Acid-base Balance

Question 1

pH

Answer 1

Negative log [H+]

Question 2

Acidosis

Answer 2

Disorder tending to make blood more acid than normal

Question 3

Alkalosis

Answer 3

Disorder tending to make blood more alkaline than normal

Question 4

Acidemia

Answer 4

Low blood pH

Question 5

Alkalemia

Answer 5

High blood pH

Question 6

Factors affecting pH

Answer 6

Respiratory component: CO2 concentration
Metabolic component:
- intrinsic acid = products of metabolism
- extrinsic acid = diet, toxins
- buffering capacity = bicarbonate, other buffers

Central relationship between bicarbonate and CO2

Question 7

Standard bicarbonate

Answer 7

Measures of metabolic component of any acid-base disturbance

Question 8

What is standard bicarbonate standardised to

Answer 8

Bicarbonate concentration standardised to pCO2 5.3kPa and temp 37

Question 9

Absolute bicarbonate is affected by

Answer 9

Respiratory and metabolic components

Question 10

Base excess

Answer 10

Quantity of acid required to return pH to normal under standard conditions

Question 11

What is standard base excess corrected to

Answer 11

Hb 50g/L

Question 12

What can base excess be used to calculate

Answer 12

Bicarbonate dose to correct acidosis
0.3 x Wt xBE

Question 13

Base deficit

Answer 13

Base excess is negative in acidosis

Question 14

What is measured by an arterial blood gas

Answer 14

pH
pO2
pCO2
STD HCO3-
STD base excess
May include other measures eg lactate, Na+, K+

Question 15

How is acid-base status interpreted

Answer 15

Henderson approach
Stewart’s theory (strong ion difference)

Question 16

Henderson-Hasselbalch equation

Answer 16

pH = pKa + log([A-]/[HA])
pH = pKaH2CO3 + log([HCO3-]/[H2CO3])
pH = 6.1 + log([HCO3-]/0.03 x pCO2)

Question 17

What can effect acid-base status

Answer 17

Lungs- CO2 excretion
Kidneys- H+ excretion and HCO3- recycling

Question 18

2 major organs involved in acid-base balance

Answer 18

Lungs
Lidneys

Question 19

Stewart’s strong ion difference

Answer 19

Principle: pH and HCO3- are dependent variables governed by:
pCO2
Concentration of weak acids (ATOT)
ATOT = Pi + Pr + Alb
Strong ion difference (SID)
SID = Na+ + K+ + Mg2+ + Ca2+ – Cl- – other strong anions (eg lactate, ketoacids)

Question 20

Evaluation of Stewart’s strong ion difference

Answer 20

Identifies the factors controlling pH
Calculation can be very problematic- measurements introduce errors
Probably adds little in practice

Question 21

Diagnosing acid-base disorders

Answer 21

Acidoses or alkaloses
Respiratory or metabolic

Can co-exist

Question 22

Causes of metabolic acidosis

Answer 22

Dilutional
Failure of H+ excretion
Excess H+ load
HCO3- loss

Question 23

Failure of H+ excretion

Answer 23

Renal failure
Hypoaldosteronism
Type 1 renal tubular acidosis

Question 24

Excess H+ load

Answer 24

Lactic acidosis
Ketoacidosis
Ingestion of acids eg salicylate, ethylene, glycol

Question 25

HCO3- loss

Answer 25

Diarrhoea Type 2 renal tubular acidosis

Question 26

Clinical features of metabolic acidosis

Answer 26

Sighing respirations (Kussmaul’s resps) Tachypnoea

Question 27

Compensatory mechanism for metabolic acidosis

Answer 27

Hyperventilation to increase CO2 excretion

Question 28

Anion gap

Answer 28

Difference between measured anions and cations = [Na+] + [K+] - [Cl-] - [HCO3-] Useful to identify cause of acid-base disorders

Question 29

Normal anion gap

Answer 29

10-16

Question 30

Wide anion gap causes

Answer 30

Lactic acidosis Ketoacidosis Ingestion of acid Renal failure

Question 31

Causes of narrow anion gap (ie high chloride)

Answer 31

GI HCO3- loss Renal tubular acidosis

Question 32

Causes of metabolic alkalosis

Answer 32

Alkali ingestion Gastrointestinal acid loss - vomiting Renal acid loss- hyperaldosteronism, hypokalaemia

Question 33

Compensatory mechanism for metabolic alkalosis

Answer 33

Hypoventilation (limited by hypoxic drive) Renal bicarbonate excretion

Question 34

Respiratory acidosis

Answer 34

CO2 retention leading to increased carbonic acid dissociation

Question 35

Causes of respiratory acidosis

Answer 35

Any cause of type 2 respiratory failure

Question 36

Compensatory mechanism of respiratory acidosis

Answer 36

Increased renal H+ excretion and bicarbonate retention (only if chronic)

Question 37

Respiratory alkalosis

Answer 37

CO2 depletion due to hyperventilation

Question 38

Causes of respiratory alkalosis

Answer 38

Type 1 respiratory failure Anxiety/panic

Question 39

Compensation of respiratory alkalosis

Answer 39

Increased renal bicarbonate loss (if chronic)

Question 40

ABG interpretation

Answer 40

What is the pH? What is the respiratory component (ie pCO2)? What is the metabolic component (std HCO3-, base excess)? Which component is congruent with the pH?

Question 41

If pH and metabolic component are opposite

Answer 41

Respiratory problem- body trying to compensate

Question 42

If pH and metabolic component going same way

Answer 42

Metabolic component driving factor/problem

Question 43

If pH and pCO2 are opposite

Answer 43

Compensatory

Question 44

If pH and pCO2 are same way

Answer 44

Causing factor

Question 45

Low pCO2

Answer 45

Alkalosis

Question 46

Normal pCO2 range

Answer 46

4.6-6.0

Question 47

Normal base excess range

Answer 47

0-2

Question 48

Normal Std HCO3- range

Answer 48

22-26

Question 49

Normal pH range

Answer 49

7.36-7.44

Question 50

Normal pO2 range

Answer 50

9.5-12

Question 51

Alkalosis is a condition in which the body fluids have excess base (alkali). This is the opposite of excess acid (acidosis). Which of the following would cause a metabolic alkalosis?

Answer 51

Vomiting