Acid-base

Question 1

What is an acid?

Answer 1

A H+ donor

Question 2

What is a strong acid?

Answer 2

A H+ donor that is completely dissociated at physiological pH

Question 3

What is a weak acid?

Answer 3

A H+ donor that is incompletely dissociated at physiological pH

Question 4

What is pH?

Answer 4

The reverse logarithmic representation of H+ activity

Question 5

What is a base?

Answer 5

A H+ recipient

Question 6

What does base excess represent?

Answer 6

A reflection of the metabolic component of acid-base status.

Question 7

What is HCO3-?

Answer 7

A non-linear representation of the metabolic component. It is not measured in gas samples but is derived from pH and PaCO2

Question 8

What is standard base excess?

Answer 8

The acid required when Hb= 50g/L

The reduced buffering capacity of this Hb depleted blood is more representative of extracellular acid-base status

Question 9

What is actual base excess?

Answer 9

Actual base excess is the amount of acid that must be added to return the pH in vitro to 7.40 under the standard conditions of T=37 and PaCO2=5.33Kpa

Question 10

What are the two approaches to acid-base balance

Answer 10

Siggard-Anderson

Stewart approach - this offers more understanding of the underlying mechanism

Question 11

What is the Siggaard-Anderson approach?

Answer 11

It’s based upon the Henderson-Hasselbach equation, which arises from the dissociation equation for carbonic acid: CO2+H20H+ +HCO3-
It’s based on the premise that HCO3 and PaCO2 are the main determinants of pH
A negative BE suggests a metabolic acidosis is contributing to the acid-base state
Identification of a metabolic acidosis should prompt identification of the precipitating biochemical abnormality - calculate anion gap.

Question 12

What is the Henderson-Hasselbach equation?

Answer 12

pH= 6.1 +log HCO3/ (0.03x PaCO2)

Question 13

How do you calculate anion gap?

Answer 13

(Na+K)-(Cl+HCO3)

Question 14

What are the mechanisms behind normal anion gap metabolic acidosis?

Answer 14

Bicarbonate loss
Decreased renal excretion of H
Ingestion/infusion of acid

Question 15

What is the mechanism behind raised anion gap acidosis?

Answer 15

Strong acid accumulation

Question 16

What are the causes of raised anion gap acidosis?

Answer 16

Lactic acidosis
Ketoacidosis
AKI/CKD
Toxins - methanol, ethylene glycol
Chronic paracetamol use
Salicylate poisoning 
(MUDPILES)

Question 17

What are the causes of normal anion gas acidosis?

Answer 17

Diarrhoea
Ileostomy
Renal tubular acidosis
TPN
Dilutional
Colonic ureteric implant/diversion

Question 18

What are the concepts that the Stewart approach to acid-base balance are based on?

Answer 18

1. Acids increase the [H+] and bases decrease the [H+] of a solution
2. HCO3- is not independent of pH/H+, rather it is dependent on it
3. Acids are said to be stong if the fully dissociate in plasma and weak if they are only partially dissociated
4. It takes into account the influence of albumin on acid-base disturbances

Question 19

According to the Stewart theory, what are the 3 major contributors to acid-base balance?

Answer 19

1. Strong ion difference
2. Concentration of non-volatile weak acids in the plasma
3. PaCO2

Question 20

What is the strong ion difference?

Answer 20

Reflects the difference between the number of strong cations and the number of strong anions
Strong cations: Na, K, Ca, Mg
Strong anions: Cl, lactate

Question 21

How do you calculate the ‘apparent’ SID?

Answer 21

([H+] + [K+] + [Ca2+] + [Mg2+]) - ([Cl-] + [lactate-])

Question 22

What does an increase in SID mean?

Answer 22

An increase in strong cations relative to anions leads to an alkalosis

Question 23

What does a decrease in SID mean?

Answer 23

Acidosis

Question 24

What is the ‘apparent’ SID?

Answer 24

SID is referred to as apparent if it only include the strong ions which are routinely measured

Question 25

What is the ‘effective’ strong ion difference?

Answer 25

Can be calculated using a different equation that takes into account PaCo2 and albumin

Question 26

What is the strong ion gap?

Answer 26

The difference between the SIDeff and the SIDapp

Question 27

What is the significance of the SIG?

Answer 27

If elevated (>2) it signifies the presence of unmeasured strong cations contributing to the acidosis e.g. ketones, salicylates, paraldehyde

Question 28

According to the ‘Emcrit’ method how do you interpret an ABG?

Answer 28

1. Look at pH - acidosis or alkalosis
2. Look at PaCO2 - respiratory or metabolic
3. Calculate SID (Na-Cl)
   
   • > if < 38 this is a metabolic acidosis
   • > if > 38 this is a metabolic alkalosis
4. Look at the lactate if > 2 then think sepsis, or ischaemia, or drug induced
5. Calculate the strong ion gap
   
   • > if > 2 this is a SIG acidosis (MUDPILES)
   • > if negative causes include high ca, mg or K, immunoglobulins, nitrates, lithium OD
6. Think about compensations

Question 29

How do you calculate the SIG?

(the emcrit method)

Answer 29

[base deficit] + [SID - 38] + 2.5(42-albumin) - lactate

Question 30

Compensation:
If the primary defect is respiratory and you think it’s chronic how do you calculate the expected metabolic compensation?

Answer 30

Expected change in BE = 0.4 x chronic change in CO2

Question 31

If the primary problem is a metabolic acidosis how do you calculate the expected decrease in CO2?

Answer 31

= base deficit

Question 32

If the primary problem is a metabolic alkalosis how do you estimate the expected increase in CO2?

Answer 32

0.6 x base excess