eLFH - Respiratory Physiology Part 3 Flashcards
Hypoxia definition
Deficiency of oxygen for tissue respiration
Hypoxaemia definition
Arterial PO2 < 12 kPa
Hypoxia classification
Hypoxic hypoxia
Anaemic hypoxia
Circulatory (stagnant) hypoxia
Cytotoxic (histotoxic) hypoxia
Delivery of oxygen equation and relation to classification of hypoxia
Cytotoxic (histotoxic) hypoxia
Occurs at cellular level with no deficit in O2 delivery
E.g. cyanide poisoning
Oxygen cascade
Causes of Hypoxic hypoxia
Low FiO2
Hypoventilation
Diffusion defect
V/Q mismatch / shunt
Causes for low FiO2
Inadvertent hypoxic gas mixture
High altitude
Diffusion hypoxia following nitrous oxide administration
Part of oxygen cascade affected by low FiO2
Air
Part of oxygen cascade affected by hypoventilation
Alveolus
Pulmonary capillary
Artery
Organ capillary
Mitochondria
(I.e. alveolus onwards)
Part of oxygen cascade affected by diffusion defect
Pulmonary capillary onwards
Part of oxygen cascade affected by V/Q mismatch / shunt
Artery onwards
Typical O2 consumption in an adult
250 ml/min
Alveolar gas equation
How does hypoventilation lead to hypoxia
Usually secondary to hypercarbia (see alveolar gas equation) as room air with hypoventilation typically sufficient to meet oxygen consumption of 250 ml/min
Causes of hypoventilation
Reduced central respiratory drive
Impaired peripheral mechanisms of breathing
Increased dead space
Causes of reduced central respiratory drive causing hypoventilation
Drugs
Metabolic alkalosis
Intracranial pathology
Alveolar hypoventilation syndrome
Hypothermia
Drugs which reduce central respiratory drive
Opiates
Benzodiazepines
Causes of impaired peripheral mechanisms of breathing causing hypoventilation
Airway obstruction
Restriction
Chest disease
Muscular weakness
Neuromuscular junction impairment
Nerve lesions
Examples of lung restriction
Pain
Obesity
Ascites
Examples of chest diseases
COPD
Asthma
Flail chest
Examples of muscular weakness
Dystrophies
Electrolyte imbalance
Critical illness neuropathies
Examples of neuromuscular junction impairment
Muscle relaxants
Myasthenia gravis
Examples of nerve lesions
Phrenic nerve / spinal cord injuries
Guillain-Barre syndrome
Polio
Causes of increased dead space
Use of anaesthetic equipment
Causes of diffusion defect
Pulmonary fibrosis
Pulmonary oedema
How diffusion defect causes hypoxia
Diffusion defects increases time spent in capillary required for O2 diffusion to be complete
When pulmonary blood flow increases (e.g. exercise), time spent in capillary reduces can can cause clinically relevant hypoxia
Severely abnormal cases are apparent at rest
How far along pulmonary capillary is O2 transfer complete in healthy individuals
~ one third of the way along pulmonary capillary
How much does exercise reduce time blood spends in pulmonary capillary by
Around one third reduced
Normal healthy lung unit with ventilation and perfusion matched
End capillary blood and alveolus have the same PO2 and PCO2
How does dead space manifest clinically
Increased PaCO2-EtCO2 difference
Dead space alveoli not involved in gas exchange so no CO2, therefore dilutes EtCO2 from other alveoli
How does shunt manifest clinically
Hypoxaemia
Unventilated lung unit means end pulmonary capillary blood has same PO2 and PCO2 as venous blood, therefore dilutes PO2 in arterial blood
Lung volumes top vs bottom of lung
Top > Bottom
due to gravity
Ventilation top vs bottom of lung
Bottom > top
Therefore V/Q matched
Perfusion top vs bottom of lung
Bottom > top
due to gravity
Therefore V/Q matched
Effect of decreased FRC under GA on V/Q matching
FRC reduced
Lung moves down compliance curve
V/Q no longer match as ventilation now better at top (as compliance curve steeper) but perfusion still best at bottom of lung
I.e. a shunt has developed
Closing capacity definition
Volume at which airway closure occurs
Closing capacity = Closing volume + Residual volume
I.e. lung volume above residual volume at which airway closes
Relevance of closing capacity
Usually CC much lower than FRC
If CC increases, or FRC decreases, eventually airway closure occurs earlier causing shunt
Factors which increase closing capacity
Age
Increased intrathoracic pressure (e.g. asthma)
Smoking
At which ages does closing capacity = FRC
Neonates and infants
Supine 45 year old
Upright 65 year old
Factors which decrease closing capacity
PEEP / CPAP
Factors which reduce FRC
Head down position
Obesity / pregnancy
GA
Restrictive lung disease
Female
Youth
Measuring closing volume
Fowler’s method
After plateau phase, EtN2 rises again - this stage represents closing volume
Clinical relevance of closing volume
Used more often than closing capacity as can be measured with Fowler’s method
Residual volume needs helium dilution or total body plethysmography to be measured and therefore closing capacity
Why does second N2 rise occur on graph using Fowler’s method
Ventilation better at bottom of lungs
Therefore with only a single breath of 100% O2, bottom alveoli have greater proportion of O2
Top alveoli have higher N2 proportion as have greater volume
Therefore as closing volume reached, bottom alveoli collapse first as they are lower volume
When this occurs, higher proportion of N2 exhaled with lower proportion of O2, therefore get second rise in graph
Usual PaCO2-EtCO2 difference in healthy adults
0.7 kPa
Reason for there being a PaCO2-EtCO2 difference in healthy adults
There is always some dead space
Why does V/Q ratio decrease in bottom vs top of lung
Both ventilation and perfusion increase in bottom of lung, but perfusion more so than ventilation
Vice versa for top of lung
Therefore bottom of lung tends more towards shunt and top of lung tends more towards dead space
Non respiratory functions of the lung
