80 - Physiological Consequences of Increased Fluid Movement across Pulmonary Membranes

Question 1

Features of pulmonary circulation

Answer 1

Low pressure, low resistance

Question 2

Why can the pulmonary circulation be low pressure?

Answer 2

Mostly at the level of the heart, so don’t need pressure to push fluid against gravity

Question 3

Pulmonary artery pressure

Answer 3

25/8 (systolic/diastolic)

Mean is 15mmHg

Question 4

Capillary pressure

Answer 4

12/8 mmHg

Question 5

What would happen if pulmonary circulation were high pressure?

Answer 5

Fluid would leak from vessels into the alveoli.

This would impair gas exchange

Question 6

Components of pulmonary circulation

Answer 6

Thin-walled vessels, thin right atrium, ventricle

Question 7

Capillary volume in pulmonary circulation at rest

Answer 7

60-80mL

Question 8

Effect of exercise on pulmonary circulation pressure

Answer 8

No effect.

Vasodilation prevents pressure rise with increased CO

Question 9

Effect of inspiration on pulmonary circulation

Answer 9

Pooling of blood

Question 10

Amount of deoxyhaemoglobin for cyanosis to be visible

Answer 10

4g/L

Question 11

Signs of a metabolic acidosis

Answer 11

Acidosis without elevated blood CO2.
Low bicarbonate (EG: loss from diarrhoea, or acting as a buffer for elevated lactic acid)

Question 12

Signs of a respiratory acidosis

Answer 12

Elevated CO2 with decreased pH

Question 13

Anion gap

Answer 13

Difference in concentrations of commonly-measured anions and cations in the blood (Anions and cations won’t be different, just measuring the common ones, EG: K+, Na+, Cl-, HCO3-).

Used to measure electrolyte imbalance.

Question 14

Sum of the commonly measured anions and cations

Answer 14

Difference in charge is normally under 15.

Question 15

How does the anion gap appear with metabolic acidosis, from introduced acid

Answer 15

Bicarbonate decreases (neutralising acid).
Anion gap goes above 15.

Question 16

How does the anion gap appear with metabolic acidosis from diarrhoea

Answer 16

Gap stays the same, as although HCO3- is being lost, the body tries to retain Cl-, to maintain anion gap

Question 17

Clinical features of pulmonary oedema
1
2
3

Answer 17

Impaired gas exchange
Metabolic acidosis
Tissue hypoxia

Question 18

Factors determining fluid movement across pulmonary capillaries
1
2
3

Answer 18

Hydrostatic pressure inside and outside the capillary (Pc, Pi)
Oncotic pressure inside and outside the capillaries (Oc, Oi)
Permeability of the capillary (sigma)

Net fluid out = K[(Pc-Pi)-sigma(Oc-Oi)]

Question 19

What drives alveolar oedema in pulmonary oedema?

Answer 19

Increased hydrostatic pressure within capillaries

Question 20

What causes respiratory alkalosis?

Answer 20

Decreased blood CO2.

Exhaling too much CO2

Question 21

How to distinguish between metabolic and respiratory alkalosis

Answer 21

Observe whether CO2 is too low, or normal

Question 22

ARDS

Answer 22

Adult respiratory distress syndrome.
Respiratory capillaries become hyper-permeable.
Often in response to inflammation of some sort

Question 23

Effects of adult respiratory distress syndrome

Answer 23

Decrease V/Q units, shunt, leading to decreased PaO2.

Stiff lungs increases elastic work, leading to increased O2 demand.

Question 24

Difference between cardiogenic pulmonary oedema and ARDS

Answer 24

Very similar in symptoms (flooded alveoli), but ARDS can be unilateral, doesn’t have cardiomegaly.
Cardiogenic is from elevated hydrostatic pressure, ARDS from increased capillary permeability

Question 25

Normal rate of lymph flow

Answer 25

~20mL/hour from lungs

Question 26

When does fluid begin to accumulate in interstitial spaces and alveoli?

Answer 26

When it overwhelms the rate of lymphatic drainage. | Alveoli begin flooding after the interstitium is flooded with fluid.

Question 27

``` Mechanical changes in pulmonary oedema. 1 2 3 4 ```

Answer 27

Decreased lung compliance Decreased lung volumes (collapse of alveoli) Increased airway resistance Increased work of breathing (can hear wheezing)

Question 28

Effect of pulmonary oedema on gas exchange

Answer 28

Hypoxaemia due to shunt, low V/Q, diffusion impairment.

Question 29

``` Effect of pulmonary oedema on blood gasses 1 2 3 4 ```

Answer 29

Decreased PaO2 Decreased PaCO2 Increased pH If very severe, increase in PaCO2, decrease pH (metabolic and respiratory acidosis)

Question 30

Effect of pulmonary oedema on pulmonary circulation

Answer 30

Increases resistance

Question 31

Permeability of capillary endothelium

Answer 31

Capillary endothelium is highly permeable to water, ions and small molecules (not protein)

Question 32

Permeability of alveolar epithelium

Answer 32

Alveolar epithelium is not and actively pumps water from the alveoli into the interstitial spaces

Question 33

Is interstitial oedema more or less severe than alveolar oedema?

Answer 33

Interstitial oedema causes little functional effect but alveolar oedema has a large effect on lung function

Question 34

Causes of pulmonary oedema 1 2

Answer 34

1. Increased capillary hydrostatic pressure eg left ventricular dysfunction, mitral stenosis, fluid overload, pulmonary veno-occlusive disease 2. Increased capillary permeability eg toxins, sepsis, multiple trauma, aspiration of gastric acid ? high altitude, ? heroin, ? neurogenic

Question 35

Does altered colloid pressure lead to pulmonary oedema?

Answer 35

In theory, but not in practise.