Basic Sciences - Respiratory Physiology Flashcards
SI unit for airway pressure
cm H2O
SI unit for partial pressure of gas
kPa
(kilo Pascal)
Definition of cm H2O / mmHg
Equivalent pressure exerted by a column of fluid of a given height, being acted on by gravity
kPa equivalent to 1 Atmosphere at sea level
101
cm H2O equivalent to 1 Atmosphere at sea level
1030
m H2O equivalent to 1 Atmosphere at sea level
10
mmHg equivalent to 1 Atmosphere at sea level
760
Approximate oxygen requirement increase during exercise
Up to 10x increased
Alveolar surface area for gas exchange in adults
80-90 metres squared
Approx half singles tennis court
Approx tidal volume in adults
6-8 ml/kg
Approx 500ml
Anatomical dead space approximate volume
150 ml
Alveolar ventilation approximate volume
350 ml
(Tidal volume - Anatomical dead space)
Minute ventilation calculation
Minute ventilation = Tidal volume x RR
Eg. 500 x 14 = 7000 ml/min
Alveolar minute ventilation calculation
Alveolar minute ventilation = Alveolar ventilation x RR
Eg. 350 x 14 = 4900 ml/min
Spirometry trace and volume terms
Tidal volume
Inspiratory reserve volume
Expiratory reserve volume
Vital capacity
Total lung capacity
Residual volume
Functional residual volume
Approximate inspiratory reserve volume for young fit male
3 L
Approximate expiratory reserve volume for young fit male
1.5 L
Approximate residual volume for young fit male
1 L
Approximate total lung capacity volume for young fit male
6 L
Approximate vital capacity volume for young fit male
5 L
Approximate functional residual capacity volume for young fit male
2.5 L
Functional residual capacity definition
Quantity of gas in the lungs at end of normal expiration
Why functional residual capacity is important in anaesthetics
Provides oxygen reservoir during apnoea
FRC influences distribution of ventilation within lung by determining starting position of each lung area on the compliance curve
Factors which reduce functional residual capacity
Reduced muscle tone:
- General anaesthesia
Increased intrathoracic pressure:
- Supine from standing
- Obesity
- Pregnancy
Negative pressure generated during inspiration in alveoli
Around 2 to 3 cm H2O below atmospheric pressure
What generates positive pressure during expiration in alveoli
Elastic recoil of lungs and chest wall
Dominant muscle contributor during quiet breathing
Diaphragm > intercostal muscles
How does Intermittent Positive Pressure Ventilation generate respiration
Increases pressure at mouth to create pressure gradient
Approximate pressures usually required during IPPV to deliver same tidal volume as normal breathing
Higher pressures needed
Around 10 to 15 cm H2O
Undesirable effect of IPPV on circulation
- Increase in mean intrathoracic pressure
- Reduces venous return to heart
- Fall in cardiac output
Similar effect to Valsalva Manoeuvre
Location of respiratory centres
Medulla and Pons
Description of respiratory centres
Groups of neurons which act together to control respiration
Factors which impact respiratory centres
Central chemoreceptors (increase in CO2)
Peripheral chemoreceptors (decrease in O2)
Cortex (volume control)
Stretch receptors (muscle activity)
Most important factor in controlling respiration and reference range
Arterial PaCO2
Usually kept between 5.1 and 5.5
CO2 response curve
As arterial PaCO2 rises, Minute ventilation increases
Lower end of curve flattens as automatic firing of respiratory centre maintains minimum minute ventilation
Effect of opioids and GA on CO2 response curve
Depress CNS and make chemoreceptors less sensitive to PaCO2
Curve shifts to right and becomes less steep
Two factors which need to be overcome for gas to reach the alveoli
Resistance (obstructive airway disease)
Compliance (restrictive lung disease)
Compliance curve
Initial part of curve - high pressures needed to inflate alveoli from collapsed state
Once inflated lungs become compliant until chest wall reaches limit of expansion
Forces involved in expiration
Passive process usually
Energy used to overcome compliance in inspiration is stored as potential energy in the elastic tissues
This energy used to generate pressure gradient for expiration
Distribution of ventilation in the lungs in normal patient breathing spontaneously at rest
Apices more expanded than bases
Therefore greater compliance at bases and mid zone of lungs
Bases and mid zones receive greater ventilation
Distribution of perfusion in the lungs in normal patient breathing spontaneously at rest
Due to gravity, perfusion pressure reduces by 1 cmH2O for every cm in height above heart level, and increases by 1 cmH2O for every cm below heart level
Ventilation / Perfusion matching in lungs (V/Q)
Both ventilation and perfusion are greater towards bases of lungs
Therefore ventilation and perfusion are well matched
Definition of shunt
Area of lung becomes occluded (airway closure or secretions)
No ventilation to this area of lung
Effect of a lung shunt
Unoxygenated blood mixes with blood from ventilated blood to give arterial hypoxaemia
Increasing inspired O2 will not correct hypoxaemia as area not ventilated
Need to clear obstruction
Definition of alveolar dead space
Perfusion ceases due to occlusion (thrombus or air or drop in cardiac output)
No perfusion to ventilated area
Effect of alveolar dead space
Drop in end tidal CO2
Gas not involved in respiratory exchange dilutes CO2 concentration from rest of lung gas
Treatment of incomplete alveolar obstruction
Increase FiO2 as some ventilation occurs so aim to increase O2 in alveoli for gas exchange
How does general anaesthesia produce a degree of V/Q mismatch
- Fall in FRC
- Lung moves down compliance curve
- Lung bases have less ventilation
- Perfusion distribution unchanged as gravity influenced
Results in V/Q mismatch
