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Hypoxia and Hypercapnoea

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Presentation on theme: "Hypoxia and Hypercapnoea"— Presentation transcript:

1 Hypoxia and Hypercapnoea
Causes Effects Compensatory Mechanisms 12-Apr-19 Hypoxia

2 Hypoxia 12-Apr-19 Hypoxia

3 Hypoxia Inadequate tissue oxygen utilization
Lower than normal oxygen pressure in the tissue Tissue damage occurs When arterial oxygen saturation  50% And PaO2 30 mm Hg A PaO2 <20 mm Hg is intolerable to life 12-Apr-19 Hypoxia

4 Causes of Hypoxia Reduced partial pressure of oxygen in air
Inadequate alveolar ventilation Impaired pulmonary uptake  PO2 in air Inadequate alveolar ventilation Impaired O2 uptake Inadequate blood flow to tissues Poisoning or inadequate cellular enzymes 12-Apr-19 Hypoxia

5 Causes of Hypoxia Venous – arterial shunt
Inadequate flow of blood to tissues Poisoning or inadequate cellular enzymes  PO2 in air Inadequate alveolar ventilation Impaired O2 uptake Inadequate blood flow to tissues Poisoning or inadequate cellular enzymes 12-Apr-19 Hypoxia

6 Classification Hypoxic hypoxia Causes of hypoxic hypoxia
Low PO2 in the atmosphere Hypoventilation Neuromuscular disorders Polio Spinal cord injuries Depression of respiratory center Drugs 12-Apr-19 Hypoxia

7 Hypoxic Hypoxia Mismatch of ventilation/perfusion  Airway resistance
Asthma Emphysema  Lung compliance Pulmonary fibrosis Lung abnormalities Restrictive diseases Kyphoscoliosis Mismatch of ventilation/perfusion  Physiological dead space 12-Apr-19 Hypoxia

8 Hypoxic Hypoxia All the above conditions cause
 In respiratory membrane diffusion capacity  In area  In thickness Venous - arterial shunts All the above conditions cause Inadequate oxygenation of the blood in the lungs 12-Apr-19 Hypoxia

9 Anaemic Hypoxia Fall in oxygen carrying capacity of the blood
PO2 remains near normal level 12-Apr-19 Hypoxia

10 Anaemic Hypoxia Causes  In concentration of active Hbg Anaemia
Poisoning with substances which combine with Hbg Limit uptake of O2 by Hbg Carbon monoxide, carboxyhemoglobin Methaemoglobin 12-Apr-19 Hypoxia

11 Stagnant Hypoxia Interference with circulation of blood
Subnormal flow of Hbg molecules through the tissues capillaries 12-Apr-19 Hypoxia

12 Stagnant Hypoxia When occlusion is from Arterial end of capillaries
 Amount of blood in the capillaries Ischaemia 12-Apr-19 Hypoxia

13 Stagnant Hypoxia When interference with flow In either situation
Is at the venous end Capillaries are congested with blood In either situation There is excessive extraction of oxygen from blood leaving the tissues 12-Apr-19 Hypoxia

14 Histotoxic Hypoxia Oxygen supply is normal Ability of tissues
To utilize oxygen is interfered with Interference with the ability of Cell enzymes to use molecular O2 Cyanide poisoning Oxygen toxicity Excessive pressure of O2 Inactivates oxidative metabolism 12-Apr-19 Hypoxia

15 Effects of Hypoxia Tissue damage occurs when CNS
Arterial O2 saturation  <50% PO2 30 mm Hg CNS Brain O2 consumption 3ml O2/100g/min Average O2 consumption 45 ml/min 18% of total basal O2 consumption 12-Apr-19 Hypoxia

16 Effects of Hypoxia O2 depletion leads Cessation of cerebral blood flow
Abnormal cerebral function Cessation of cerebral blood flow Loss of function within 4 to 6 secs Loss of consciousness in 10 to 20 secs Irreversible changes in 3 to 5 min 12-Apr-19 Hypoxia

17 Effects of Hypoxia Heart Basal cardiac O2 consumption
10 ml/100g/min Total myocardial O2 consumption 35ml/min 14% of total basal O2 consumption 2/3 of O2 consumed for contraction 1/3 of O2 consumed for conduction 12-Apr-19 Hypoxia

18 Effects of Hypoxia There is almost complete Chronic myocardial hypoxia
O2 extraction from arterial blood Hence  need for O2 by heart Can be met by  blood flow only Chronic myocardial hypoxia Severe structural alteration Dilatation Hypertrophy Fibrosis 12-Apr-19 Hypoxia

19 Effects of Hypoxia Pulmonary vascular bed
Vasoconstriction of small pre-capillary vessels  Pulmonary vascular resistance Pulmonary hypertension 12-Apr-19 Hypoxia

20 Effects of Hypoxia Kidney Renal O2 consumption Renal blood flow
6 ml/100g/min 18 ml/min 7.2% of total basal O2 consumption Renal blood flow High O2 extraction low Renal A-V PO2 and O2 is high 12-Apr-19 Hypoxia

21 Effects of Hypoxia Liver Blood supply Portal blood flow is
Approx 1.5 liters/min (about 25% CO) 70% from portal vein 30% from hepatic artery Hepatic artery provides 50% of O2 supply 12-Apr-19 Hypoxia

22 Effects of Hypoxia Liver O2 extraction 55 ml/min
22% total basal O2 consumption 12-Apr-19 Hypoxia

23 Compensatory Mechanisms in Hypoxia
 Pulmonary ventilation in PaO2 leads to Chemoreceptor hypoxic stimulation  pulmonary ventilation The  in alv ventilation decrease CO2  PCO2 &  pH both Inhibit respiration Tend to oppose stimulatory effect of hypoxia 12-Apr-19 Hypoxia

24 Compensatory Mechanism in Hypoxia
After 2 to 5 days this inhibition fades away Ventilation  3 to 7 times the normal level Active pumping of HCO3- from CSF Cause  pH in the CSF Chemoreceptors are once more stimulated 12-Apr-19 Hypoxia

25 Compensatory Mechanism in Hypoxia
O2 transport Haemoglobin Hypoxia causes Release of erythropoietin Increase production of RBC (polycythemia) Haematocrit increase From 40 –45% to 60-65% Hb increase from 15 gm% to 22 gm% 12-Apr-19 Hypoxia

26 Compensatory Mechanism in Hypoxia
Total result is Increase in circulating Hb of 50 to 90% more than normal Polycythemia Increase viscosity However, these adaptive changes Slow to develop Takes about 2 – 3 weeks 12-Apr-19 Hypoxia

27 Compensatory Mechanism in Hypoxia
O2 transport  affinity of Hb for O2 Effect of increased 2,3 DPG (from 85 g/ml to 140 g/ml of blood) within 1st 2 days Shifts O2 - Hb dissociation curve to the right Diffusion capacity of lung for O2 (21 ml/mm Hg/min) Increases 3 times more 12-Apr-19 Hypoxia

28 Compensatory Mechanism in Hypoxia
Increased capillary density Cardiac output Increase by 20 – 30% immediately But after few days Return back to normal May fall slightly below normal after few months There is increase in blood flow to Muscles, heart, brain 12-Apr-19 Hypoxia

29 Compensatory Mechanism in Hypoxia
Cellular level Enzyme adaptation to hypoxia Increase in number of mitochondria Increase in cellular oxidative enzymes 12-Apr-19 Hypoxia

30 Compensatory Mechanism in Hypoxia
Hypoxaemia , acidosis Vasoconstriction of pulmonary vasculature  Pulmonary resistance Also in hypoxaemia  Haematocrit, viscosity Pulmonary hypertension Right ventricular hypertrophy Right ventricular failure 12-Apr-19 Hypoxia

31 Hypercapnia 12-Apr-19 Hypoxia

32 Hypercapnia Presence of excess CO2 in the body fluids Retention of CO2
PCO2 >45 mm hg 12-Apr-19 Hypoxia

33 Hypercapnia Retention of large amounts of CO2
Produce symptoms due to depression of CNS Confusion  Sensory acuity Eventual coma Respiratory depression & death 12-Apr-19 Hypoxia

34 Hypercapnia In patients with these symptoms
PCO2 is markedly elevated Severe respiratory acidosis Plasma HCO3- may >40 mEq/l Large amounts of HCO3- are excreted But more HCO3- is reabsorbed Hence  plasma HCO3- Partially compensating for the acidosis 12-Apr-19 Hypoxia

35 Hypercapnia CO2 is much more soluble than O2
Hypercapnia is rarely a problem In patients with pulmonary fibrosis It occurs in V/Q inequalities Cause  alveolar ventilation 12-Apr-19 Hypoxia

36 Hypercapnia It is made worse when CO2 production is increased Fever
13%  in CO2 production for every 1o C rise in temperature 12-Apr-19 Hypoxia

37 Causes of Hypercapnia Causes Pulmonary diseases Hypoventilation
CNS depression Acute severe pulmonary oedema 12-Apr-19 Hypoxia

38 Causes of Hypercapnia Neuromuscular abnormalities
Bony abnormalities of thorax Defective instruments Anaesthesia, scuba diving Limestone caves  Co2 12-Apr-19 Hypoxia

39 Effects of Hypercapnia
Peripheral vasodilatation Hypoxia Respiratory acidosis Stimulation of ventilation Cerebral depression 12-Apr-19 Hypoxia

40 Vasodilatation Effect on vascular smooth muscle
Peripheral vasodilatation results in Cerebral vasodilatation  Cerebral blood flow  Cerebral capillary pressure Cerebral oedema  CSF pressure Papilloedema 12-Apr-19 Hypoxia

41 Vasodilatation Overall effects of excess CO2 Hypertension
Reflex stimulation of sympathetic Via chemoreceptors 12-Apr-19 Hypoxia

42 Hypoxaemia  In PCO2 can cause hypoxemia
PAO2 = FIO2 * (PB – 47) – PaCO2/RQ PAO2 = alveolar partial pressure of O2 FIO2 = inspired oxygen fraction PaCO2= arterial blood partial pressure of CO2 PB = barometric pressure RQ = respiratory quotient 12-Apr-19 Hypoxia

43 Hypoxaemia Thus A rise in paco2 will cause A fall in PAO2 Hypoxaemia
12-Apr-19 Hypoxia

44 Respiratory Acidosis Acute hypercapnia Buffers provide compensation
Acute respiratory acidosis Buffers provide compensation Retained CO2  Blood H2CO3  H+ activity CO2 + H2O  H2CO3  H+ + [HCO3-]  PCO2 Intracellular Prot- buffers Na+ K+ 12-Apr-19 Hypoxia

45 Respiratory Acidosis H+ are buffered By intracellular buffers
HCO3- remain in plasma  Plasma HCO3- level CO2 + H2O  H2CO3  H+ + [HCO3-]  PCO2 Intracellular Prot- buffers Na+ K+ 12-Apr-19 Hypoxia

46 Respiratory Acidosis In the RBC CO2 +H2O  H2CO3  H+ + HCO3-
H+ cannot readily move out of RBC It is buffered by Hb buffer H+ + HbO2 ⇌ H+.Hb + O2 HCO3- leaves the RBC In exchange of Cl- Cl- HCO3- CO2 Cl- CO2 + H2O2⇌ H2CO3⇌ H+ + HCO3- H++ HbO2 HHb + CO2 HHb + O2 Carbamino-Hb O2 12-Apr-19 Hypoxia

47 Respiratory Acidosis Prolonged hypercapnia
Chronic respiratory acidosis Cellular buffers exhausted Compensation for the acidosis Depends on ability of kidney to Secrete H+ Produce & retain  quantities of HCO3- 12-Apr-19 Hypoxia

48 Respiratory Acidosis Elevated PaCO2 facilitates
The  H+ secretion by the kidney Elevation of Tm of HCO3- The  Tm of HCO3- permits Maintenance of  serum HCO3- As long as CO2 retention persist 12-Apr-19 Hypoxia

49 Effect on Consciousness
 PaCO2 to 90 – 120 mm Hg Carbon dioxide narcosis occurs Alteration of intracellular pH Derangement of metabolic process 12-Apr-19 Hypoxia

50 Effect on ANS  Paco2 causes The effects of  catecholamines
 Plasma levels of Adrenaline, noradrenaline The effects of  catecholamines To a certain extent are offset by  Sensitivity of target organs when pH Direct depressant effect of CO2 on target organs 12-Apr-19 Hypoxia

51 Effects on ANS Acetylcholine hydrolysis
 By low pH Certain parasympathetic activity May be enhanced during hypercapnia Anterior pituitary Stimulated by CO2  ACTH secretion 12-Apr-19 Hypoxia

52 Effects on Circulatory System
Myocardial contractility Direct effects  Myocardial contractility  HR However, these direct depressant effects Overshadowed by stimulatory effects Mediated by sympathetic system 12-Apr-19 Hypoxia

53 Effects on Circulatory System
Blood pressure Tend to rise with  in PCO2 However, at very high PCO2 BP decline and may be the cause of death 12-Apr-19 Hypoxia

54 Effects on Circulatory System
Regional blood flow Brain, coronary & skin Blood flow  with increase in PCO2 Skeletal muscle blood flow Is reduced 12-Apr-19 Hypoxia

55 Effects on Kidney Renal blood flow & GFR
Little influenced by minor changes in PCO2 At high levels of PCO2 there is Constriction of glomerular afferent arterioles Leading to anuria 12-Apr-19 Hypoxia

56 Effects on Kidney Chronic hypercapnia results in
 Re-absorption of HCO3- by the kidneys Leads to  plasma HCO2- A secondary or compensatory metabolic alkalosis 12-Apr-19 Hypoxia

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