Gas exchange & transport

Question 1

How to measure the arterial pressure of CO2 PACO2?

Answer 1

• End Tidal CO2 measurement this is the same as the alveolar pressure -ETCO2

Question 2

What factors determine the PAO2 and are used in the alveolar gas equation?

Answer 2

= [(Patm – PH20) x FiO2] – (PaCO2/R) - partial pressure of inspired oxygen- partial pressure of leaving oxygen

Question 3

What conditions can cause reduction in CaO2 (arterial oxygen content)?

Answer 3

• competitive inhibitor i.e CO
• Fe2+ oxidixed to Fe3+, as it is ineffective at binding and delivering oxygen

Question 4

What controls respiration?

Answer 4

• the PONS and the Medulla

Question 5

What are the inputs that modify or modulate the breathing rhythm?

Answer 5

• the cerebral cortex - lung mechanoreceptors - chemoreceptors )central and peripheral)

Question 6

How do chemoreceptors in the brain work? Central Chemoreceptors

Answer 6

• clusters of cells by the CSF - detects pH through the carbonic anhydrase - CO2 from the blood is converted - slightly slower than peripheral receptors

Question 7

How do peripheral chemoreceptors work? Carotid and Aortic Bodies

Answer 7

• As arterial blood PO₂ decreases below its normal value of about 95 mmHg, there is a progressive increase in receptor firing rate.
• Any elevation of PCO₂ above a normal value of 40 mmHg, or a decrease in pH below 7.4 enhances receptor firing at a given PO₂.
• The cardiovascular response to arterial chemoreceptor activation is determined, in part, by the respiratory response.
  
  • If respiratory activity is not allowed to change during chemoreceptor stimulation (thus removing the influence of lung mechanoreceptors), then chemoreceptor activation of the carotid bodies causes bradycardia and coronary vasodilation (both via vagal activation) and systemic vasoconstriction (via sympathetic activation).
  • If respiratory activity increases in response to the chemoreceptor reflex, then increased sympathetic activity stimulates both the heart and vasculature to increase arterial pressure.

Question 8

How do opioids affect the alveolar ventilation?

Answer 8

• morphine reduces the alveolar ventilation - hypercarbic respiratory failure

Question 9

What is the ventilatory response to PO2?

Answer 9

• much less sensitive than PCO2

Question 10

What is ABG?

Answer 10

• Arterial Blood Gas

Question 11

What are the six steps to interpreting an ABG?

Answer 11

1) pH, PCO2, HCO3- 2) alkaline or acidic pH 3) metabolic acidiosis, anion fap 4) compensatory process? 5) mixed acid-base disorder is present 6) cause?

Question 12

Explain what Non-anion gap metabolic acidosis is?

Answer 12

• the loss of bicarb

Question 13

What is the anion gap?

Answer 13

• cations must equal anions - K+ and Na+, Cl- HCO- - the gap between the number of uncounted anions and uncounted cations - shouldn’t be greater than 12

Question 14

What are the main causes for an elevated anion gap?

Answer 14

• Glycols, Oxoproline, Lactate, D Methanol, Aspirin, Renal failure , Ketoacidosis -

Question 15

What are the compensation for respiratory acidosis?

Answer 15

• increased HCO3 reabsorption by the kidney

Question 16

What are the compensation for respiratory alkalosis?

Answer 16

• reduced H+ excretion in the kidney
• increased HCO3 excretion

Question 17

What are the compensation for metabolic acidosis?

Answer 17

• tachypnoea/ hyperventilation

Question 18

What are the compensation for metabolic alkalosis?

Answer 18

• hypoventilation

Question 19

What are the causes of Metabolic acidosis?

Answer 19

• increased acid production due
• hyperchloremic metabolic acidosis are gastrointestinal bicarbonate loss,
• renal tubular acidosis,
• drug-induced hyperkalemia,
• early renal failure
• administration of acids.

Question 20

What are the causes of Metabolic alkalosis?

Answer 20

• increased HCO3-
• excess antiacids
• diuretics
• hypokalemia
• heart, kidney or liver failure
• Genetics

Question 21

What are the causes of respiratory acidosis?

Answer 21

• reduced ventilation
• toxins or CNS disease
• airflow obstruction:- COPD, asthma, sleep apnea, airway edema

Question 22

What are the causes of respiratory alkalosis?

Answer 22

• hyperventilation
• stress/panic

Question 23

What is Venous admixture?

Answer 23

The mixing of highly oxygenated arterial blood mixing with oxygen-poor venous blood due to shunting.

Question 24

What does Fick’s law of Diffusion state and how does this relate to shunting?

Answer 24

The rate of diffusion is proportional to the pressure difference across the membrane

• the large difference in PaO2 and PvO2 drives the transport of oxygen
• the drop in intravascular PO2 favours Hb leading to abundant transfer of )2 out of the blood an into the tissue

Question 25

What 3 ways can CO2 be transported in the blood?

Answer 25

• Bicarbonate (90%)
• Carbamino compounds (~5%), mostly Hb-CO2
• soluble gas (5%)