Lecture 26 - Acid Base and Control of Ventilation

Question 1

Henderson-Hasselbach equation?

Answer 1

pH = 6.1 + log (HCO3-)/0.03 PCO2

Question 2

Result of rise in CO2?

Answer 2

Respiratory acidosis

Question 3

Result of fall in CO2?

Answer 3

Respiratory alkalosis

Question 4

Result of rise in HCO3-?

Answer 4

Metabolic alkalosis

Question 5

Result of fall in HCO3-?

Answer 5

Metabolic acidosis

Question 6

How does the transport of CO2 in blood affect blood?

Answer 6

Powerful effect on the acid-base balance of the blood

Question 7

Normal pH?

Answer 7

7.4

Question 8

“-emia”?

Answer 8

Blood

Question 9

What does alkalemia mean?

Answer 9

High blood pH > 7.4

Question 10

What does acidemia mean?

Answer 10

Low blood pH < 7.4

Question 11

What does alkalosis mean?

Answer 11

Disease leading to alkalemia

Question 12

What does acidosis mean?

Answer 12

Disease leading to acidemia

Question 13

What 2 systems manage the acid-base balance of the blood? Compare them.

Answer 13

1. Lung with CO2 management: QUICK

2. Renal metabolic system with HCO3- management: SLOW

Question 14

6 primary disorders of acid base balance?

Answer 14

1. Acute respiratory acidosis
2. Chronic respiratory acidosis
3. Acute respiratory alkalosis
4. Chronic respiratory alkalosis
5. Metabolic acidosis
6. Metabolic alkalosis

Question 15

Change in pH due to acute change in PaCO2 of 10 mmHg in clinically relevant pH range?

Answer 15

Change pH by 0.08 in opposite direction

Question 16

Change in pH due to acute change in PaCO2 of 20 mmHg in clinically relevant pH range?

Answer 16

Change pH by 0.16 in opposite direction

Question 17

How do the kidneys function to manage the acid-base balance of the blood when hyper or hypoventilation alter the CO2 content in blood? What is this called? What to note?

Answer 17

Over hours/days they retain/release HCO3- leading to a compensatory acidosis or alkalosis correcting the pH partially = chronic compensated respiratory alkalosis/acidosis

Never fully compensated because the abnormality is causing the compensation so when pH reaches the normal range the compensatory mechanism stops instead of continuing until 7.4 => pH will still be acidemic or alkalemic

Question 18

2 types of respiratory acidosis or alkalosis?

Answer 18

1. Acute uncompensated

2. Chronic compensated

Question 19

Change in pH due to chronic change in PaCO2 of 10 mmHg in clinically relevant pH range?

Answer 19

Change pH by 0.03-0.05 in opposite direction

Question 20

Does the pH ever reach 7.4 in chronic compensated respiratory alkalosis/acidosis?

Answer 20

NEVER

Question 21

Describe metabolic acidosis.

Answer 21

Drastic reduction in HCO3- => acidosis => lungs quickly react and hyperventilate to partially correct it => pH remains low (acidemia) + HCO3- is low (metabolic acidosis) + PCO2 is low (respiratory compensation)

Question 22

Can one distinguish an acute metabolic derangement from a chronic one based on blood gas alone? Why?

Answer 22

NOPE cause lungs are so quick to compensate

Question 23

Describe metabolic alkalosis.

Answer 23

Drastic increase in HCO3- => alkalosis => lungs quickly react and hypoventilate to partially correct it => pH remains high (alkalemia) + HCO3- is high (metabolic alkalosis) + PCO2 is high (respiratory compensation)

Question 24

How can acid base status of blood be measured?

Answer 24

Using an arterial blood gas test to measure:

1. PaO2
2. PaCO2
3. pH
4. HCO3- (computer calculate using HH equation)

Question 25

How to determine which primary disorders of acid base balance it is? What 2 numbers are needed?

Answer 25

1. Primary disorder: alkalemia or acidemia determines acidosis or alkalosis OR if pH is normal but CO2 is not: that means there are 2 primary disorders working in opposite directions 2. Respiratory derangement? If it does not match the primary disorder than the primary disorder must be metabolic 3. If primary disorder is respiratory, is it acute or chronic? OR if pH change is higher than 0.8, both respiratory and metabolic disorders happening in same direction? 4. If chronic, what is the compensatory metabolic disorder (always opposite the primary disorder)? ONLY pH and PaCO2 needed

Question 26

If the pH change is HIGHER than 0.8 per 10 mmHg, what can we assume?

Answer 26

BOTH metabolic and respiratory disorders happening together in the same direction

Question 27

What are the axes of a Davenport diagram?

Answer 27

- X-axis: pH - Y-axis: plasma HCO3- - Z-lines: PCO2 (number to left of line)

Question 28

How to read a point on a Davenport diagram to determine primary disorder?

Answer 28

1. Track to x-axis to read pH 2. Follow the z-line for PCO2 Follow the technique!

Question 29

Normal plasma HCO3-?

Answer 29

24 mEq/L blood

Question 30

What does it mean if the pH is deranged, but the CO2 is normal?

Answer 30

Metabolic disorder + respiratory issue so lungs cannot compensate

Question 31

Davenport diagram: jump from one line to line to the right?

Answer 31

Respiratory alkalosis

Question 32

Davenport diagram: jump from one line to line to the left?

Answer 32

Respiratory acidosis

Question 33

Which nerves are responsible for controlling respiratory output? What brain center controls these?

Answer 33

Phrenic nerves controlled by pons and medulla, which can be overriden by the cortex

Question 34

From what do the pons and medulla receive input to control respiratory output?

Answer 34

1. Chemoreceptors 2. Lungs 3. Cortex

Question 35

2 types of chemoreceptors regulating respiratory output? Describe each.

Answer 35

1. Central chemoreceptors in medulla responding to changes in blood pH and PaCO2 2. Peripheral chemoreceptors in aortic and carotid bodies responding to hypoxia (low PaO2 causing low CaO2), pH, and PaCO2

Question 36

What are peripheral chemoreceptors most sensitive to? List the 3 factors in order. Which one determines minute ventilation? Why?

Answer 36

1. Hypoxia 2. pH 3. PaCO2*** Because the hypoxia has a lower threshold to kick in (PaO2 of 50 mmHg) and the body maintains constant pH

Question 37

Trick to be able to hold your breath for longer? Why is this dangerous?

Answer 37

1. Hyperventilate to lower CO2 2. Hold your breath => PaCO2 threshold of chemoreceptors will take longer to be reached Dangerous because person may not have enough time to surface to avoid drowning secondary to hypoxia

Question 38

Main 2 causes of COPD?

Answer 38

1. US: smoking | 2. Most of the world: cooking over biomass fuels

Question 39

2 types of COPD? Describe each.

Answer 39

1. Emphysema: damaged alveoli | 2. Chronic bronchitis: more narrow airways

Question 40

In patients with COPD, which factor determines the minute ventilation? Big treatment mistake if they come into hospital being sick?

Answer 40

Because breathing is difficult, the patient’s body will adjust over time to decrease respiratory rate => decreased respiratory rate will augment CO2 retention => respiratory acidosis => over time the kidneys adjust to release HCO3- in attempt to maintain pH => chronic compensated respiratory acidosis => dangerous cycle where the chemoreceptors become increasingly desensitized to a continuous increase in PaCO2 as the patient breathes less over time => silenced CO2 alarm => after a certain period, the patient’s oxygen levels will become so low that hypoxic drive becomes the main input controlling respiration Big treatment mistake if they come in to hospital hypoxic because they are sick and you give them 100% O2 they will stop breathing because no more respiratory drive = hypercapnic coma

Question 41

Which response is slower: central or peripheral chemoreceptor response to increase PaCO2?

Answer 41

Central chemoreceptor response is SLOWER

Question 42

How does the ventilatory response vary during sleep?

Answer 42

It decreases

Question 43

How does the ventilatory response vary if the work of breathing is increased?

Answer 43

It decreases

Question 44

What is the major stimulus to ventilation at high altitude?

Answer 44

Hypoxia

Question 45

PaO2 in mild CO poisoning?

Answer 45

Normal