How respiratory function during anaesthesia and its effect.

1. RESPIRATORY
FUNCTION DURING
ANAESTHESIA
Dr. Parth Patel

2. INTRODUCTION
• Anaesthesia impairs pulmonary function whether
patient is breathing spontaneously or on
mechanical ventilation.
•Impaired oxygenation of blood occurs in most
subjects who are anaesthetised. (so supplemented
with FiO2 0.3 to 0.4)
•Despite this Mild to Moderate hypoxaemia
(arterial O2 saturation between 85 % and 90 %)
occurs in half of the patients undergoing elective
surgery(can last from few seconds to upto 30 min.

3. • The first phenomenon that might be seen with
anaesthesia is loss of muscle tone with
subsequent change in the balance between
outward forces( i.e. Respiratory muscles) and
inward forces (i.e., elastic tissue in the lung)
leading to FALL IN FRC,Reduced compliance
and increased airway resistance.
• Fall in FRC affects the patency of the lung
tissue with formation of ATELECTASIS and
airway closure.
• This will alter the distribution and matching of
ventilation which will impede Oxygenation of
blood and removal of CO2.

4. LUNG VOLUME AMD RESPIRATORY MECHANICS
DURING ANAESTHESIA
1. LUNG VOLUME: From upright to supine
position FRC reduces by 1 L.
• Induction of anaesthesia will decrease it
further 0.5 L.
• Thus FRC reduced from 3.5 L to 2.0 L
• 20 % fall in FRC during General anaesthesia
(spontaneous or controlled ventilation/
Intravenous or inhalational agents and muscle
paralysis)

5. • The decrease in FRC- loss of Respiratory
muscle tone, cranial displacement of
diaphragm and decrease in transverse
diameter of chest.
• Maintenance of muscle tone as during
Ketamine anaesthesia does not reduce the
FRC.
2) COMPLIANCE AND RESISTANCE OF
RESPIRATORY SYSTEM
• Compliance of the total respiratory system (lungs
and chest wall) is reduced on average from 95 to 60
ml/cm H2O.

6. • Decreased compliance (CL) :decreased
ventilated volume (Atelectasis and Airway
closure).
• Airway resistance increased: decreased airway
dimension by lowered FRC

8. 3)ATELECTASIS AND AIRWAY CLOSURE
• is cause of Impaired oxygenation,Reduced
compliance of the Respiratory system
• 90 % of the patients irrespective to the choice of
anaesthesia(IV/Inhalational agents or
Spontaneous /controlled ventilation)
• 15 to 20 % lung atelectatic during uneventful
anaesthesia and 50 %of the lung in case of
Thoracic surgery and cardiopulmonary bypass.
• The amount of atelectasis decreases towards
apex which is mostly aerated.
• Idependent of age.
• Obese patients more atelectasis

9. • COPD patient: Less or no atelectasis may be b’coz of
airway closure occurs before alveolar collapse or it
may be due to an altered balance between chest wall
and the lung.
• PREVENTION OF ATELCTASIS
(I) PEEP: complex effects of higher PEEP as SHUNT
is not reduced proportionately.
This persistent of shunt is due to
REDISTRIBUTION BLOOD FLOW towards the more
dependent part of lung which are not aerated or
towards the atelectatic area.
Decrease in venous return and cardiac output as
a result of increased intrathoracic pressure.
Lung collapses rapidly after discontinuation of
PEEP.

10. (II) RECRUITMENT MANEUVER
• Sigh maneuver or double Vt.
• For complete reopening of all collpased lung
tissue 40 cm H2O of inflation pressure( not more
than 7-8 sec) is required. Such large inflation
corresponds to maximum spontaneous
inspiration- VC maneuver
(III)MINIMIZING GAS RESORPTION
• Continuous PEEP is required to prevent rapid
recurrence of the atelectasis.
• 40 % O2 and 60 % N2(mixture of gas) reduces the
propensity for reaccumulation of atelectasis with
20 % reappearing 40 minutes after recruitment.

11. • Thus ventilation during anaesthesia should be
done with moderate FiO2(i.e.,0.3 to 0.4) and
should only be increased if arterial
oxygenation is compromised.
• Alternatively CPAP with 100 % FiO2 can be
used without significant degree of atelectasis.
(IV) MAINTENANCE OF MUSCLE TONE
• Use of an anaesthetic agent that allows
maintenance of respiratory muscle tone will
prevent atelectasis formation. (Ketamine)

12. (4)AIRWAY CLOSURE
• Propensity increases with AGE
• Intermittent airway closure reduces the
ventilation of the affected alveoli-low V/Q regions
• Decrease in FRC which will result in airway
closure.
(5)DISTRIBUTION OF VENTILATION
• Recruitment maneuvers increase dependent lung
ventilation in anaesthetized subjects in the lateral
and supine position, restoring the distribution of
ventilation to that in awake state.

13. (6)DISTRIBUTION OF LUNG BLOOD FLOW
• From upper to lower regions perfusion
improves with slight drop in lowermost
portion of the lung.
• PEEP will cause the redistribution of blood
flow towards the dependent lung region.
• Thus upper lung regions will be poorly
perfused and there’s increase in the dead
space.

14. (7)HYPOXIC PULMONARY VASOCONSTRICTION
• Refers to reflex contraction of vascular smooth
muscle in pulmonary circulation in response to low
regional partial pressure of oxygen(occurs when
PAO2<100 mm Hg,maximum at 30 mm Hg)
• Auto –regulatory mechanism which reduces the
blood flow in hypoxic lung regions.
• Several inhalational anaesthetic agents (Halothane
and isoflurane) inhibits the HPV.

15. • CO2 elimination: anaesthesia impairs CO2
elimination(reduced Minute volume b’coz of
increase in Vd/Vt) and oxygenation of blood.
• OXYGENATION:Impairment in arterial
oxygenation during anaesthesia is more
marked with increased age,obesity, smoking
and venous admixture.

16. FACTORS INFLUENCING RESPIRATORY
FUCNTION DURING ANAESRTHESIA
1. Spontaneous breathing: During spontaneous
breathing the lower dependent portion of the
diaphragm moves the most whereas with
muscle paralysis the upper non dependent part
showed largest displacemnt
2. Increased oxygen fraction: increased shunt
possibly by attenuation of HPV or development
of atelectasis and shunt in low V/Q regions.
3. Age :Impaired Oxygenation with increasing age

17. • V/Q mismatch increases with the age
• Atelectasis formation and shunt doesn not
incease with age in adults.
• Major cause of impaired gas exchange during
anaesthesia at <50 yr is SHUNT and >50 yr it’s
MISMATCH
(4)OBESITY: worsens oxygenation(FRC),atelctasis
formation and shunting
(5) PRE EXISTING LUNG DISEASE:
• SMOKER and chronic lung disease have
impaired gas exchange.

18. • Smokeers with moderate airflow limitation
may have less shunting as compared to
healthy one and very less atlectasis formation
( d/t chronic hyperinflation and tendency to
collapse reduced).
• However there’s considerable amount of V/Q
mismatch with large perfusion fraction to low
V/Q regions.
(6) POSITION :
• SUPINE:FRC decreases by 0.5-1.0 L
• Cranial displacement of the diaphragm

19. • Gravity increases perfusion of dependent
region
• Spontaneous respiration favours dependent
segments and controlled ventilation favours
anterior segments.
• PRONE: decrease in total lung compliance and
increase in work of breathing(d/t compression
of abdomen and thorax)
• Mechanical ventilation improves oxygenation
in ALI/ARDS.
• LATERAL :decrease volume of dependent lung
but increase in perfusion

20. • TRENDELENBERG: decrease in lung capacity
due to shift of abdominal viscera,increase V/Q
mismatch and atlectasis,decreased FRC and
compliance.
(7) REGIONAL ANAESTHESIA
• Depends on type and extension of motor
blockade.
• If all thoracic and lumbar segment involved
Inspiratory capacity reduced by 20 % and ERV
reaches zero.
• Arterial oxygenation and CO2 elimination is
well maintained during spinal and epidural
anaesthesia.

21. RESPIRATORY FUNCTION DURING ONE
LUNG VENTILATION
• One lung is non- ventilated but still perfused
which will lead to shunting and deceased
PaO2.
• Dependent ventilated lung further impedes
oxygenation by atelectasis formation in the
dependent region.
• Recruitment maneuver are also considered in
one lung ventilation.

22. • RIGHT TO LEFT SHUNT: if blood passes through
the lung without contacting the ventilated alevoli
no gas exchange.
• No roxygenation and relase of O2
• This condition called a shunt.
• Po2 decreases and Pco2 increases.
• Healthy people have small shunt(2-3 %)
• Shunt is caused by complete cessation of
ventilation in particulae region e.g,Pneumonia as
result of consolidation whereas V/Q mismatch is
d/t reduced ventilation and blood flow in some
area and increased in other.

23. THANK YOU