Physiology of respiratory failure:
S.Mahadevan,MD,
V.R.Pattabhiraman,MD,DNB,FCCP
Arjun Srinivasan,MD,DM
Pulmonology Associates
KMCH
• Hypoxia
• Hypoxaemia
Low oxygen in the blood.
Low oxygen in the tissues.
Sepsis
Hypotension
Low cardiac output state
Poisoning, cellular toxins
Low hemoglobin
KMCH
• Ventilation
Respiratory
Alveolar
• Perfusion
• The blood that are in the
pulmonary arteries.
The air that you breathe that
takes part in respiration.
Minute Ventilation=Tidal
volumeX Respiratory rate.
KMCH
Dead space
• Anatomical dead space
• Physiological
• The air in the conducting
space of the airways like the
nasal cavity , trachea , main
bronchus , bronchioles,
alveolar ducts.
• The air in the air sacs that
do not take part in gas
exchange.
KMCH
KMCH
Muscles of respiration:
KMCH
Diaphragmatic response to respiratory failure:
•
•
•
•
•
Maximal blood flow due to greater capillary density.
Oxygen uptake is greater.
Increased mitochondrial density.
Oxidative fibers are more (80% vs 40%).
Fast twitch muscle fibers are designed to contract at
low stimulus.
KMCH
KMCH
Respiratory physiology:
KMCH
Alveolar gas exchange.
KMCH
Clinical scenario:
• 40 year old male , BMI 33,underwent emergency
upper abdominal surgery under GA , shifted.
• 9PM,breathless,severe pain,Sao2 89%,RR 24,HR 100
• Tried NIV and oxygen , Saturation dropped to 85%
and hence he was put on 10 liters O2 and for pain
morphine was given.
• Oxygen monitored ,> 97%
• At about 4AM he was unresponsive ,with shallow
breathing ????
KMCH
Respiratory Failure
• Clinical syndrome
• Failure of the lung to fulfill it’s function
Oxygenation
Carbon dioxide elimination
KMCH
Classification of Respiratory Failure
Fig. 68-2
KMCH
Hypoxemia : Physiological causes
•
•
•
•
•
High Altitude
Diffusion
Hypoventilation
Ventilation : Perfusion mismatch
Shunting
KMCH
Altitude and pAo2 relationship:
KMCH
Hypoxemia due to high altitude
• As you go up in altitude the oxygen content in the
blood is lesser.
• Total barometric pressure (air pressure ) at sea level
is 760 mm Hg
• Oxygen is 21%
• (760mmHg-47mmHg) x .21=150 mmHg pA O2
• At 19,000 feet,
• (380mmHg-47mm Hg)x.21=70 mmHg
KMCH
Hypoxemia due to high altitude
•
•
•
•
•
•
•
Low Total Barometric pressure.
100% oxygen Pa02 responds.
Normal A-a gradient.
Body responds with hyperventilation.
paCo2 goes down.
Can affect normal individuals.
Acclimatization by climbing slowly can reduce the
incidence of pulmonary edema.
KMCH
Diffusion Limitation
Fig. 68-5
KMCH
Diffusion abnormality and hypoxemia.
• Pulmonary fibrosis.
• Hypoxemia during exercise.
• Red cells doesn't have enough contact time for
oxygenation.
• Increased A-a gradient.
• Hypoxemia responds to 100% oxygen.
KMCH
Hypoventilation and hypoxemia.
Respiratory rate and tidal volume are lesser.
Increased pCO2.
Seen in narcotics, obesity, brainstem stroke.
Normal A-a gradient.
KMCH
KMCH
Regional difference in perfusion
V/Q
KMCH
Hypoxemic pulmonary vasoconstriction:
• Blood chasing oxygen.
• Alveoli sends neural
impulse to produce
vasoconstriction of the
adjacent arterioles.
• This ensures optimization of
V/Q ratio on other units.
KMCH
V:Q mismatch and hypoxemia
• Pulmonary artery pressures are low and hence
gravity accounts for distribution of blood in lungs.
• Top part of the lung has high V/Q ratio due to lower
perfusion and higher ventilation.
• Zero ventilation is shunt and zero perfusion is dead
space.
• Certain areas of lung have high V/Q ratio and certain
areas of lung have low V/Q ratio and is known as V/Q
mismatch.
KMCH
FIO2
Ventilation
without
perfusion
(deadspace
ventilation)
Hypoventilation
Diffusion
abnormality
Normal
Perfusion
without
ventilation
(shunting)
KMCH
• V/Q is the most common cause for hypoxemia.
• Pneumonia, COPD , Bronchial Asthma, Pulmonary
embolism , COPD ,pulmonary fibrosis , PHT .
• It responds to 100 % oxygen
• Increased A-a gradient
KMCH
Shunting and hypoxemia:
• Shunt refers to perfusion without ventilation.
• Intrapulmonary shunt refers to areas in the lung
where perfusion exceeds ventilation.
• Pulmonary shunting is minimized by the normal
reflex pulmonary vasoconstriction to hypoxia.
• Because shunt represents areas where gas exchange
does not occur, 100% inspired oxygen is unable to
overcome the hypoxia caused by shunting
KMCH
FIO2
Shunting:
Ventilation
without
perfusion
(deadspace
ventilation)
Hypoventilation
Diffusion
abnormality
Normal
Perfusion
without
ventilation
(shunting)
KMCH
75%
75%
87.5%
KMCH
100%
75%
FIO2 space ventilation and hypoxemia:
Dead
Ventilation
without
perfusion
(deadspace
ventilation)
Hypoventilation
Diffusion
abnormality
Normal
Perfusion
without
ventilation
(shunting)
KMCH
• Alveolar-capillary interface destroyed e.g
emphysema
• Blood flow is reduced e.g CHF, PE
• Overdistended alveoli e.g positive- pressure
ventilation
KMCH
Clinical signs:
Hypoxemia.
• Breathlessness.
• Tachypnea and RR > 30.
• Tachycardia and PR > 110.
• Anxious,Restless.
• Cyanosed.
• Sweating.
• Accessory muscles of
breathing.
• Silent Chest.
Hypercapnea.
• Confusion.
• Unresponsiveness.
• Shallow breathing.
• Agitation.
• Hypotension.
KMCH
Basic assessment of a breathless patient:
• Quick record of vitals including
pulse,BP,RR,Temperature,Oxygen saturation.
• Look out for possible drug overdose.
• Quick enquiry of prior admission.
• Clinical assessment.
• Arterial blood gas analysis.
• Oxygen supplementation if hypoxemic and IV access.
• ECG.
KMCH
Clinical scenario:
• 40 year old male , BMI 33,underwent emergency
upper abdominal surgery under GA , shifted.
• 9PM,breathless,severe pain,Sao2 89%,RR 24,HR 100
• Tried NIV and oxygen , Saturation dropped to 85%
and hence he was put on 10 liters O2 and for pain
morphine was given.
• Oxygen monitored ,> 97%
• At about 4AM he was unresponsive ,with shallow
breathing ????
KMCH
Thank you:
KMCH