80 - Physiological Consequences of Increased Fluid Movement across Pulmonary Membranes Flashcards
Features of pulmonary circulation
Low pressure, low resistance
Why can the pulmonary circulation be low pressure?
Mostly at the level of the heart, so don’t need pressure to push fluid against gravity
Pulmonary artery pressure
25/8 (systolic/diastolic)
Mean is 15mmHg
Capillary pressure
12/8 mmHg
What would happen if pulmonary circulation were high pressure?
Fluid would leak from vessels into the alveoli.
This would impair gas exchange
Components of pulmonary circulation
Thin-walled vessels, thin right atrium, ventricle
Capillary volume in pulmonary circulation at rest
60-80mL
Effect of exercise on pulmonary circulation pressure
No effect.
Vasodilation prevents pressure rise with increased CO
Effect of inspiration on pulmonary circulation
Pooling of blood
Amount of deoxyhaemoglobin for cyanosis to be visible
4g/L
Signs of a metabolic acidosis
Acidosis without elevated blood CO2. Low bicarbonate (EG: loss from diarrhoea, or acting as a buffer for elevated lactic acid)
Signs of a respiratory acidosis
Elevated CO2 with decreased pH
Anion gap
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.
Sum of the commonly measured anions and cations
Difference in charge is normally under 15.
How does the anion gap appear with metabolic acidosis, from introduced acid
Bicarbonate decreases (neutralising acid). Anion gap goes above 15.
How does the anion gap appear with metabolic acidosis from diarrhoea
Gap stays the same, as although HCO3- is being lost, the body tries to retain Cl-, to maintain anion gap
Clinical features of pulmonary oedema
1
2
3
Impaired gas exchange
Metabolic acidosis
Tissue hypoxia
Factors determining fluid movement across pulmonary capillaries
1
2
3
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)]
What drives alveolar oedema in pulmonary oedema?
Increased hydrostatic pressure within capillaries
What causes respiratory alkalosis?
Decreased blood CO2.
Exhaling too much CO2
How to distinguish between metabolic and respiratory alkalosis
Observe whether CO2 is too low, or normal
ARDS
Adult respiratory distress syndrome.
Respiratory capillaries become hyper-permeable.
Often in response to inflammation of some sort
Effects of adult respiratory distress syndrome
Decrease V/Q units, shunt, leading to decreased PaO2.
Stiff lungs increases elastic work, leading to increased O2 demand.
Difference between cardiogenic pulmonary oedema and ARDS
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
Normal rate of lymph flow
~20mL/hour from lungs
When does fluid begin to accumulate in interstitial spaces and alveoli?
When it overwhelms the rate of lymphatic drainage.
Alveoli begin flooding after the interstitium is flooded with fluid.
Mechanical changes in pulmonary oedema. 1 2 3 4
Decreased lung compliance
Decreased lung volumes (collapse of alveoli)
Increased airway resistance
Increased work of breathing (can hear wheezing)
Effect of pulmonary oedema on gas exchange
Hypoxaemia due to shunt, low V/Q, diffusion impairment.
Effect of pulmonary oedema on blood gasses 1 2 3 4
Decreased PaO2
Decreased PaCO2
Increased pH
If very severe, increase in PaCO2, decrease pH (metabolic and respiratory acidosis)
Effect of pulmonary oedema on pulmonary circulation
Increases resistance
Permeability of capillary endothelium
Capillary endothelium is highly permeable to water, ions and small molecules (not protein)
Permeability of alveolar epithelium
Alveolar epithelium is not and actively pumps water from the alveoli into the interstitial spaces
Is interstitial oedema more or less severe than alveolar oedema?
Interstitial oedema causes little functional effect but alveolar oedema has a large effect on lung function
Causes of pulmonary oedema
1
2
- Increased capillary hydrostatic pressure
eg left ventricular dysfunction, mitral stenosis, fluid overload, pulmonary veno-occlusive disease - Increased capillary permeability
eg toxins, sepsis, multiple trauma, aspiration of gastric acid
? high altitude, ? heroin, ? neurogenic
Does altered colloid pressure lead to pulmonary oedema?
In theory, but not in practise.
