Respiratory physiology Flashcards
How is DLCO measured?
Patient inhales carbon monoxide which diffuses across the alveolar membrane and is then exhaled and amount of exhaled is measured
What does DLCO tell you?
integrity of alveolar membrane
What can cause decreased DLCO?
Reduction in lung surface area (emphysema)
Increased thickness of membrane (ILD)
Pulmonary hypertension
Anaemia (due to decreased partial pressure gradients, not enough Hb drawing O2 across into alveoli)
What are causes of increases DLCO?
Excercise, lying supine
asthma
pulmonary haemorrahge (increased blood in alveoli bind CO2 before diffusing across membrane)
polycythemia
Mild left heart failure with increased left sided pressures
Obesity - controversial
When is DLCO measurement useful?
in determining cause of restriction:
- pulmonary (ILD) vs non pulmonary (obesity, chest wall disorders, neuromuscular weakness
in those with normal lung functions who you are suspicious have pulmonary hypertension - pulmonary HTN causes reduced DLCO
Examples of variable extrathoracic obstruction?
Vocal cord paralysis (most common)
Extrathoracic goitre
tracheomalacia
laryngeal tumors
Pathogenesis behind variable extrathoracic obstruction?
tumor/obstruction gets sucked inwards towards trachea in inspiration and pushed outwards on expiration
- flow volume loop shows normal expiration but flattened off inspiration
Examples of variable intrathoracic obstruction?
Tumor in intrathoracic portion of trachea, tracheomalacia (affected intrathoracic portion)
Findings in variable intrathoracic obstruction?
Opposite to extrathoracic
- gets pushed outwards during inspiration due to increasing intrathoracic volume and gets pulled in during expiration due to decreasing volumes
- flow volume loop shows normal inspiration but flattened off expiration
What is an example of a fixed airway obstruction?
Circumferential tracheal tumor
Tracheal stenosis from intubation etc.
Both inspiration and expiration are flat on flow volume loop
What is the algorithm for evaluating lung function tests?
FEV1/FVC ratio - less than 0.7 obstruction - greater then 0.7 restrictive or normal FVC - reduced - possible mixed or restrictive - normal - normal or obstructive TLC - reduced - restrictive - normal or enlarged - obstructive DLCO - reduced - restrictive, obstructive, pulmonary vascular - normal or high with restrictive pattern suggest extra-pulmonary restriction
What spirometry measurement is most affected by obstructive airways disease?
FEV1
- unable to move air quickly past obstructed airways
What spirometry measurement is most affected by restrictive airways disease?
FVC
- reduced as lung volumes are reduced
What is the concept of psuedorestriction?
In severe obstruction testing with spirometry only may falsely show restriction as RV is so markedly increased due to air trapping that all other lung volumes are low including FVC
What is tests can assess residual volume and total lung capacity?
Helium dilution
Nitrogen washout
Body plethysmography - most accurate, one we do withing a box
Estimation from CT or CXR
What is the residual volume?
The amount of air left in the lung at the end of inspiration
What is the total lung capacity?
The maximum inspiration and expiration + RV
- total gas holding capacity of lung
What is the tidal volume?
amount of air breathed in and out at relaxed breathing
What is the vital capacity?
The maximum inspiration to expiration (maximum volume that you can breathe at)
TLC - RV = VC
What is the functional residual capacity?
From the bottom of the tidal volume to the end of RV
The residual lung capacity at the end of expiration of normal breathing
What measurements increase with air trapping?
RV, FRC and TLC
How is CO2 carried in the blood?
Predominantly by bicarbonate ions
What causes a shift of the Hb dissociation curve to the right?
Increased: Temperature Excercise Co2 Acid (lower pH) 2,3 DBG
What causes a shift in the Hb dissociation curve to the left?
Decreased: Temperature Co2 2,3 DBG Acid (higher pH)
What does a shift in the Hb dissociation curve to the right mean?
Easier for Hb to release oxgyen to the tissues
Harder for hit to bind oxygen molecules
What dose a shift in the Hb dissociation curve to the left mean?
Easier for Hb to bind oxygen molecules (higher affinity for O2)
Harder for it to release it to tissues
What does cooperative binding of O2 to Hb mean?
once a single oxygen molecule has bound it induces a confirmational change so that the Hb binds oxygen more readily
What is responsible for the steep part of the oxygen Hb curve?
Cooperative binding of oxygen to Hb - binding of a single oxygen molecule causes increased rate of binding of others
What cells produce surfactant?
Type 2 alveolar cells
What is involved in inspiration?
Contraction of diaphragm pulls downwards and ribs upwards
External intercostals pull ribcage out and upwards
Increased intrathoracic volume, decreased intrapulmonary pressure which draws air into lungs
Use of accessory muscles in excercise or disease states
What is involved in expiration?
Usually a passive process
Diaphragm relaxes, thoracic cavity decreases in volume and air is pushed out of the lungs
Can use the abdominal muscles and internal intercostals in excercise or disease
What is the usual pressure in the pleural cavity?
-4-5mmhg in relation to atmospheric pressure
Why is pleural pressure important?
Negative pressure in pleura compared to intrapulmonary causes a transpulmonary pressure which prevents lungs from collapsing
Carbon monoxide affect on oxygenation?
binds irreversibly to Hb to form carboxyhaemoglobin so Hb cannot carry O2
Has higher affinity for Hb by 250x then oxygen
How many oxygen molecules can 1 Hb bind?
4
What is the main form of carbon dioxide transport?
Bicarbonate
What is the normal anion gap?
4-12
What are some causes of high anion gap acidosis?
MUDPILES methanol Uremia Diabetes propylene glycol Iron/isoniazid lactic acidosis ethlyne glycol salycilates
What are some causes of normal anion gap acidosis?
ABCD Addisons Bicarbonate loss (GI - diarrhoea, renal - prox RTA) Chloride excess Dieuretics, acetazolimide
What are causes of hypoxia with a normal Aa gradient?
alveolar hypoventilation
low FiO2
What are some causes of hypoxia with increased Aa gradient?
diffusion deficit
VQ mismatch
R to left shunt
What is the alveolar gas equation?
PAO2 = 150 - (PCO2/0.8)
What is the normal Aa gradient?
5-10 but increases with age
What is the delta ratio used for?
in a high anion gap metabolic acidosis to determine if there is a co-existing respiratory acidosis or metabolic alkalosis
change in anion gap divided by change in bicarbonate
What is physiological VQ mismatch?
ventilation higher at apexes
Perfusion higher at bases
