Acute Respiratory FailureAcute Respiratory Failure•Is a clinical condition in which the Is a clinical condition in which the

pulmonary system fails to maintain pulmonary system fails to maintain adequate gas exchange PaO2 value adequate gas exchange PaO2 value less than 60 mmHg while breathing less than 60 mmHg while breathing air or PaCo2 of more than 50 mmHgair or PaCo2 of more than 50 mmHg..

•RFRF•Respiratory failure exists whenever the Respiratory failure exists whenever the

exchange of O2 for Co2in the lungs exchange of O2 for Co2in the lungs can’t keep up with the rate of O2 can’t keep up with the rate of O2 consumption & CO2 production in the consumption & CO2 production in the cells of the bodycells of the body..

•This results in a fall in arterial O2 This results in a fall in arterial O2 tension (hypoxemia) & arise in arterial tension (hypoxemia) & arise in arterial CO2 tension (hypercapnia)CO2 tension (hypercapnia)..

Causes of acute Respiratory Causes of acute Respiratory FailureFailure

**intrapulmonaryintrapulmonary**Lower air way and alveoli

Pulmonary circulation

Alveolar capillary membrane

Extra pulmonary causesExtra pulmonary causesBrain,spinalcord,neuromuscular Brain,spinalcord,neuromuscular system, thorax pleura, upper air way system, thorax pleura, upper air way obstructionobstruction..

Classifications of A.R.FClassifications of A.R.FType1{hypoxemic R.F}, in which the PaO2 is less Type1{hypoxemic R.F}, in which the PaO2 is less than50 mmHg and the PaCO2 is normal or lowthan50 mmHg and the PaCO2 is normal or low..

**the major pathophysiologic mechanisms are the major pathophysiologic mechanisms are ventilation-perfusion (V/Q) mismatching and Right ventilation-perfusion (V/Q) mismatching and Right to Left shuntingto Left shunting

Classifications of A.R.FClassifications of A.R.FType2{hypercapnic/hypoxaemic Type2{hypercapnic/hypoxaemic respiratory failure}, in which the respiratory failure}, in which the Paco2>45 mmHg, accompanied by a Paco2>45 mmHg, accompanied by a lower than normal PaO2lower than normal PaO2 . .

Pathophysiology caused by alveolar Pathophysiology caused by alveolar hypoventilationhypoventilation..

PathophysiologyPathophysiology•Hypoxemia is the result of impaired Hypoxemia is the result of impaired

gas exchange and the hall mark of gas exchange and the hall mark of acute respiratory failure. acute respiratory failure. Hypercapnia may be present, Hypercapnia may be present, depending on the causedepending on the cause..

PathoPatho•The main cause of hypoxemia are The main cause of hypoxemia are

alveolar hypoventilation, ventilation / alveolar hypoventilation, ventilation / perfusion (V/Q) mismatching and perfusion (V/Q) mismatching and intrapulmonary shuntingintrapulmonary shunting..

PathPath•**Type1 respiratory failure results Type1 respiratory failure results

from V/Q mismatching and from V/Q mismatching and intrapulmonary shuntingintrapulmonary shunting..

•**Type2 respiratory failure results Type2 respiratory failure results from alveolar hypoventilation which from alveolar hypoventilation which may be accompanied by V/Q may be accompanied by V/Q mismatching and intrapulmonary mismatching and intrapulmonary shuntingshunting..

11--Alveolar hypoventilationAlveolar hypoventilationOccurs when the amount of oxygen being Occurs when the amount of oxygen being brought into the alveoli is insufficient to brought into the alveoli is insufficient to meet the metabolic needs of the babymeet the metabolic needs of the baby..

This can be the result of increasing This can be the result of increasing metabolic oxygen needs or decreasing metabolic oxygen needs or decreasing ventilationventilation..

Alveolar hypoventilationAlveolar hypoventilationHpoxemia caused by alveolar Hpoxemia caused by alveolar hypoventilation is characterized by hypoventilation is characterized by hypercapnia and commonly results hypercapnia and commonly results from extra pulmonary disorderfrom extra pulmonary disorder..

Alveolar hypoventilationAlveolar hypoventilationIt is characterized by ABG values that It is characterized by ABG values that reflect an increase in PaCO2, and a reflect an increase in PaCO2, and a decrease in SaO2,and a decrease in decrease in SaO2,and a decrease in PaO2 , or hypercapnic/hypoxemic PaO2 , or hypercapnic/hypoxemic respiratory failurerespiratory failure..

Ventilation-perfusion Ventilation-perfusion mismatchingmismatchingNormally alveolar ventilation is Normally alveolar ventilation is approximately 4 L/m & capillary approximately 4 L/m & capillary perfusion is approximately 5 L/m . perfusion is approximately 5 L/m . Thus the normal ventilation to Thus the normal ventilation to perfusion ratio is 4:5perfusion ratio is 4:5. .

Ventilation – perfusion Ventilation – perfusion mismatchingmismatching• V/Q may vary from low to high ratios. V/Q may vary from low to high ratios.

Situations in which ventilation Situations in which ventilation exceeds perfusion (V/Q>o.8)are exceeds perfusion (V/Q>o.8)are considered dead space producing. considered dead space producing. (high v/q units).(high v/q units).

Ventilation-perfusion Ventilation-perfusion mismatchingmismatching• Situations in which perfusion exceeds Situations in which perfusion exceeds

ventilation (V/Q<o.8) are considered ventilation (V/Q<o.8) are considered shunt producing.shunt producing.

• Significant alterations in relationship Significant alterations in relationship result in hypoxemia. (low V/Q).result in hypoxemia. (low V/Q).

Ventilation-perfusion Ventilation-perfusion mismatchingmismatching• V/Q mismatching occurs when V/Q mismatching occurs when

ventilation and blood flow are ventilation and blood flow are mismatched in various regions of the mismatched in various regions of the lung in excess of what is normal i.e. lung in excess of what is normal i.e. (there is a greater than normal (there is a greater than normal variability in these V/Q matching the variability in these V/Q matching the overall ratio of ventilation to perfusion overall ratio of ventilation to perfusion is disturbed and hypoxemia develops).is disturbed and hypoxemia develops).

Ventilation-perfusion Ventilation-perfusion mismatchingmismatching• V/Q mismatching is the most V/Q mismatching is the most

common cause of hypoxemia and is common cause of hypoxemia and is usually the result of alveoli that are usually the result of alveoli that are partially collapsed or partially filled partially collapsed or partially filled with fluid.with fluid.

V/Q MismatchV/Q Mismatch• Hypoxemia results when alveoli are Hypoxemia results when alveoli are

under ventilated relative to the under ventilated relative to the normal amount of perfusion (blood normal amount of perfusion (blood flow) they receive. flow) they receive.

• Un oxygenated blood passing by Un oxygenated blood passing by under-ventilated alveoli mixes with under-ventilated alveoli mixes with oxygenated blood and lowers the oxygenated blood and lowers the PaO2.PaO2.

V/Q MismatchV/Q Mismatch• Decreased ventilation relative to Decreased ventilation relative to

perfusion is the mechanism of perfusion is the mechanism of hypoxemia is such conditions as hypoxemia is such conditions as Asthma , COPDAsthma , COPD and pulmonary edema.and pulmonary edema.

• Bronchospasm , mucus plugging and Bronchospasm , mucus plugging and atelectasis can also reduce ventilation atelectasis can also reduce ventilation of well perfused alveoli,of well perfused alveoli, resulting in resulting in impaired oxygenation.impaired oxygenation.