physiology of respiratory failure: s.mahadevan,md, v.r.pattabhiraman,md,dnb,fccp arjun...
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Physiology of respiratory failure:
S.Mahadevan,MD,V.R.Pattabhiraman,MD,DNB,FCCPArjun Srinivasan,MD,DMPulmonology Associates
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• Hypoxia • Hypoxaemia
Low oxygen in the tissues.
SepsisHypotensionLow cardiac output statePoisoning, cellular toxinsLow hemoglobin
Low oxygen in the blood.
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• Ventilation
The air that you breathe that takes part in respiration.
Minute Ventilation=Tidal volumeX Respiratory rate.
• Perfusion
• The blood that are in the pulmonary arteries.Respiratory
Alveolar
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Dead space
• Anatomical dead space
• The air in the conducting space of the airways like the nasal cavity , trachea , main bronchus , bronchioles, alveolar ducts.
• Physiological
• The air in the air sacs that do not take part in gas exchange.
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Muscles of respiration:
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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.
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Respiratory physiology:
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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 ????
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Respiratory Failure
• Clinical syndrome• Failure of the lung to fulfill it’s function
OxygenationCarbon dioxide elimination
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Classification of Respiratory Failure
Fig. 68-2
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Hypoxemia : Physiological causes
• High Altitude• Diffusion• Hypoventilation• Ventilation : Perfusion mismatch• Shunting
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Altitude and pAo2 relationship:
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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
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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.
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Diffusion Limitation
Fig. 68-5
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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.
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Hypoventilation and hypoxemia.
Respiratory rate and tidal volume are lesser.Increased pCO2.Seen in narcotics, obesity, brainstem stroke.Normal A-a gradient.
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Regional difference in perfusion
V/Q
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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.
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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.
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FIO2
Ventilation without
perfusion(deadspace ventilation)
Diffusion abnormality
Perfusion without
ventilation (shunting)
Hypoventilation
Normal
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• 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
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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
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FIO2
Ventilation without
perfusion(deadspace ventilation)
Diffusion abnormality
Perfusion without
ventilation (shunting)
Hypoventilation
Normal
Shunting:
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75% 75%
100% 75%
87.5%
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FIO2
Ventilation without
perfusion(deadspace ventilation)
Diffusion abnormality
Perfusion without
ventilation (shunting)
Hypoventilation
Normal
Dead space ventilation and hypoxemia:
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• Alveolar-capillary interface destroyed e.g emphysema
• Blood flow is reduced e.g CHF, PE• Overdistended alveoli e.g positive- pressure
ventilation
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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.
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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.
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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 ????
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Thank you: