Acid-Base regulation

Question 1

Recall typical values for blood gases once it is pumped into the arteries

Answer 1

PaO2 >10kPa
SaO2 >95%
PaCO2- 4.7-6.4kPa

Question 2

Recall typical values for blood gases in the veins

Answer 2

5.3 kPa
~75%
6.1 kPa

Question 3

Which other factor is important in the acid-base equilibrium

Answer 3

Temperature

Question 4

Define alkalaemia

Answer 4

Refers to high-than-normal pH of blood

Question 5

Define acidaemia

Answer 5

Refers to lower-than-normal pH of blood

Question 6

Define alkalosis

Answer 6

Describes circumstances that will decrease [H+] and increase pH

Question 7

Define acidosis

Answer 7

Describes circumstances that will increase [H+] and decrease pH

Question 8

How can you easily distinguish between -osis and -aemia

Answer 8

• osis- what causes the change in pH

- aemia- conc in blood at that time

Question 9

What is measured in the arterial blood gas test

Answer 9

PO2 – the partial pressure of oxygen: How much oxygen is dissolved in the arterial blood.
PCO2 – the partial pressure of carbon dioxide: How much CO2 is dissolved in the arterial blood.
pH – the ‘power of hydrogen’: Describes the [H+] of the blood.
HCO3- - plasma bicarbonate: Describes how much bicarbonate is dissolved in the arterial blood.
BE – base excess: Describes the concentration of bases compared to the ‘expected concentration’. An exact match is zero.

Question 10

What is meant by an acid

Answer 10

An acid is any molecule that has a loosely bound H+ ion that it can donate
H+ ions are also called protons (because an H atom with a +1 valency has no electrons or neutrons)
PARADOX: A greater concentration of H+ ions refers to a lower pH (discussed next)

Question 11

Why is it important that the acidity of the blood is tightly regulated

Answer 11

The acidity of the blood must be tightly regulated, marked changes will alter the 3D structure of proteins (enzymes, hormones, protein channels)
Ultimately, this is achieved by the actions of buffers of the respiratory and renal systems.

Question 12

What is meant by a base

Answer 12

A base is an anionic (negatively charged ion) molecule capable of reversibly binding protons (to reduce the amount that are ‘free’)
H+A- H+ and A-

Question 13

What is the equation for the acid-base equilibrium

Answer 13

H2O + CO2 H2CO3 H+ + HCO3-
changes in CO2, HCO3- will have an effect on pH.
CO2 regulated by ventilation
HCO3- regulated by kidneys

Question 14

What did the pitts and swan experiment show

Answer 14

The Pitts and Swan experiment discovered the buffering capacity of the blood when they injected 14 molar acids into a dog and found it
The blood has an ENORMOUS buffering capacity that can react almost IMMEDIATELY to imbalances
s blood pH barely changed at all.

Question 15

What are the concentrations of H+, Na+ and K+ in the ECF

Answer 15

H+ = 0.00000004 Eq/L
Na+ = 140 Eq/L  and  K+ =  4 Eq/L

Question 16

Describe the work of Sorenson

Answer 16

Sørensen scaled the data using a log10 transformation
This was great as it made numbers much more manageable, although negative . This was easily fixed by applying a minus sign to the equation
The inverse function of log is (10x), using this we can calculate [H+] from pH

-log10[H+] = 7.4
[H+] = 10-pH

Question 17

What are the sources of acid

Answer 17

Respiratory- CO2
Metabolic- HCL from stomach, lactic acid, phosphate, sulphate, bicarbonate
Respiratory acids have a much larger effect on the pH of the blood.
Small amounts from diet too

Question 18

What is the Henderson equation

Answer 18

To calculate the dissociation constant (Ka)

K = [H+][HCO3-]/[CO2][H2O]

Question 19

What is the Henderson-hasselbach equation

Answer 19

pH = pK + log10([HCO3-]/[CO2])

combination of Henderson and Sorenson equations

Question 20

Using Sørensen’s equation and your gas transport notes, calculate the change in [H+] between arterial blood and venous blood. What is it as a percentage?

Answer 20

a. pHarterial = 7.4, pHvenous = 7.36. This 0.04 change in pH is equivalent to a 10% increase in acidity.

Question 21

Using the notes from the gas transport lecture, estimate the volume of respiratory acid (i.e. CO2) produced in a typical adult over a 24-hour period.

Answer 21

a. CO2 total flux is 200mlCO2min-1 so in a day that is 200x60x24 = 288Lday-1 of respiratory acid.

Question 22

Calculate the pH of an arterial blood gas sample if the [H+] is 48 nmol/L.

Answer 22

a. pH = -log10[H+] = -log10(48 x 10-9) = 7.32.

Question 23

Outline a systematic approach for analysing arterial blood gases

Answer 23

Is a low blood pH acidosis or acidaemia?
[H+] only? You’d better convert!
Is the PaCO2 normal, high or low?
This is assessing the respiratory component
Is the BE excess high or low?
This is assessing the metabolic component
Is the patient hypoxaemic?
No, mildly, moderately or severely?

Question 24

What are the guidelines for hypoxaemia

Answer 24

Basic guidelines for PaO2
>10 kPa is normal
8-10 kPa is mild hypoxaemia
6-8 is moderate hypoxaemia
<6 kPa is severe hypoxaemia

Different values for different locations

Question 25

What are the values for pH in different locations of the body

Answer 25

``` Intracellular fluid: 7.0. Extracellular fluid: 7.4. Arterial blood: 7.4. Venous blood: 7.36. Stomach: 2.4. ```

Question 26

Summarise compensatory mechanisms

Answer 26

Changes in ventilation can stimulate a RAPID compensatory response to change CO2 elimination and therefore alter pH Changes in HCO3- and H+ retention/secretion in the kidneys can stimulate a SLOW compensatory response to increase/decrease pH An acidosis will need an alkalosis to correct An alkalosis will need an acidosis to correct

Question 27

What are the causes of respiratory acidosis

Answer 27

Hypoventilation V/Q mismatch An increase in PCO2 will increase an increase in hydrogen ion concentration. Plasma bicarbonate ion cocn will increase to compensate.

Question 28

Describe the renal compensations for respiratory acidosis

Answer 28

Increased filtration of hydrogen ions in the kidneys increased HCO3- reabsorption and production Raises the pH back towards normal

Question 29

List some conditions linked to respiratory acidosis

Answer 29

``` Asthma COPD Blocked airway (tumour or foreign body) Spontaneous lung collapse, brain lesion Injury to the chest wall. Drugs that reduce ventilation include: morphine, barbituarates and general anaesthetics. ```

Question 30

Describe respiratory alkalosis

Answer 30

Decrease in PCO2 Hyperventilation- hypoxic drive in pneumonia, diffuse interstitial lung diseases, high altitude, mechanical ventilation, brainstem damage, hysteria, drugs (aspirin) and infections causing fever

Question 31

Describe the renal compensation for respiratory alkalosis

Answer 31

Reduced hydrogen ion filtration in the glomeruli Reducing HCO3- reabsorption and production Renal compensation reduces pH towards normal.

Question 32

Describe metabolic acidosis

Answer 32

Excess hydrogen, which reduces the bicarbonate conc (think to the equilibrium). Respiration is unaffected therefore PCO2 is initially normal. Exogenous acid loading (aspirin overdose) Endogenous acid production (ketogenesis in diabetes) Loss of HCO3- from kidneys or gut (diahhroea) Renal failure- metabolic production of H+ ions, the kidney may not be able to excrete the excess hydrogen ions immediately.

Question 33

Describe respiratory compensation for metabolic acidosis

Answer 33

Decrease in pH is detected by peripheral chemoreceptors- increasing ventilation and lowering PCO2. This shifts the equilibrium to the left, reducing H+ conc and bicarbonate Raising pH towards normal.

Question 34

Can respiratory compensation fully correct metabolic acidosis

Answer 34

No. There is a limit to how far PCO2 can fall with hyperventilation. Correction can only be carried out by removing the excess H+ from the body or restoring the lost bicarbonate.

Question 35

Describe metabolic alkalosis.

Answer 35

Increase in bicarbonate or fall in hydrogen concentration (removing H+ will shift the equilibrium to the right, increasing the conc of bicarbonate). Initially PCO2 is normal Causes; vomiting (hcl loss from stomach) ingestion of alkaline substances potassium depletion (diuretic, excess aldosterone)

Question 36

Describe respiratory compensation for metabolic alkalosis

Answer 36

increase in pH is detected by peripheral chemoreceptors. This causes a decrease in ventilation- increasing PCO2. Equilibrium is shifted to the right pH returns to normal.

Question 37

Can respiratory compensation fully correct metabolic alkalosis

Answer 37

No Full correction can only be carried out by removing the problem of either reduced hydrogen ion conc or increased bicarbonate conc. This is done by reducing renal hydrogen ion secretion. more bicarbonate is excreted because more is filtered at the glomerulus and less is reabsorbed in combination with hydrogen ions.

Question 38

What is meant by base excess

Answer 38

1. The ABG analyser calculates hypothetical [HCO3-] based on PCO2 and assumes no metabolic or renal disturbance. 2. The machine then ELIMINATES the changes in [HCO3-] due to PCO2 so that any change in [HCO3-] is due to metabolic acid base disturbance or a change in renal excretion of acid.

Question 39

What can change base excess

Answer 39

A rise in base excess is due to an increase in renal excretion of acid, ingestion/administration of a base or loss of acid from vomiting. The result is a metabolic alkalosis. A fall in base excess is due to the overproduction of metabolic acids, the ingestion of acid, a reduction/failure of acid excretion by the kidney or excessive loss of alkali from intestines with diarrhoea. The result is a metabolic acidosis.

Question 40

Describe the chronic and acute phases of respiratory acidosis

Answer 40

Partial Compensation: will have lower pH, high PCO2 and high base excess Acute phase: CO2 moves into erythrocytes, combines with H2O in presence of carbonic anhydrase to form bicarbonate, which moves out of cell by AE1 transporter; increased bicarbonate leads to raised base excess, shifting equilibrium backwards to carbonic acid and reducing [H+] Chronic phase: increases bicarbonate reabsorption in kidneys to stabilise pH Full Compensation: will normalise pH with large PCO2 and base excess

Question 41

Describe the chronic and acute phases of respiratory alkalosis

Answer 41

Partial Compensation: will have higher pH, low PCO2 and low base excess Acute phase: none Chronic phase: reduces bicarbonate from nephrons and increases secretion in collecting duct, causing more carbonic acid dissociation, reducing base excess Full compensation: will normalise pH with low PCO2 and base excess

Question 42

Summarise the partial and full compensations for metabolic acidosis

Answer 42

Partial compensation: will have a lower pH, low PCO2 and low base excess; occurs by increasing ventilation rate to increase diffusion gradient and reduce PCO2, causing shift to left on equilibrium, forming carbonic acid, and then CO2 Full compensation: will normalise pH with low PCO2 and base excess

Question 43

Summarise the partial and full compensations for metabolic alkalosis

Answer 43

Partial compensation: will have high pH, high PCO2 and high base excess; reducing ventilation rate to increase arterial PCO2 drives equation to right to increase protons and bicarbonate Full compensation: will normalise pH with high PCO2 and base excess

Question 44

Outline an interpretation procedure

Answer 44

Type of imbalance? Acidosis (or acidaemia) / Alkalosis (or alkalaemia) / Normal Aetiology of imbalance? Respiratory (acidosis or alkalaemia) / Metabolic (acidosis or alkalosis) / Mixed (respiratory and metabolic) / Normal Any homeostatic compensation? Uncompensated / Partially compensated / Fully compensated Oxygenation? Hypoxaemia / Normoxaemia / Hyperoxaemia

Question 45

List the normal values

Answer 45

[Hb] 130 to 170 g/L pH 7.35 to 7.45 PCO2 4.7 to 6.4 kPa (35 - 48 mm Hg) PO2 >10 kPa (>80 mmHg) HCO3- 22-26 mEq/L Base excess -2 to +2 mmol/L or mEq/L (This variable is calculated) Standard BE BE for 5 g/dL [Hb]

Question 46

What should you look at when assessing acidosis

Answer 46

f acidosis has been established: 1. Assess the PaCO2: a. Elevated = respiratory acidosis. b. Low = metabolic acidosis. 2. Assess the BE: a. Low: i. With low PaCO2 = partially compensated metabolic acidosis. ii. With normal PaCO2 = uncompensated metabolic acidosis. iii. With high PaCO2 = uncompensated mixed acidosis. b. Normal: i. With low PaCO2 = N/A to this lecture. ii. With high PaCO2 (this is usually associated) = uncompensated respiratory acidosis. c. High: i. With low PaCO2 = N/A to this lecture. ii. With high PaCO2 (this is usually associated) = partially compensated respiratory acidosis.

Question 47

What should you look at when assessing alkalosis

Answer 47

Assess the PaCO2: a. Elevated or normal = metabolic alkalosis. b. Low = respiratory alkalosis. 2. Assess the BE: a. Low: i. With low PaCO2 (this is usually associated) = partially compensated respiratory acidosis. ii. With high/normal PaCO2 = N/A to this lecture. b. Normal: i. With low PaCO2 (this is usually associated) = uncompensated respiratory alkalosis. ii. With high/normal PaCO2 = N/A to this lecture. c. High: i. With low PaCO2 = uncompensated mixed alkalosis. ii. With normal PaCO2 = uncompensated metabolic alkalosis. iii. With high PaCO2 = partially compensated metabolic alkalosis.

Question 48

What can explain a normal pH

Answer 48

1. Assess the PaCO2 AND BE together: a. Both within range = patient is normal. b. Both low – one of either: i. Fully compensated respiratory alkalosis. ii. Fully compensated metabolic acidosis. c. Both high – one of either: i. Fully compensated respiratory acidosis. ii. Fully compensated metabolic alkalosis.