FunMed: PBL 4 (Acid-Base Problems - Cystic Fibrosis)

Question 1

What type of acid-base disorder is cystic fibrosis and why?

Answer 1

Respiratory acidosis due to decreased ability to remove carbon dioxide from the body (hypoventilation) causing the extracellular fluid to become acidic (CO2 is acidic)

Question 2

What is the cause of cystic fibrosis?

Answer 2

Autosomal recessive genetic disorder on chromosome 7 in gene which codes for the CFTR channel (inherits 2 faulty versions)

Question 3

What is the role of the CFTR ion channel?

Answer 3

Cystic fibrosis transmembrane conductance regulator - chloride channel which pumps sodium and water out of cells into the extracellular space

Question 4

Outline how CFTR channels function

Answer 4

ATP-gated anion (chloride) cahnnel which allows Cl- to flow down electrochemical gradient (out of cell) which creates a positive flow for Na+ and water to follow to create more watery mucus

Question 5

Where are CFTR channels located?

Answer 5

Lungs, liver, pancreas, digestive tract, reproductive tract and skin

Question 6

How is cystic fibrosis diagnosed?

Answer 6

Heelprick/Guthrie test (newborns) that detects trypsin in blood released by damaged pancreas, sweat test (saltier), carrier testing if family member has CF, or antenatal testing

Question 7

What are the symptoms of cystic fibrosis?

Answer 7

Chronic lung infections, persistent coughing, shortness of breath, difficulty breathing, infertility (all men, most women), diabetes from pancreatic damage

Question 8

Define ‘bronchiecstasis’

Answer 8

abnormal widening of the bronchi or their branches, in CF due to chronic thick mucus and repeated chest infections

Question 9

What is the heterozygote advantage of a CFTR mutation?

Answer 9

May influence survival of people affected by diseases which involve loss of body fluid (cholera or typhoid etc.)

Question 10

What are the most common types of bacteria which cause lung infections in CF patients?

Answer 10

Staphyloccus aureus, (MRSA), P. aeruginosa, H. influenza, S.maltophilia

Question 11

Describe the treatment of cystic fibrosis

Answer 11

Daily physiotherapy (remove mucus and encourage exercise), pancreatic enzyme tablets (to take with fat-containing foods), higher calorie/protein diets, vitamin supplements (especially fat-soluble vitamins), oral/nebulised/IV antibiotics, transplants and potential gene therapy

Question 12

Outline some of the complications of cystic fibrosis

Answer 12

diabetes, low bone mineral density (bone thinning), nephron/ototoxicity (hearing and kidney function loss) due to antibiotics taken, distal intestinal obstruction syndrome (blockages in stomach –> stomach pain, bloating, nausea and weight loss)

Question 13

Which GI organ is affected by CF and how?

Answer 13

Due to CFTR mutation, the duct system of the pancreas is blocked by viscous secretions so digestive enzymes produced don’t reach duodenum –> self-digestion and can damage the Islets of Langerhans –> (can cause) diabetes

Question 14

Why are CF patients so prone to lung infections?

Answer 14

Thick mucus lining of airways is warm and moist, and thick enough to resist ciliary action so mucus isn’t swept up –> optimal conditions for bacteria to culture and invade cells of airways

Question 15

Why do pH levels have to be strictly controlled?

Answer 15

Enzyme function effectively to facilitate all essential metabolic processes

Question 16

What are the 3 main mechanisms by which the body handles changes in serum pH?

Answer 16

Physiologic (chemical) buffers, renal compensation, pulmonary compensation

Question 17

What are the 3 types of physiologic/chemical buffers?

Answer 17

Bicarbonate-carbonic acid (blood)
Intracellular proteins (between cells)
Phosphate buffers (bone)

Question 18

Define ‘buffer’

Answer 18

Base and conjugate acid OR acid and conjugate base salt

Question 19

Describe how renal compensation balances pH

Answer 19

Starts when other methods ineffective (6 hours later).
In acidosis: excrete H+ and retain HCO3-
In alkalosis: excrete HCO3- and retain H+

Question 20

Describe how bone acts a buffer?

Answer 20

Contains large reservoir of bicarbonate and phosphate so can buffer significant acute acid load

Question 21

Define alkalosis

Answer 21

pH > 7.45

Question 22

Define acidosis

Answer 22

pH

Question 23

What is normal serum pH?

Answer 23

7.40

Question 24

Describe respiratory acidosis

Answer 24

pH less than 7.35, caused by alveolar hypoventilation (not enough CO2 - acid - removal). Relationship between pCO2 and HCO3- determine arterial pH

Question 25

Outline acute respiratory acidosis

Answer 25

accompanied by minimal changes in serum HCO3-, and over 1-3 days renal HCO3- conservation restored pH

Question 26

Outline chronic respiratory acidosis

Answer 26

Occurs secondary to a chronic reduction in alveolar ventilation as seen in COPD

Question 27

Describe respiratory alkalosis

Answer 27

pCO2 reduction –> increase in pH due to increased alveolar ventilation

Question 28

What is the treatment of respiratory alkalosis?

Answer 28

Discovery and correction of underlying etiology

Question 29

Describe metabolic acidosis

Answer 29

Increase in absolute body acid from excess production of acids OR excessive loss of bicarbonate, sodium and potassium caused by lactic acid, diarrhoea etc.

Question 30

Describe the role of the kidney in electrolyte balance

Answer 30

Retain cellular sodium by exchanging it for H+ or potassium (to be excreted) in renal fluid. In the presence of a H+ load, H+ moves into cells, and potassium has to move out to maintain electro neutrality.

Question 31

Outline severe metabolic acidosis

Answer 31

Significant depletion of total body potassium (due to removal in exchange for H+) even if there is serum hyperkalaemia.

Question 32

Describe metabolic alkalosis

Answer 32

When HCO3- is increased,, usually due to excessive loss of acids due to diuretics, prolonged vomiting etc.

Question 33

What is the treatment for metabolic alkalosis?

Answer 33

If chloride concentration is

Question 34

What is the equation for hydrogen bicarbonate as a buffer?

Answer 34

H20 + CO2 H2C03 HCO3- + H+

Question 35

Why do bicarbonate levels not rise in acute respiratory acidosis?

Answer 35

The compensation for an acute respiratory acidosis is by intracellular buffering (such as proteins and phosphates) so plasma bicarbonate rises only slightly and so it does not contribute to the buffering