integrated control of ventilation

Question 1

what is respiratory acidosis ?

Answer 1

• occurs when ventilation is insufficient relative to the metabolic demands of the body
  Insufficient ventilation
  ↓
  ↑PACO2
  ↓
  ↑PaCO2 (hypercapnia)
  ↓
  ↑[H+] = ↓pH (acidosis)
• this leads to increased chemoreceptor activity and increased respiratory rate
• In the event of chronic hypo-ventilation, HCO3- excretion must decrease to maintain normal pH.

Question 2

what is respiratory alkalosis ?

Answer 2

• occurs when ventilation is excessive relative to the metabolic demands of the body
  Excessive ventilation
  ↓
  ↓PACO2
  ↓
  ↓PaCO2 (hypocapnia)
  ↓
  ↓[H+] = ↑pH (alkalosis)
• this results in increased HCO3 excretion via kidney decreased chemoreceptor activity and decreased respiration rate.
• In the event of chronic hyperventilation, HCO3- excretion must increase to maintain normal pH.

Question 3

what is are some causes of hyperventilation/ respiratory alkalosis ?

Answer 3

anxiety:
Anxiety trigger → panic attack
↓
Increased ventilation (tachypnoea) without increased metabolic demand
↓
Hyperventilation → Decreased PaCO2
↓
Respiratory alkalosis

altitude:
At sea level, PAtm ≈ 100kPa
↓
At 8000ft, PAtm ≈ 75kPa
↓
↑ alveolar ventilation to try and maintain normal PAO2 & PaO2 (via hypoxic drive)
↓
Increased ventilation = ↓PaCO2
↓
Respiratory alkalosis

Question 4

what is metabolic acidosis ?

Answer 4

• occurs when metabolic acid production is excessive and/or [HCO3-] is deficient
• Unless PaCO2 changes in proportion with [HCO3-] (in the opposite direction), acidosis (↓pH) will occur.
• Respiratory compensation (↑ventilation) occurs almost instantly, and attempts to return pH back to normal range. However, if patients have simultaneous respiratory pathology, this may not be possible)

Question 5

what are some causes of metabolic acidosis ?

Answer 5

sepsis:
Infection
↓
Pathological immune response
↓
Septic shock
↓
↓oxygen delivery to tissues (hypoxia)
↓
↑Lactic acid production & ↓pH
Respiratory compensation to ↑O2 delivery and ↑lactic acid conversion

diabetic ketoacidosis:
↑ fatty acid release from liver due to insulin deficiency =
↑ (acidic) ketone body production

renal failure:
decreased HCO3- reabsorption in proximal tubule)

diarrhea
decreased HCO3- absorption in colon

Question 6

what is metabolic alkalosis ?

Answer 6

• occurs when metabolic acid excretion is excessive and/or [HCO3-] becomes excessive
• Unless PaCO2 changes in proportion with [HCO3-] (in the opposite direction), alkalosis (↑pH) will occur.
• Respiratory compensation (↓ventilation) attempts to return pH back to normal range.

Question 7

what causes metabolic alkalosis ?

Answer 7

• Diuretics (changes to H+ and HCO3- reabsorption)
• Antacid “abuse” (↑HCO3- consumption)
• Vomiting (loss of H+ in stomach acid)

Question 8

what is acidosis induced hyperkalemia ?

Answer 8

• The homeostasis of potassium levels within the blood is dependent on pH.
• Cells have multiple mechanisms for indirectly exchanging H+ for K+
• one mechanism is via initial exchange of H+ and Na+ (hydrogen efflux, sodium influx), followed by Na+ and K+ exchange (sodium efflux, potassium influx) – the net effect is that potassium ions enter the cell in exchange for hydrogen ions leaving.
• However the initial step involving diffusion of H+ out of the cell in exchange for Na+, relies on the presence of a H+ concentration gradient – the concentration of H+ inside the cell must be greater than the concentration outside the cell.
• In the event acidosis, this process breaks down as pH falls, representing an increase in blood and extracellular [H+].
• The overall effect is that uptake of K+ by cells is reduced in response to acidosis, leading to potassium accumulation within the extracellular fluid in blood.
• Because K+ have critical roles in membrane potential and muscle function, cells such as skeletal muscle and cardiac pacemaker cells are adversely effect, as can be observed by the symptoms of cardiac arrhythmia and muscle weakness observed in patients with hyperkalaemia.

Question 9

what is alkalosis induced vasoconstriction ?

Answer 9

• The contractile tone of vascular smooth muscle is sensitive to changes in pH, with cerebral arteries being particularly sensitive.
• Carbon dioxide (via conversion to H+) acts as vasodilator, relaxing smooth muscle and increasing blood flow.
• In the event of hyperventilation-induced alkalosis, reductions in PaCO2 and H+ (↑pH) induce vasoconstriction of cerebral arteries, reducing blood flow to the brain and causing symptoms of headache, light-headedness, and (eventually) seizures or loss of consciousness.

Question 10

summary of acidosis and alkalosis

Answer 10