HYPOXIA RESPIRATORY FAILURE

M. Tatar Dept. of Pathophysiology

HYPOXIA hypoxemia anoxia ischemia

glucose

38 ATP

Krebs´s cycle CO 2 lactate glucose pyruvate

2 ATP

O 2 H 2 O

The aim of oxygen transport

to preserve high mitochondria PO 2 gradient between capillaries and

Q x Hb conc. x (SaO 2 – SvO 2 ) O 2 c m

ADP V O 2

Classification of hypoxia (1)

1.

2.

Hypotonic hypoxemic hypoxia  PaO 2 ,  CaO 2 ; Q . Hb . ( 

SaO 2

– SvO 2 ) - carotid body stimulation, hyperventilation - pulmonary hypertension in chronic form - respiratory failure Izotonic hypoxemic hypoxia - normal PaO 2,  CaO 2 ; Q . 

Hb

- anemia, carboxyhemoglobin . (  SaO 2 – SvO 2 ) - chemoreceptors are not stimulated, lack of dyspnea

200 150

Hb concentration and CaO

2

interrelationship

polycythemia Hb = 20 normal Hb = 15 anemia Hb = 10 100 100 100 20 60 100 120 Pa O 2 , mmHg

Classification of hypoxia (2)

3. Hypoextractive hypoxia - increased Hb afinity to O 2 - Q . Hb . (SaO 2 – 

SvO 2

) 100 50 pH = 7,4; t = 37 °C pH  7,4; t  37 °C 6 Pa O 2 , kPa 14

Classification of hypoxia 3

4. Hypocirculatory hypoxia 

Q

. Hb . (SaO 2 – SvO 2 ) - ischemic, congestive; local, general 5. Overutilization hypoxia  demand of tissues for O 2 excesses the available supply - angina pectoris, epilepsy (fatigue and cerebral depression) 6. Histotoxic hypoxia - disturbed ATP production, blocked oxidative phosphorylation - Q . Hb . (SaO 2 – 

SvO 2

) - cyanide

Respiratory failure - definition

Syndrome characterized by disturbed exchange of oxygen and carbon dioxide in lung

Consequences: PaO 2 PaCO 2  60 mmHg (8.0 kPa) with or without > 50 mmHg (6.7 kPa) - under resting condition - breathing atmospheric air at sea level Classification: 1. Hypoxemic (hypoxemia with normal or  2. Hypercapnic (hypoxemia and hypercapnia) PaCO 2 )

Respiratory failure

Factors determining oxygenation and ventilation are different PaCO 2 must be regarded as a function of the overall ventilation of the entire lung, without regard to local inequalities of distribution of ventilation and perfusion PaO 2 , on the other hand, depends not only on the amount of alveolar ventilation but also on the matching of ventilation and perfusion

Respiratory failure

Mechanisms responsible for gas exchange disturbances A. intrinsic lung disorders (airways, lung parenchyma) 1. Ventilation/perfusion (V´/Q´) mismatch 2. Venous admixture 3. Diffusion impairment B. extrinsic lung disorders (respiratory centre, nerve pathways, respiratory muscles, thoracic cage, pleural space) 1. Alveolar hypoventilation (overall)

PaO 2 PaCO 2 SaO 2  ventilatory drive

100% 50 50 70% hypoxemia hypercapnia

chemoreceptors

10 20 C D C A C B C V´ A = 1 / 2 low V´ A /Q´ Q´=1 high V´ A =1 1 / 2 V´ A /Q´ mormal V´ V´ A =1 Q´=1 A /Q´ V´ A ? Q´=1 =1 Q´=1 20 40 60 80 100 120 mmHg

PaO 2

B 50 A C 25 V´ A =1 High Q´=1 V´ A /Q´ Normal V´ A /Q´ 1 1 / 3 + 2 / 3 = 2 Q´=1 V´ A =1 1 / 3 low V´ A = 2 / 3 V´ A /Q´ Q´ = 1 20 40 PaCO 2 V´ A =1 Q´=1 60 mmHg

Respiratory failure Mechanisms of hypoxemia

1. alveolar hypoventilation

2. compartments with low V´/Q´ ratio

3. right-to-left shunting of blood in compartments with zero V´/Q´ratio

4. diffusion impairment due to thickening of the alveolar-capillary membrane

Diffusion impairment – oxygen saturation of arterial blood 12 normal impaired PcO 2 4

exercise

0.8 s

rest

PvO 2 Er contact time with A-c membrane

Respiratory failure Mechanisms enhancing hypoxemia Pure oxygen breathing:

 hypoxic pulmonary vasoconstriction resorptive atelectasis (  P A N 2 ,  resorption of O 2 )  central inspiratory drive

Respiratory failure

Mechanisms of hypercapnia

1. overall alveolar hypoventilation

2. critical amount of the compartments with low

V´/Q´ ratio

overall ventilation must increase to maintain effective alveolar ventilation (normal CO 2 exchange)

limits of effective alveolar ventilation:

 work of breathing respiratory muscle fatigue  dead space ventilation