Physiological Consequences of Airway Obstruction Flashcards
What happens to breathing when there is airflow obstruction?
- increased sensation of breathing
- increased respiratory muscle effort
- active exhalation
- longer time to inspire
- longer time to exhale
- reduced maximum ventilation
- altered pattern of breathing
- sometimes gas trapping
What two factors influence work of breathing?
Load and Drive
What causes increased load?
stiff lungs, narrow airways, chest wall, diaphragm
What causes increased drive?
higher centres, mechanoreceptors, irritant receptors, chemoreceptors, baroreceptors, temperature
What is increase work of breathing?
Where there is an increased respiratory muscle effort
When is mechanical ventilation required?
if PaO2 is less than 60mmHg and PaCO2 is greater than 50mmHg
What are the consequences of increased work of breathing?
recruitment of accessory muscles of breathing, increased oxygen consumption by respiratory muscles, risk of respiratory muscle fatigue
What are the accessory muscles of breathing?
Sternocleidomastoid and scalenes
What happens to expiration in airflow obstruction?
It becomes active
What happens to inspiratory pressure in airflow obstruction?
Have to generate higher than normal inspiratory pressure
What is pulses paradoxis?
During airflow obstruction there is a need to generate greater intrapleural pressure which influences the systolic BP and brings it down
What happens to FEV1 and FVC in airflow obstruction?
FEV1 is decreased but FVC remains the same
What is the FEV1/FVC ratio which indicates airflow obstruction?
less than 70%
How can you tell if airflow obstruction is reversible?
By seeing if a bronchodilator/steroids improves the FEV1/FVC
What is a flow-volume loop?
A measurement of flow rate vs volume in forced inspiration and expiration - used to determine whether it is a lower airway obstruction or upper airway obstruction
What is minute ventilation?
Tidal volume x frequency
What influences pattern of breathing?
The pattern which minimises work of breathing
What is the usual pattern of breathing in airway obstruction?
Deep, slow breaths
What is the usual pattern of breathing in someone with stiff lungs?
Rapid small breaths
What is maximum ventilation rate?
The amount you can breath in one minute - approximately FEV1 X 35
What limits exercise in normal people, ventilation rate or heart rate?
Heart rate
What limits exercise in someone with airflow obstruction?
ventilation rate
What is gas trapping?
In severe airflow obstruction where air can be inspired but not exhaled so is trapped behind the obstruction
What is the result of gas trapping?
High TLC, RV and RV/TLC
What happens to ventilation and perfusion in airflow obstruction?
Inhomogeneity
Why is perfusion uneven in airflow obstruction?
As a compensatory mechanism there is vasoconstriction to reduce V/Q mismatch
What can happen as a result of vasoconstriction?
Increase pulmonary arterial pressure
What is the Alveolar-arterial gradient (A-a gradient)?
The difference between Alveolar PO2 and arterial PO2 - it is a measure of efficiency of gas exchange
How is PAO2 measured?
PAO2 = PiO2 - PaCO2/RQ (RQ = 0.8)
What is the A-a gradient normally?
less than 15
What causes a larger A-a gradient?
V/Q mismatch
