Respiratory - Static lung volumes Flashcards
Draw the spirometry trace with the various volumes and capacities included. List the volumes and capacities and their numerical values in a 70kg Adult male
Page 50 chambers
VOLUMES Tidal volume - 500 mls Inspiratory Reserve Volume - 2500 mls Expiratory Reserve Volume - 1500 mls Residual Volume - 1500 mls
CAPACITIES Inspiratory Capacity - 3000 mls Functional Residual Capacity - 3000 mls Vital Capacity - 4500 mls Total Lung Capacity - 6000 mls
Define volutrauma and barotrauma and how to prevent these phenomena
Volutrauma (VT 6 ml/kg)
Diffuse alveolar damage caused by over-distension of the lung.
Barotrauma (Ppeak < 35 cmH2O and Pplat < 30 cmH2O)
Damage to the lung as result of high airway and/alveolar pressure.
Lung protective ventilation: Vt 4 - 6 ml/kg Pplat < 30 cmH2O Ppeak < 35 cmH2O Permissive hypercapnoea (In poorly compliant lungs such as in ARDS).
Define functional residual capacity
It is the starting point of tidal respiration.
At end expiration and beginning of inspiration the inspiratory and expiratory muscles are relaxed - inward parenchymal elastic forces = outward chest wall outward force.
Give 3 reasons why FRC is physiologically important.
- O2 BUFFER - continues diffusion of O2 into capillaries during inspiration and expiration. without FRC, diffusion would stop during expiration.
- PREVENTION OF ALVEOLAR COLLAPSE - Atelectasis –> V/Q mismatch –> hypoxaemia. Re-expansion of atelectatic alveoli –> Increase WOB
- OPTIMAL LUNG COMPLIANCE - Lung compliance is optimal at FRC. Vascular resistance PVR is also at its lowest at FRC.
Give 2 reasons why FRC is vital to anaesthetists
- APNOEA - O2 reservoir
- SMALL AIRWAY CLOSURE - If FRC falls below a certain volume (the closing capacity), small airways close, resulting in a V/Q mismatch and hypoxaemia.
List 5 factors that decrease FRC
- Position - supine –> FRC down by 1000ml
- Raised IAP (Pregnancy/Obese/acute abdo/lapscope surgery)
- Anaesthesia - unknown cause: ? reduced chest wall muscle tone and loss of physiological PEEP.
- Decreasing age - neonates, infants and young children have lower FRC
- Lungs disease
- pulmonary fibrosis
- pulmonary oedema
- Atelectasis
- ARDS
List 4 factors that increase FRC
- PEEP
- Emphysema - lost elastic tissue –> chest wall outward spring dominant
- Increasing age - Reduced elastic tissue
- Asthma - air trapping and intrinsic PEEP
Why do we pre-oxygenate patients? give normal FRC, supine FRC and time to hypoxaemia with and without pre-oxygenation.
Preoxygenation is breathing FiO2 100% for 3 minutes. displaces nitrogen from FRC and replaces it with O2
- Normal FRC = 3000 mls (upright)
- Supine patient FRC = 2000 mls
- O2 consumption 250 ml/minute
NO PREOXYGENATION
- FRC contains 450 mls O2
- Hypoxaemia in 1 - 2 minutes of apnoea
PREOXYGENATION
- FRC contains 1800 ml O2
- Hypoxaemia in 6 - 9 minutes of apnoea
Which lung volumes and capacities can be measured with a spirometer? Which lung volumes/capacities cannot be measured using a spirometer?
Vt
IRV
ERV
CANNOT BE MEASURED
RV
Therefore Capacities that include RV cannot be measured by spirometry:
TLC
FRC
As FRC cannot be measured using spirometry, list 3 methods of calculating FRC.
- Gas dilution
- Body plethysmography
- Multiple-breath N2 washout
Summarise the gas dilution method and equation for calculating the FRC
- Helium - does not cross alveolar/capillary
- Known concentration C1 at known volume V1 of Helium is released to patient after tidal expiration (at FRC).
- VT continues to allow He to equilibrate
and new conc. reached C2.
C1V1 = C2(V1 + FRC)
FRC = V1 (C1 - C2)
_________
C2
- Sometimes UNDERESTIMATES as some alveoli closed off esp. in COPD.
Explain the use of plethysmography in calculating FRC
The body plethysmograph is an instrument used for measuring changes in volume within an organ or whole body.
The body plethysmograph applies Boyle’s law to measure functional residual capacity. Boyle’s law states that at constant temperature the volume of a fixed mass of gas is inversely proportional to the absolute pressure. PV = k. Plethysmography applies Boyles law to calculate FRC.
Braw two closed boxes with mouthpiece connected to lungs with shutter and lungs within a box.
P3 x FRC = P4 x (FRC + delta V)
P3, P4, delta V are measured and FRC can therefore be calculated.
Page 53 chambers.
How is FRC measured using the multiple-breath nitrogen washout method?
- Patient breathes room air
- At FRC (end tidal expiration) switch to 100% O2.
- From next expiration all expired gases pass through N2 analyzer and are collected.
- Breathing: N2 is replaced by O2.
- Total volume of expired N2 is calculated from the volume of expired gas multiplied by the concentration of N2 within the collected gas..
- FRC = total expired N2 volume x [N2]f/[N2]i
What is closing capacity
Lungs affected by gravity
Upright –> basal compression + apical stretching.
Basal compression –> closure of small airways here during forced expiration.
The lung volume at which these airways close is called the closing capacity which is made up of the residual volume and the closing volume
CC = CV + RV
What happens if closing capacity is greater than FRC.
Under which circumstances could CC be greater than functional residual capaicity?
Closure of small airways (particularly in the basal region) during tidal breathing –> V/Q mismatch –> hypoxaemia
CC > FRC
- FRC is lower than normal (neonates have reduced FRC)
- CC is increased - CC increases with age, encroaching on FRC at age 45 when supine and age 60 when standing
