18 Flashcards
what’s the fick principle
The volume of gas per unit time which diffuses across a membrane surface (eg the alveolar membrane) is
- proportional to the area of the surface,
- inversely proportional to the thickness
- Proportional to the difference in partial pressure of the gas on the two sides
- Dependant upon the permeability coefficient for that gas in the membrane (related to the solubility)
what key aspects of lungs make it good for gas transport
Large exchange area
think diffusion membrane
high partial pressure difference
high permeability coefficient
when is gas transfer from alveoli to capillaries reduced
Reduced membrane surface area, (lobectomy or reduced effective membrane area with pneumonia)
Decreased diffusion across membrane with pulmonary fibrosis, and acute lung injury
Decreased partial pressure of oxygen as in high altitude
Collapse of alveoli during expiration due to lack of surfactant
Poor pulmonary perfusion of alveolar membrane
pathophysiology of pneumonia
alveoli become filled with bacteria which make toxins. inflammation. so alveolar sac becomes packed w fluid and cant take in oxygen
why does radiating therapy result in upper respiratory tract infections
bc kills of macrophages on alveoli periphery
pathophysiology of pulmonary fibrosis
fibroblast proliferate in the lungs (lung ‘scarring’) and reduce alveolar size and numbers
main obstructive lung disease
COPD asthma
main restrictive lung disease
fibrosis
what effect does altitude have on partial pressure
Oxygen fraction stays constant but total pressure decreases with height; therefore oxygen partial pressure decreases with height; lower partial pressure of inspired air – lower partial pressure of alveolar air – less saturation of haemoglobin in lungs. Can get altitude sickness if respiration rate not increased
what happens to your body when you’re altitude
receptors in carotid bodies sense low oxygen so start to make you breath more but then your drop in Co2 means CSF is more alkaline than it should be and bc its dominant you stop breathing fast.
which cells make surfactant
type II pneumocystis
what are the different forms of surfactant
There are different forms of surfactant (with different proportions of lipoproteins type A,B,C, D) which make with different properties:
Surfactant containing lipoproteins B &C is used to reduce surface tension and ensure proper lung function.
Surfactants containing lipoproteins A & D coat bacteria and viruses and help the immune system deal with them. (surfactant D deficiency is a particular risk factor for pulmonary tuberculosis)
‘Infant Respiratory Distress Syndrome’
fetal lung alveoli are shut closed by surface tension and NO surfactant.
surfactant made in weeks 24.28
surfactant deficiency in adults cause
Bacterial or viral disease can affect type 2 pneumocyte function and thus surfactant release
how come sometimes people with COPD can have a small or no change in airway compliance
bc of an increase in compliance in the upper zones combined w a decrease in lung airway compliance at the bases of the lungs
This reduces V/Q still further at the bases and decreases oxygen uptake in the zone where the blood flow is high.
what is the PACO2 and PAO2 of blood leaving the base of the lung
low PAO2 and high PACO2
bc less good gas exchange but greater blood flow in the base
Persistent fetal circulation cause
caused by a failure in the systemic circulation and pulmonary circulation to convert from the antenatal circulation pattern to the “normal” pattern at birth.
what other condition is Persistent fetal circulation associated with
pulmonary hypertension
how to reduce or eliminate PPHN
by the addition of the vasodilator nitric oxide to the inspired gas in the infant ventilator.
why does NO work as vasodilator
increases cGAMP which sequesters Ca in smooth muscle and makes it relax
why use spirometry
Assess the prognosis of respiratory disease in a patient.
Assess whether lung disease is present at an early stage, i.e. prior to overt clinical disease.
Assist in quantifying the severity of airway disease.
Assess the effect of therapy, such as corticosteroids, bronchodilators
Delineate risk factors, e.g. the odds of developing future respiratory disease, or operative risks.
Monitor whether the pattern of lung growth or aging is normal.
lung function test measurement
spirometry
Total lung capacity
inclues forced vital capacity plus residual volume plus physiological dead space.
TLC, the total volume of the lungs, cannot be measured directly by spirometry.
It is measured by inspiration of a fixed volume of a gas mixture which includes helium, followed by rebreathing the mixture until the helium is evenly distributed in the lung gases. Helium does not get absorbed into the blood, so if we sample the rebreathed gas it will contain a lower concentration of helium than the original inspired gas as it has penetrated the residual volume. Let C1 be concentration of helium in inspired gas & V1 volume of inspired gas. Similarly let C2 be concentration in rebreathed gas and V2 the volume (ie V2 = TLC)
Peak flow measurement
measures FEV1
The peak flow rate assess airway resistance
The test is useful in patients with obstructive lung disease (e.g. asthma and COPD)
It is measured by the patient taking a full inspiration and then giving a short sharp blow into the peak flow meter
The average of three attempts is usually taken
The peak flow rate in normal adults vary with age and height
which is done more easily and commonly peak flow or total vital capacity
peak flow
what test would you do to test for COPD
peak flow
peak flow for man
660 l/min
peak flow for woman
320 l/min
Lung Function testing
4) Tests of ventilation/perfusion (V/Q) matching
if there is poor saturation of arterial haemoglobin (hypoxaemia), V/Q ratio can be measured
V/Q ratios can be analysed using isotope scanning
Whole lung reduced ventilation with normal perfusion (reduced V/Q ratio) in pneumonia causes hypoxaemia
Decreased perfusion with normal ventilation (too high V/Q ratio) in pulmonary emboli also causes hypoxaemia
in COPD Regional V/Q mismatch due to regional emphysema reduces the efficiency of gas transfer and thus hypoxaemia
5) Measurement of gas transfer
The efficiency of oxygen transport across the alveolar membrane can be measured by comparing alveolar oxygen levels (PAO2) with arterial levels (PaO2)
Alveolar oxygen partial pressure can be obtained by analysis with an oxygen meter of the ‘end tidal ‘ expired gas, ie the last portion of expired gas, which will have the same composition as alveolar gas.
Arterial oxygen partial pressure can be measured with a pulse oximeter which measures the saturation of the haemoglobin with oxygen. The haemoglobin oxygen saturation is related to the oxygen partial pressure by the oxygen-haemoglobin saturation curve
exercice training
How far can the patient walk, is it enough to carry out essential tasks ?
Physiotherapy and OT assessment
Assess breathlessness on exercise with visual analogue score
Treadmill or bicycle exercise tests show ventilation as a function of work and anaerobic threshold, which correlates with cardiopulmonary capacity and operative risk
Main Tests used to assess lung function
Tests of lung volumes (spirometer)
Tests of total lung capacity using helium
Tests of airflow (peak flow meter/vitalograph)
Tests of V/Q mismatch using isotopes
Tests of gas transfer (compare PO2 at different sites)
Tests of challenge by exercise
