KMCH

Physiology of respiratory failure:

S.Mahadevan,MD,V.R.Pattabhiraman,MD,DNB,FCCPArjun Srinivasan,MD,DMPulmonology Associates

KMCH

• Hypoxia • Hypoxaemia

Low oxygen in the tissues.

SepsisHypotensionLow cardiac output statePoisoning, cellular toxinsLow hemoglobin

Low oxygen in the blood.

KMCH

• 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

KMCH

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.

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

OxygenationCarbon 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)

Diffusion abnormality

Perfusion without

ventilation (shunting)

Hypoventilation

Normal

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

Ventilation without

perfusion(deadspace ventilation)

Diffusion abnormality

Perfusion without

ventilation (shunting)

Hypoventilation

Normal

Shunting:

KMCH

75% 75%

100% 75%

87.5%

KMCH

FIO2

Ventilation without

perfusion(deadspace ventilation)

Diffusion abnormality

Perfusion without

ventilation (shunting)

Hypoventilation

Normal

Dead space ventilation and hypoxemia:

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: