Acid base and electrolytes

Question 1

What pH ranges are not compatible with life?

Answer 1

<6.8 or >8

Question 2

Carbonic anhydrase equation

Question 3

What is the definition of increased/decreased base excess?

Answer 3

Decreased base excess indicates low HCO3 or increased H+
Increased BE indicates increased HCO3 or low H+

Question 4

Anion gap calculation and normal ranges for dogs and cats

Answer 4

Dog: 12 - 25 mmol/l
Cat: 17 - 31 mmol/l

Question 5

What are the main ways by which a metabolic acidosis develops and how would these be distinguished?

Answer 5

Can either be due to loss of HCO3 from GI or renal which will result in increased HCO3 production and therefore increased chloride production (hperchloraemic metabolic acidosis)

Or can be due to a gain of additional acids (high anion gap) which may result in reduced chloride and increased HCO3

Question 6

What are the main types of metabolic alkalosis and why do these occur?

Answer 6

1. Loss of H+ via the GIT or renal systems –> hypochloraemic metabolic alkalosis (as Cl is often also lost in these mechanisms)
2. Gain of base (HCO3) which will mean that the body remains normochloraemic.

Question 7

DDx. respiratory acidosis

Answer 7

◦ Increased CO2 production by tissues
‣ Hyperthermia
‣ Seizures
‣ Fever
‣ Malignant hyperthermia
◦ Decreased ventilation by lungs
‣ Airway obstruction
‣ Brain injury
‣ Cervical spinal cord disease
‣ Peripheral neuropathy affecting respiratory muscles

Question 8

DDx. respiratory alkalosis

Answer 8

◦ Decreased tissue production of CO2
◦ Increased ventilation by lungs
‣ Hyperventillation
‣ Diseases causing hypoxaemia –> hyperventillation
‣ Pulmonary parenchymal disease

Question 9

DDx. metabolic acidosis

Answer 9

Hyperchloraemic (normal AG):
Diarrhoea, RTA, compensation, hypoA, 0.9% NaCl administration

Increased AG: lactate, uraemic toxins, ketoacidosis, EG

Question 10

DD. metabolic alkalosis

Answer 10

Hypochloraemic: uper GIT obstruction, loop diuretics

Normochloraemic: HAC, hyperaldosteronism, HCO3 administration, IVFT (CSL)

Question 11

Normal Pa02 on room air, what is the disadvantage of this value?

Answer 11

80 - 150mmHg

This does not take intoaccount supplemental oxygen or hypoventillation

Question 12

Calculation of and normal values for:
1. P/F ratio
2. 120 rule
3. A-a gradien

Answer 12

P/F = Pa02/Fio2 normal >400 - 500
120 rule = PaO2 + PC02 which should be >120
Aa gradient = [150-(1.2xPC02)]-Pa02
- Normal <15
- 15 - 20 = grey zone
- Abnormal >20 - 25

Question 13

Calculation for plasma osmolarity?

Answer 13

mOsm/L = 2[Na] + [BUN] + [Glucose]
Glucose is often ignored in non-diabetic animals as insulin allows it to move freely between compartments

Question 14

Reasons for falsely increased or decreased sodium using non-ion electrode measurements?

Answer 14

Increased: hypoproteinaemia, low lipids (reverse true for decreased sodium)

Question 15

What are the DDx for the following?
- Hypervolaemic hypernatraemia
- Normovolaemic hypernatraemia
- Hypovolaemic hyponatraemia

Question 16

Outline the pathway to diagnosis of disorders of hyponatraemia

Answer 16

1. Rule out pseudohyponatraemia = hyperproteinaemia or hyperlipidaemia. In this scenario plasma tonicity should be normal
2. Consider hypertonic hypernatraemia = essentially where there is a retention of free water due to the presence of other osmotic solutes in the ECF such as glucose, mannitol, ethylene glycol or alcohol
3. Consider hypotonic hyponatramia = a situation where there is either a gain of free water in excess of sodium or a loss of sodium in excess of free water

Question 17

DDx for hyponatraemia by volume status

Answer 17

see pic

Question 18

What are the SIADH criteria (5)

Answer 18

1. Hyponatraemia with plasma hypotonicity
2. Urine osmolality is greater than plasma
3. Renal sodium excretion is increased
4. Absence of oedema or volume depletion (normovolaemia)
5. Normal renal and adrenal function

Question 19

Weird DDx for hyponatraemia

Answer 19

Sodium deficient diet
Babesiosis
Sepsis
Toxoplasmosis
Alaskan sled dogs

Question 20

What is the corrected Cl calculation and how does this help you?

Answer 20

[measured Cl] x [mid-reference Na]/[measured Na]

Decreased measured chloride with normal corrected chloride suggests increased ECF water. Therefore, follow DDx for hyponatraemia

Decreased measured and corrected chloride is often accompanied by hyponatraemia (as chloride has been lost for some reason)

Question 21

DDx/ corrected hypochloraemia

Question 22

DDx for chorected hyperchloraemia (7 categories)

Answer 22

see pic

Question 23

How to treat hypernatraemia

Answer 23

1. Correct any fluid deficity (e.g. hypovolaemia)
2. Give free water:
   - ([Current Na]/[Normal/Target Na]-1) x (0.6 BWT)
3. Decrease by no more than 0.5 - 1 meq/L/h depending on severity

Question 24

How to treat hyponatraemia

Answer 24

1. If caused by eccess circulating fluid volume then correct the underlying cause, avoid use of fluids with Na less than that of the patient
2. If asymptomatic and not meeting criteria 1 then treat with fluids with more sodium than that of the patient
3. Symtomatic hyponatraemia should be treated more aggressively
   a) Frusemide + mannitol to remove excess fluid (increase sodium by no more than 10meq/l/day
   b) Hypertonic saline: aim to raise sodium by 10 - 15% and no quicker than 2mEq/l/h initially. This is often achieved with 3% NaCl and use sodium eficit calculation:

([Target Na - patient Na]) x [0.6 x patient bodyweight). This determines the amount of hypertonic saline to be given increase the sodium by no more than 2mEq/L/h