Acid-base, electrolytes and fluid balance

Question 1

Acidosis vs Acidaemia

Answer 1

acidiosis is an abnormal condition which tends to decrease arterial pH, and similarly alkalosis is an abnormal condition which tends to increase arterial pH

Question 2

Compensation

Answer 2

Normal body process which attempt to return the arterial pH to normal - uses respiratory and renal buffering mechanisms

Question 3

Why is the number of H+ important?

Answer 3

Cellular machinery is very sensitive to changes in H+

Intracellular H+ is related to extracellular H+ and Na and K+ ions

Question 4

Acid-base balance

Answer 4

At normal pH [H+] is 40nmol/L, as pH is -log10[H+] = 7.4
Between pH 6-7 there is a 10x change in H+
Normal pH is 7.35-7.45

Question 5

Behavior of weak acids

Answer 5

HA H+ & A-
The law of mass action –> [H+][A-]/[HA] = K
Henderson Hasselbach equation: pH=pKa + log10[A-]/[HA]

Question 6

Buffering theory

Answer 6

Enables the body to handle H+ without a change in pH

This is most effective when th pKa of the buffer is close to the working pH

Question 7

Buffering molecules

Answer 7

Haemoglobin
Proteins
Phosphate  (PO4--)
Bicarbonate (HCO3-) (Renal)
Carbon Dioxide (about 60%) (Respiratory)

Question 8

Carbonic acid and bicarbonate system

Answer 8

CO2 + H20 HCO3- + H+ (speeded by carbonic anhydrase) –> very rapid changes
Rise in PCO2 increases [H+] and lowers pH
Decrease in PCO2 reduces [H+] and increases pH

Question 9

Renal compensation mechanisms

Answer 9

H+ is secreted and HCO3 is reabsorbed by renal tubules
(dependent on filtered load of HCO3 and GFR)
H+ secretion is dependent on arterial PCO2
In a respiratory acidosis more H+ is secreted and in a alkalosis secretion is decreased and HCO3 reabsorption is depressed

Question 10

Clinical Signs of Acidosis

Answer 10

CVS –> neg. inotropic effect, catecholamine release, tachy, arrhythmias, peripheral vasodilation, renal and GI vasoconstriction
GI –> Decreased Gut motility
RS –> pulmonary vasoconstriction, increased ventilatory drive until PCO2>13Kpa, Bronchodilation,
Electrolytes –> High Ca++ and K+
CNS –> reduced GCS, Changes in CBF/ICP,

Question 11

Clinical Signs of Alkalosis

Answer 11

CVS –> Increased coronary & systemic vascular resistance, Left shift in Hb/O2 dissociation curve, decreased DO2
Electrolytes –> Low Ca++ and K+
CNS –> epilepsy

Question 12

Causes of Respiratory alkalosis

Answer 12

Cental –> CNS lesions, aspirin, anxiety, pregnancy, septicaemia, liver failure
Pulmonary –> pneumonia, asthma, CCF, PE, hyperventilation

Question 13

Causes of Respiratory acidosis (Hypoventilation)

Answer 13

CNS trauma –> trauma/infection/tumor/etc
Drugs –> sedatives or narcotics
Neuromuscular compromise
Airway obstruction –> FB, asthma, COPD

Question 14

Causes of Metabolic alkalosis

Answer 14

Loss of H+ –> gastric (vomiting or drainage), urine (Cushings or diuretics), Potassium deficiency (drives H+ into cells)
Excessive intake of HCO3 –> As bicarbonate, lactate or citrate

Question 15

Causes of Metabolic acidosis

Answer 15

Increased H+ –> renal failure, ketoacidosis, lactate acidosis, ingesting aspirin/glycol/ethanol
Decreased H+ secretion –> renal failure, renal tubular acidosis, mineralocorticoid deficiency
Loss of bicarbonate –> diarrohea or pancreatic fistula, proximal RTA

Question 16

Blood gas interpretation

Answer 16

1. pH –> acidosis or alkalosis
2. CO2 –> if it agrees with the pH it is resp, if not metabolic
3. BE –> HCO3 -25 is BE, -ve is acidotic, +ve is alkalotic

Question 17

Electrolyte normal values

Answer 17

Na+ 134-141mmol/L
K+ 3.5-4.5 mmol/L
Cl- 100-105mmol/L
HCO3— 24-28mmol/L

Question 18

Electrolyte daily intake

Answer 18

Na+ –> 80mmol/day (0.5L of 0.9% saline)
K+ –> 80mmol/day
Water –> ~ 3L per day

Question 19

Fluid compartments of the body

Answer 19

The whole body is: 18% protein, 7% mineral, 15% fat, 60% water
Intracellular water is 40% of body weight
Extracellular water is 20% of body weight

Question 20

Causes of Hyponatraemia

Answer 20

Artifact or contamination of the sample
Severe hyperlipidaemia or hyperproteinaemia
Reduced Na intake is rare but increased Na loss is common

Question 21

Hyponatraemia with normal ECF volume

Answer 21

Water excess –> psychogenic polydipsia, disorders of the thirst centre, beer drinkers potomania or normal intake with poor renal function
SIADH –> CNS disease, tumor, chest disease, carbamazapine
Or increased sensitivity to ADH (or oxytocin) or abnormal ADH release (hypothyroid, Addisons, vagal neuropathy)

Question 22

Hyponatraemia with decreased ECF volume

Answer 22

GI –> Vomiting, diarrhoea, haemorrhage

Kidney –> osmotic diuresis, diuretics, recovery of acute tubular necrosis, renal artery stenosis

Question 23

Management of Hyponatraemia

Answer 23

Na, limit fluid, enhance water excretion, demeclocycline or hypertonic saline

Question 24

Demeclocycline

Answer 24

An antibiotic which is actually used to induce nephrogenic diabetes insipidus by reducing the sensitivity of collecting duct cells to ADH and so it is used to treat hyponatraemia

Question 25

Hypernatraemia

Answer 25

``` ADH deficiency (diabetes insipidus) or insensitivity (Li, ATN or tetracyclines) Iatrogenic from hypertonic solution Osmotic duresis (TPN or hyperosmolar coma) Deficient water intake ```

Question 26

Management of Hypernatraemia

Answer 26

Na>145mmol/L thirst, confusion, coma, cerebral thrombosis, Treat with slow correction with 0.9% saline and beware cerebral oedema

Question 27

Causes of Hypokalaemia

Answer 27

``` reduced intakes (anorexia) or artefact Usually due to a shift into cells --> alkalosis, insulin, catecholamines, paralysis (rare) Increased loss--> Renal (alkalosis, diuretics, tubular damage, hyperaldosteronism, hypomagnesaemia) or GI (vomiting, diarrhoea or laxatives) ```

Question 28

Management of Hypokalaemia

Answer 28

Symptoms --> weakness and lethargy, arrhythmias, | Treatment --> careful supplementation and treat underlying cause

Question 29

Causes of Hyperkalaemia

Answer 29

Artefacts (common) -- haemolysis, contaminated sample Increased intake from supplements (rare) Shift out of cells --> acidosis, cell lysis (burns or crush injuries) Decreased Loss --> acidosis, renal failure, K+ sparing diuretics, addisons, ACEis, cyclosporin

Question 30

Management of Hyperkalaemia

Answer 30

Symptoms --> Ca gluconate --> protect the myocardium, arrhythmias or cardiac arrest, ECG changes Treatment --> Oral cation exchange resin (polystyrene sulphonate) to increase gut excretion, glucose and insulin

Question 31

Insulin management of hyperkalaemia

Answer 31

1. Sodium bicarbonate 50ml of 8.4% IV - -> Ca gluconate (protect the myocardium) 2. Glucose (200ml of 10%) plus 10 units of insulin

Question 32

ECG changes in severe hyperkalaemia

Answer 32

Broad QRS Peaked T ST depression Absent P

Question 33

Perioperative fluid management

Answer 33

Aim to maintain circulating volume Normal electrolyte balance, with normal Hb and glucose Replace ongoing fluid losses with fluid or blood

Question 34

Replacing fluid deficit due to starvation

Answer 34

Hourly maintenance x hours starved

Question 35

Hourly maintenance of fluid

Answer 35

0-10kg = 4ml/kg/hr 10-20kg = (40ml + 2ml/kg)hr >20kg = (60ml + 1ml/kg)hr Eg 85kg man requires 125ml/hr maintenance

Question 36

Where does fluid go?

Answer 36

Water or detrose will distribute across all body fluid, only 5-10% stays in blood 30-35% of saline stays in blood while all of blood stays in the blood

Question 37

Causes of anion gap metabolic acidosis (CAT MUDPILES)

Answer 37

``` Cyanide, CO, congestive heart failure Aminoglycosides Theophylline Methanol Uraemia Diabetic (EtOH/starvation) ketoacidosis Paraldehyde, paracetamol, phenformin Iron, isoniazid, inborn errors Lactic acidosis Ethanol/ethylene glycol Salicylate ```