Fluid management and blood transfusion Flashcards
(116 cards)
Describe water distribution in the body by weight
Water is about 60% of weight so for a 70 kg male
Intracellular 65% 28L
Extracellular 35% 14L
— Interstitial 27.5% 11L
— Plasma 7.5% 3L
TBW 100% 42L
In general how is total body water regulated
ECF as ICF follows changes in the ECF
There are no “water pumps” in the body
ECF is controlled by the movement of Na and thus water as Na is responsible for 90% of the body’s osmotic activity
Na intake (diet/IV)
Na EXTRA-Renal loss (Sweating/faeces)
Na renal excretion
What is transcellular fluid
CSF | Synovial fluid | Ocular fluid
Glandular secretions (lungs/GIT)
Bile
Total transcellular fluid is ± 2 % TBW (± 800 ml)
What is the equation for plasma osmolarity
Plasma osmolarity = 2[Na] + 2[K] + glucose + urea
Sodium and potassium are doubled to account for their conjugate anions - glucose and urea don’t have conjugate anions
What is the difference between molarity and osmolarity
Osmolarity refers to the number of osmotically active particles per litre of solution
Molarity refers to the the number of moles of solute per litre of solution i.e. the concentration
NaCl – contributes 2 osmoles
But the molarity can only represent either Na+ or Cl-
How is osmolality different from osmolarity
Osmolality is the number of dissolved osmotically active particles per unit MASS of solution
Osmolarity is the number of osmotically active particles dissolved per unit VOLUME of solution
The problem with osmolarity (vs osmolality) is that the volume of the solvent (water) changes with the addition of solute and with temperature
Osmolality is independent of changes in mass of solute and temperature.
Why do we use 5% dextrose for infusions instead of just infusing water
Water infusion would lead to acute drop in osmolarity of the fluid surrounding the red blood cells. The circulating RBCs would find themselves surrounded by a hypotonic solution, causing them to swell. RBC can only hold a certain amount of water before haemolysis occurs. For this reason 5% dextrose infusion are used
Osmolarity of 5% Dextrose is 278 mOsm/L and plasma osmolarity is normally 285 - 295 mOsm/L
Once the glucose is metabolized, it is as if free water has been infused but without the acute drop in osmolarity.
How does ADH work in the kidneys to increase H2O reabsorption
- Luminal wall CD is impermeable to water
- Basolateral wall CD has aquaporins 3 and 4 and is permeable to water
- ADH binds to V2 receptors in the CDs –> cAMP 2nd messenger –> aquaporin 2 being inserted into the luminal walls of the CD
- Water moves down Conc gradient created by the high osmolarity in the renal medulla.
- High osmolarity in the renal medulla is generated by active reabsorption of Na in the aLOH and DCT, counter current mechanism and urea cycling.
How is ECF volume controlled
- Brain
- Hyperosmolarity (also stress and pain)
- Hypothalamus: osmoreceptors
- Posterior pituitary: ADH release
- Thirst stimulated - Heart
- Baroreceptors in atria (low P) tonic firing with normal P
- Low P –> low firing –> decreased inhibition ADH release–> increase ADH
- RA stretch –> release ANP –> Natriuresis
- Reduced RAP –> less ANP –> Na retention - Kidneys
- Decreased arterial filling pressure sensed by juxtaglomerular apparatus
- Macula densa cells release renin
- RAAS activated
- Angiotensin 2 restores filling P in renal arterioles
- Aldosterone increases Na retention
How does increased SNS tone affect renal perfusion
- Global RBF decreased
2. Redistribution of RBF to inner juxtamedullary nephrons: improved Na and water retention
Classify the “stress response”
Neurohumoral
Neural –> SNS activation
Humoral –> Glucocorticoids / Thyroid H / GH / CAT = anti-insulin hormones
Describe the neurohumoral response
Neuro
Stress –> CNS (brainstem) –> SNS activation with PSNS inhibition
Adrenalin: Predominant beta agonist effects
- -> Increased CO
- -> Vasodilation in Coronary / Skeletal muscle BV
Noradrenalin: Predominant alpha agonist effects
–> VC - skin / kidney / liver / GIT
Although blood is diverted away from the kidney –> blood distribution in the kidney is shifted toward inner juxtamedullary nephrons which have long LOH and are more suited to Sodium and water retention resulting in accentuated sodium and water retention.
What features of the part of the nephron in the renal medulla assist with the generation of the medullary osmolar gradient?
- Differential permeability of descending and ascending limbs of LOH to water (descending) and ions (ascending)
- Countercurrent exchange mechanism
- Hair pin design of the vasa recta
- Urea reabsorption from inner medullary collecting duct.
What is diabetes insipidus vs SIADH
Central Diabetes insipidus (head injury)
- Posterior pituitary fails to secrete ADH
- Polyuria, hypvolaemia, hypernatraemia, dilute urine
Nephrogenic diabetes insipidus (Lithium toxicity)
- Insensitivity to normal levels of ADH
SIADH (Intracranial pathology vs ectopic source Small cell lung Ca)
- Hyponatraemia (headache/nausea/confusion/seizures/coma)
- Fluid overload (sometimes)
- Inappropriately high urine osmolarity
Describe the humoral aspect of the neurohumoral response
Counter-regulatory hormones: Catecholamines Glucocorticoids Growth Hormone Thyroid Hormone
Anti-insulin hormones that reduce insulin release and tissue sensitivity to preserve and increase blood glucose for use by fight or flight organs (heart/skeletal muscle/lungs and brain)
Glycogen break down initially increased
Then Gluconeogenesis accelerated
Other organs (over days become fat adapted)
Summarise the overall fluid and electrolyte outcome consequent to the stress response
- Maximal sodium and water retention RAAS/ADH
- Hypernatraemia (dilutional) - ADH
- Hypokalaemia - RAAS
- Metabolic alkalosis
- Decreased RBF: Renal failure
- Hyperglycaemia
Describe the redistribution of crystalloid solutions after IV administration and state the clinical relevance of this
1 : 3 ratio
1ml stays intravascular while 3 ml moves into ECF
This means that crystalloids are inefficient effective circulating volume expanders as 3 x the volume lost of crystalloid replacement would is required to replace the blood loss.
What is the goal of maintenance fluids
Provide sufficient water and electrolytes for a patient not taking oral fluids
What is the goal of fluid replacement therapy
Fluids should resemble fluid losses which will generally resemble ECF
Why should glucose containing solutions be avoided in resuscitation
They are usually hypotonic with minimal expansion of effective circulating volume.
Furthermore, in scenarios requiring resuscitation, the physiological response increases blood sugar levels regardless of glucose administration.
What is the daily requirement of the following
H2O Na+ K+ Ca2+ Mg2+ PO4
H2O – 30 mL/kg Na+ – 2mmol/kg K+ – 1mmol/kg Ca2+ – 0.1mmol/kg Mg2+ – 0.1mmol/kg PO4 – 0.1mmolkg
Dextrose ± 100 grams/day (Adults) to prevent ketosis
Whats the difference between normal saline, ringers lactate, modified ringers lactate (Plasmalyte L) and Plasmalyte B (Balsol)
NS: Na 154 | Cl 154 | OSM: 308
RL: Na 131 | Cl 110 | OSM: 273 | K 4 | Ca 1.8 | Lac 28
MRL: Na 131 | Cl 110 | OSM: 273 | K 4 | Ca 0 | Lac 28
Pl. B: Na 131 | Cl 98 | OSM: 273 | K 4 | Mg 1.5 | HCO3 27 (Buffered with gluconate and acetate instead of lactate) (Balsol)
Why should maintenance solutions only ever be given slowly and should never be given as resuscitation fluid
They contain up to 26 mmol/L of potassium
Historically, it was thought that low sodium solutions are preferred in the perioperative period in paediatrics. How has this thinking and practice changed
Balanced salt solutions are preferred now.
Stress response leads to a water retentive state (and possibly hyponatraemia) –> Solutions resembling ECF should preferably be used in the first 24 hours postoperatively.