Respiratory expert session - 13.11.17

Question 1

What are the 3 systems that control body pH and how do they work?

Answer 1

ACID-BASE BUFFERING
- chemical buffering by body fluids which immediately combine with acid or base to prevent excessive changes in pH

RESPIRATORY CENTRE
- regulates removal of volatile CO2 as a gas in the expired air and therefore also regulates bicarbonate from the body fluids via pulmonary circulation - response occurs in minutes

KIDNEYS
- excrete either acid or alkaline urine, thereby adjusting pH of blood- this takes place over hours/days but is a more powerful regulatory system

Question 2

What are some potential causes of metabolic alkalosis?

Answer 2

abnormal loss of acid (vomiting gastric HCL) or addition of weak base

Question 3

What are some potential causes of metabolic acidosis?

Answer 3

GI loss in diarrhoea or renal loss in renal disease or another alkali or addition of acids

Question 4

What happens in respiratory acidosis?

Answer 4

inability of the lungs to eliminate CO2 efficiently so the equilibrium shifts towards H+ and HCO3- and pH decreases

Question 5

What happens in respiratory alkalosis?

Answer 5

excessive loss of CO2 through ventilation driving the equilibrium to the left away from H+ and increasing pH

Question 6

What does a Davenport diagram allow a clinician to do?

Answer 6

describe blood bicarbonate concentrations and blood pH following a respiratory or metabolic acid-base disturbance and help work out the cause

Question 7

Why in an acute respiratory acidosis or alkalosis can you have a massive change in pH without a big change in HCO3-?

Answer 7

because the differences are causes by CO2

Question 8

What axis does metabolic acidosis or alkalosis in the davenport move on?

Answer 8

move on an axis related to the ability of the kidney to reabsorb HCO3- therefore massive shifts in HCO3- are associated with minimal shifts in CO2

Question 9

In the absence of compensation what can you do ?

Answer 9

Diagnose whether a changes in pH is due to a metabolic or respiratory acidosis/alkalosis by plotting the bicarbonate value against the pH

Question 10

What has caused just respiratory acidosis?

Answer 10

Lowering of pH
increase in CO2
minimal change in HCO3-

Question 11

What has caused just respiratory alkalosis?

Answer 11

Increased pH
Reduced CO2
minimal change in HCO3-

Question 12

What has caused just metabolic acidosis?

Answer 12

Decrease in pH and HCO3- wth no change in CO2

Question 13

What has caused just metabolic alkalosis?

Answer 13

Increase in pH and HCO3- with no change in CO2

Question 14

What compensation mechanisms occur in respiratory acidosis?

Answer 14

retain HCO3- to push pH up

Question 15

What compensation mechanisms occur in respiratory alkalosis?

Answer 15

excreting more HCO3- bringing pH down

Question 16

What compensation mechanisms occur in metabolic acidosis?

Answer 16

hyperventilation to bring pH up

Question 17

What compensation mechanisms occur in metabolic alkalosis?

Answer 17

hyperventilate to bring pH down

Question 18

What does partial compensation put a patient in?

Answer 18

Chronic respiratory acidosis / alkalosis band

Question 19

What are the different types of upper airway obstructions that affect flow-volume loops?

Answer 19

Variable extra-thoracic obstructions
Variable intrathoracic obstructions
Fixed upper airway obstruction (intra or extra thoracic)

Question 20

What is a variable extra-thoracic obstruction and how does it affect flow-volume loop?

Answer 20

Expiratory part is usually normal - obstruction pushed upwards during expiration
During inspiration it is sucked into the trachea with partial obstruction and flattens inspiratory part

Causes: vocal cord paralysis o a laryngeal tumour

Question 21

What is a variable intra-thoracic obstruction and how does it affect flow-volume loop?

Answer 21

opposite to extra-thoracic
Tumour situated near the intrathoracic part of trachea is sucked outwards during inspiration so its normal but during expiration the tumour is pushed into the trachea with partial obstruction and flattening expiration

Question 22

What is a fixed extra/intra-thoracic obstruction and how does it affect flow-volume loop?

Answer 22

flattened during inspiration and expiration

cause: tracheal stenosis due to intubation, circular tracheal tumour, fixed thyroid goitre

Question 23

How and why does residual volume change in certain respiratory diseases?

Answer 23

RV = amount air left in the lungs at the end of max expiration
Increased when issues with forced expiration and removal of air from lung = emphysema and asthma

Reduced = interstitial lung disease where all lung volume parameters are reduced due to lung fibrosis and scarring

Question 24

What is transpulmonary pressure?

Answer 24

difference between the alveolar pressure and the intrapleural pressure in the pleural cavity

Question 25

Why is transpulmonary pressure not 0 when the chest walls outward pressure and the lungs elasticity recoiling inwards pressure are at equilibrium?

Answer 25

Atmospheric pressure is relatively constant, pressure in lungs must be higher of lower than atmospheric for air flow between atmosphere and alveoli

If transpulmonary pressure is 0 (alveolar pressure = intrapleural pressure), such as when the lungs are removed from the chest cavity or air enters intrapleural space (pneumothorax) the lungs collapse as a result of their inherent elastic recoil

Question 26

When does alveolar pressure equal atmospheric pressure?

Answer 26

between breaths

Question 27

What happens to intrapulmonary pressure during inspiration?

Answer 27

Thorax cavity volume increases
Intrapulmonary pressure or alveolar pressure fall below atmospheric = negative pressure
Air flows into lungs down gradient
At then end of inspiration intrapulmonary pressure again equals atmospheric pressure so airflow stops

Question 28

What happens to intrapulmonary pressure during expiration?

Answer 28

Thorax volume cavity decreases
Intrapulmonary pressure rises above atmospheric
Air flows out down gradient
At the end of expiration, intrapulmonary pressure again equals atmospheric pressure

Question 29

What is intrapleural pressure?

Answer 29

it is the pressure within the pleural cavity and is always negative so it acts like a suction to keep lungs inflated

Question 30

Why is the intrapleural pressure negative?

Answer 30

1) Surface tension alveolar fluid pulling alveoli inward and therefore pulls whole lung inward. Surfactant reduces this force
2) elasticity of the lungs - abundant elastic tissue tend to recoil and pull lung inward. As lungs move away from thoracic wall it creates a slightly larger cavity and this negative pressure created acts as a suction to keep lungs inflated
3) elasticity of thoracic wall which tend to pull away from lungs, to enlarge the pleural cavity and creating this negative pressure. Pleural fluid resists actual separation