7. How does the body detects hypoxia?
• Chemoreceptor activity: carotid bodies and the aortic bodies
• Chemoreceptors respond to a decrease in oxygen tension rather than
to a decrease in oxygen content
8. What are the defense responses against
hypoxia?
• Ventilatory compensation: tachypnea and hyperpnea
• Circulatory compensation: increase in cardiac output or an increase in
blood flow to a limited area.
• Reserves of oxygen in the body
• Tolerances to hypoxia by specific tissues Redistribution of blood
flow
9. What is oxygen reserve?
• That amount of oxygen which is available in the body for continued
metabolism, if the supply of oxygen from ambient air is interrupted.
• Should not be confused with oxygen stores, which refer to oxygen
present in the body, whether or not this oxygen is available for
metabolic purposes.
10. Four storage sites of oxygen in the body
• Alveolar air (FRC)
• Arterial blood
• Venous blood
• Tissue water contains oxygen in physical solution.
• Muscle tissue contains approximately ten times this amount of
oxygen, stored in myohemoglobin
11.
12. What are the requirements for adequate
oxygen transport?
• Fraction of oxygen in inspired air (FiO2 )
• Partial pressure of oxygen in inspired air ( PiO2 )
• Alveolar ventilation (VA)
• The relation of ventilation to perfusion in the lungs (VA/Q)
• CO, and blood volume
• Hb content
• Arterial blood pH, body temperature
14. What is the etiology of hypoxia before
anesthesia?
• Pre-existing cardiopulmonary disease
• Most important factor in promoting postoperative respiratory complications
• Already low Pa02 + Decreased respiratory and Circulatory compensation.
• Trauma :
• May involve ventilatory and circulatory systems
• May cause atelectasis and shunting imposed by immobilization, restrictive
influences, and pain medication.
15. What is the etiology of hypoxia before anesthesia?
• Old age :
• PaO2(mmHg) =106 - Age/2
• after age of 60 the range is from 70 to 80 mm Hg.
• Reduced compensatory increase in CO
• Reduced ventilatory response to hypoxia is limited by reduced pulmonary
volumes and capacities in old age
• Obesity
16. What is the etiology of hypoxia before anesthesia?
• Severe anemia :
• rarely present at operation, except in the worst emergencies
• Restriction of respiratory motion :
• in the form of tight binders or abdominal distension can cause significant
reductions in Pa02
• cause of decrease in available ventilatory compensation
• Drug administration
18. What is the etiology of hypoxia during
anesthesia?
• Decreased oxygen in inspired air
• Decreased PAO2
• Decreased oxygen tension in arterial blood Pa02
• Decreased oxygen content in arterial blood (Ca02 )
• Decreased cardiac output
• Increased tissue metabolism
19. • Classified into two
I.Problems with oxygen delivery system
II.Problems with patient
20. Problems with oxygen delivery system?
• at the level of central oxygen supplies
• at the level of pipeline distributing system
• at the level of oxygen cylinders attached to anaesthesia machine
• at the level of anaesthesia machine
• at the level of anaesthesia ventilator
• at the level of anaesthesia circuit
• at the level of endotracheal tube
21. Problems with oxygen delivery system
• At level of central Oxygen
• Liquid tank may be filled with nitrogen or argon
• Gas leak
• Inadequate pressure at central supply
• Decreased oxygen level at the tank
• Depleted cylinders
• Failure of master alarm system
• At the level of pipeline distributing system
• Leak
• Contamination of gases
• Cross connection
• Connecting wrong hose to Oxygen yoke
• Inadvertent switching of schrader adapter of piped lines
22. Problems with oxygen delivery system
• At the level of oxygen cylinders attached to anaesthesia machine
• Empty cylinders
• Substitution of non oxygen cylinder at the yoke
• Insufficiently opened cylinder
• At the level of anaesthesia machine
• Incorrect setting of flow meter
• Crack in the oxygen flow meter tubes
• Transposition of rotameter tubes
• Leak in machine
23. Problems with oxygen delivery system
• At the level of anaesthesia Ventilator
• Low tidal volume
• Low respiratory rate
• Inadequate minute volume
• Disconnection of tubing
• At the level of the anaesthesia circuit
• Disconnection
• Leak
• At the level of Endotracheal tube
• Esophageal intubation
• Endobrochial intubation
• Accidental extubation
• Kinking of tube
25. II. Problems with patient?
• A.Hypoventilation :
• A spontaneously anaesthetised patient may hypoventilate due to drug induced
respiratory depression.
• In patient who is paralysed and ventilated ,hypoventilation may occur due to
inadequate IPPV
B.Reduced functional residual capacity:
• Induction of GA will cause reduction in FRC by 15-20% invariably
• This will be more in patients with preexisting lung disease, obese patients.
• continue in post operative period.
• causes increase in PAO2 –PaO2 gradient.
• The reduction in FRC may be restored normal by application of PEEP.
26. II. Problems with patient?
C. Increased airway resistance
• Reduction in FRC
• Decrease in calibre of airways
• Endotracheal intubation
• Anaesthesia apparatus
• Laryngospasm , bronchospasm
• Obstruction of ETT
27. D. Atelectasis
• It is a condition of alveolar collapse .
• It may be micro atelectasis,macro atelectasis or lobar collapse.
• Leads to V/Q mismatch , R-L shunting and arterial hypoxemia
• occur due to airways secretions, compression packs, wedge and prolonged
procedures
• PEEP may be useful in such situation
28. E. Absorption atelectasis
• Alveloar collapse occur when the patient is getting high FiO2.
• When PAO2 rises , the rate at which O2 moves from the alveoli to
capillary blood increases.When the absorption rate is more than the
inspired flow gases , lung unit collapses
• So, absorption atelectasis occurs when
• Fio2 is high
• V/Q is low
• time of exposure of lung unit low V/Q ratio to high FiO2 is long
• CvO2 is low
29. F. Diffusion defect:
• Thickened alveloar membrane
• Thickening of air-blood interface
• Inflammation
• Edema
• Fibrosis or loss of alveolar surface area (Eg:sarcoidosis ,Emphysema)
31. H. Inhibtion of hypoxic pulmonary vasoconstriction (HPV)
• It is a protective phenomenon. When PaO2 decreases in a region
pulmonary vasoconstriction occurs at that particular region.
• HPV diverts blood flow from the hypoxic regions of the lung to better
ventilated normoxic regions , thus decreases V/Q mismatch
maintaning PaO2
• Inhibition of HPV lead to arterial hypoxemia
• Factors inhibit HPV
• Inhaled anaesthetics
• vasodilators(SNP, NTG)
• Hypocapnia
• Hypothermia
• Thrombo embolism
34. Postoperative hypoxia:
1. Diffusion hypoxia
• The solubilities of nitrogen and nitrous oxide differ sufficiently to reduce PAo2
when nitrous oxide anesthesia is stopped and the patient allowed to breathe
room air.
• Nitrous oxide comes out of solution and enters the alveoli more rapidly than
nitrogen leaves the alveoli to become dissolved.
• the period of greatest PA02 decrease is approximately 10 minutes
• after 30 minutes the impairment is insignificant
35. 2. Prolonged effect of muscle relaxants
3. Pain
• Dominant factor in determining postoperative respiratory function.
• promoting atelectasis by restricting ventilatory excursions
4. Pain relieving drugs, typified by morphine
• result in an increased incidence of respiratory complications.
• Not restricted to high doses which depress ventilation
• morphine may reduce the reflex activity, which provides for periodic
hyperinflation
37. Diagnosis:
• During early days of anaesthesia , defective oxygenation of the
patient was identified by cyanosis &dark blood in the surgical field .
• Cyanosis occurs when the deoxygenated hb is >5g /100ml
• Cyanosis usually observed when Hb saturation is 85%.this
corresponds to PaO2 of 45-50mm hg in adults 35-40mm hg in infants
• Cyanosis may be observed when there is no hypoxemia Eg :
methemoglobinemia
• Cyanosis may not be apparent in the presence of anaemia or
peripheral vasoconstriction
38. • Pulse oximeter is most commonly used one
• Other monitors –
a) oxygen analyser
b) ABG
c) Scvo2
d) Capnography
e) Airway pressure monitor
42. References:
• Bendixen, H. H., and M. B. Laver. "Hypoxia in anesthesia: A
review." Clinical Pharmacology & Therapeutics 6, no. 4 (1965): 510-
539.
• https://www.wfsahq.org/components/com_virtual_library/media/e8
cf1db1c025a18fa8a05f562082f29f-Management-of-Hypoxia-During-
Anaesthesia-Algorithm--Update-2.pdf
