of the body.
TOTAL BODY WATER = 40l (60% of body weight)
- INTRACELLULAR FLUID = 25l (40% of body weight)
- EXTRACELLULAR FLUID = 15l (20% of body weight)
—> INTERSTITIAL FLUID = 12l (80% of ECF)
—> PLASMA = 3l (20% of ECF) (+ cells = 5l of blood)
What fluid compartment is tested in dehydration?
Intravascular (all compartments affected but the intravascular compartment is easiest to test)
What are the indications for prescribing resuscitation fluids? When are they not indicated?
Indications:
- Hypovolaemic shock
- Haemorrhagic shock
Contraindicated:
- Septic shock (fluid will leak out of capillaries and cause peripheral oedema)
- Cardiogenic shock (increased fluid will make things worse)
What are the consequences of hypokalaemia on fluid balance? What complications can arise from hypokalaemia?
Activates RAAS —> increases Na+ and water retention
Impairs ADH response
- –> cannot produce concentrated urine
- –> cannot prevent interstitial oedema
S&S:
- weakness/fatigue
- increased muscle contractility —> muscle cramps/pain
- worsening diabetes/polyuria
- palpitations/arrhythmias
- psychosis, delirium, hallucinations, depression
How does the total body water vary in babies and the elderly compared to adults?
Adults = total body water is 60% of body weight
Babies = total body water is 75% of body weight
Elderly = total body water is 56% of body weight
Contrast the mechanism of the formation of oedema in different disorders.
Decreased oncotic pressure of plasma
e.g. cirrhosis, Kwashiorkor, nephrotic syndrome
Increased hydrostatic pressure of capillary
e.g. heart failure, venous thrombosis (before occlusion)
Lymph node drainage compromised
e.g. cancer, radiation poisoning
Increased permeability of capillaries
e.g. burns, inflammation, anaphylaxis
What are the consequences of hyperkalaemia?
- muscle paralysis
- dyspnoea
- palpitations
- chest pain
- N&V
- paraesthesia
What physiological mechanisms and hormones stimulate the sensation of thirst?
ADH
Angiotensin II via stimulation of:
- subfornical organ (ventral surface of fornix)
- osmoreceptors of OVLT
Osmoreceptors of organum vasculosum of lamina terminalis (OVLT - located in the median preoptic nucleus of the hypothalamus)
Arterial baroreceptors and cardiopulmonary receptors stimulate the area postrema and nucleus tractus solitarius
Outline the components and therapeutic actions of 5% dextrose solution.
Isotonic saline
50g/l of glucose
- glucose quickly taken up by cells
- water reduces osmolality of all compartments (treats dehydration, not hypotension)
- can enter the plasma, interstitial fluid, and intracellular compartments
- only ~1/12 stays in intravascular space (hence not good for replacing blood loss)
Indications:
- hypoglycaemia
- hypernatraemia
- dehydration
Cautions:
- if infusion rate exceeds the rate of glucose uptake and metabolism —> hyperglycaemia
- hypokalaemia may result
- peripheral/pulmonary oedema may result
Outline the components and therapeutic actions of 0.9% NaCL saline.
1l 0.9% saline = 155mmol Na+ & 155mmol Cl- (isotonic)
- Na+ remains in ECF —> fluid remains in ECF
- no change in osmolarity
- can enter the plasma and interstitial fluid, but not the intracellular compartments (unless via transporters)
- only ~1/4 stays in intravascular space
Indications:
- hypovolaemic shock
- metabolic acidosis
- hypercalcaemia
Cautions:
- may cause fluid overload in cardiac/renal disease
Outline the components and therapeutic actions of crystalloid.
Crystalloid = aqueous solution of mineral salts
e.g. Hartmann’s solution (Na+, lactate, etc.) (known as Ringer’s lactate in U.S.)
Indications:
- replace fluid lost via burns/trauma/fistulae/GI losses
Cautions:
- risk of interstitial oedema
- may cause lactic acidosis
- may cause liver disease (via increased lactate)
Outline the components and therapeutic actions of colloid.
Colloid = larger insoluble molecules
e.g. blood
- high in NaCl —> high oncotic pressure —> rapid intravascular expansion —> risk of volume overload
- can only enter the plasma (proteins too large to leave vasculature - usually
Not recommended at present due to:
- risk of fluid overload
- anaphylactic reaction to proteins in colloid
Outline the components and therapeutic actions of 4% dextrose/0.18% saline.
Glucose = 40g/l
30mmol/l of NaCl
Indications:
- maintenance fluid
Cautions:
- risk of hyponatraemia if too much given too quickly
- use with caution in children
What are the signs and symptoms of hypovolaemia?
Dehydration
- –> dry mucous membranes
- –> reduced urine output
Hypotension
- –> light-headedness/feeling faint
- –> syncope
- –> flattened neck veins
- –> tachycardia
- –> increased resp. rate
- –> peripheral cyanosis
- –> confusion
What is the normal osmolarity of the blood? How does this change immediately after haemorrhage?
~ 280mOsm
Osmolarity will stay the same, as isotonic fluid (blood) has been lost
Compensatory mechanisms will take place eventually which will alter the osmolarity
What is haematocrit? What might cause a reduction in haematocrit? How does it differ from the erythrocyte sedimentation rate?
Packed cell volume (% volume of blood composed of RBCs)
(NOT CHANGED WHEN RBC SIZE IS ALTERED!)
Therefore haematocrit can fall if:
- there is a loss of RBCs
- there is an increase in water in the intravascular compartment
ESR = “stickiness” of RBCs
What is the osmolarity of sweat compared to plasma? How does the production and composition of sweat vary according to the external temperature?
Isotonic when initially secreted, then modified by removal of electrolytes (becomes hypotonic compared to plasma)
Hot day: increased production but less modified —> less hypotonic
Cool day: reduced production but more modified —> more hypotonic
Why are athletes at risk of dilutional hyponatraemia? How can this be avoided?
Drinking water or isotonic sports drinks (containing glucose) whilst exercising
Replaces water lost in sweat, but not electrolytes
Increased water:Na+ —> diltutional hyponatraemia
(+ glucose —> Na+ follows glucose into cells)
This can be avoiding by using rehydration fluid or salt supplements (replace electrolytes and water)
Why is urine output a more sensitive indicator of fluid balance than thirst?
Thirst is a late response to hypovolaemia, urine concentration changes earlier
Urine output is a direct measure of fluid balance
Why are children more prone to dehydration than adults?
A higher % of total body weight is water in children
Increased s.a —> more water lost by sweating
Immature renal system (connection between kidney and brain not formed)
Reduced awareness of water intake?
Reduced control over water intake
What is the initial daily fluid and electrolyte requirement of a patient who is nil by mouth and euvolaemic?
1500ml fluid (25-30ml/kg/day) + 750ml from food
[Na+] = 1mmol/kg/day
[K+] = 1mmol/kg/day
What hormonal changes occur in hospitalised patients that influence IV fluid prescribing?
Increased cortisol and adrenaline secretion (stress response)
Increased ADH (stress response)
How would the composition of IV fluid prescribed for vomiting differ from that prescribed following an ileostomy?
Vomiting: NaCl solution prescribed
Ileostomy: HCO3- and Cl-
How do signs and symptoms of dehydration manifest in the elderly? How are the control mechanisms affected?
S&S:
- constipation
- postural hypotension
- confusion (delirium)
Reduced sensation of thirst (due to degeneration of osmoreceptors in hypothalamus)
Confusion (delirium) occurs rapidly with dehydration
Lower % of total body weight is water
Appearance of skin is less useful (reduced collagen means skin turgor is difficult to assess)
