1. RESPIRATORY PHYSIOLOGY DURING ANESTHESIA
Presenter – Hitesh Gupta
Moderater – Dr Anil Ohri

2. Anesthesia - impairment in pulmonary function whether patient is breathing spontaneously or ventilated mechanically after muscle paralysis.
20% of patients may suffer from severe hypoxemia(spo2 81% for up to 5 minutes)

3. GA produces
Fall in FRC
Fall in lung compliance
Uneven distribution of ventilation
Increased physiological dead space
Increased P(A-a)O2

4. FRC reduced by
0.8 to 1.0 L - changing body position from upright to supine
another 0.4- to 0.5-L - when anesthesia is given.
Muscle paralysis and mechanical ventilation cause no further decrease in FRC.
average reduction corresponds to around 20% of awake FRC

5. Cranial shift of diaphragm and a decrease in transverse diameter of the thorax contribute to lowered functional residual capacity (FRC).
Decreased ventilated volume (i.e. in atelectasis and airway closure ) is a possible cause of reduced lung compliance (CL).
Decreased airway dimension by the lowered FRC should contribute to increased airway resistance (Raw).

6. Causes of reduced FRC General anesthesia:
due to loss of respiratory muscle tone, which shifts the balance between the elastic recoil force of the lung and the outward force of the chest wall to a lower chest and lung volume.
Maintenance of muscle tone( ketamine anesthesia) does not reduce FRC
Supine Position:
FRC decreases by L
Diaphragm cephalad displacement

7. . Immobility, excessive intravenous fluid administration:
Dependent areas below the heart (zone3-4) are susceptible to edema
this will happen after being immobile (5 hour or more) in supine position with excess volume administration

8. . Surgical position: Supine : FRC Trendelenburg: FRC
Steep trendelenburg: FRC
Lateral decubitus : FRC in dependent lung and FRC in un dependent lung (overall FRC )
Lithotomy : FRC more than supine
Prone : FRC
Prone> lateral decubitus > supine > lithotomy> trendelenburg> steep trendelenburg

9. Ventilation pattern:
Rapid shallow breathing occurs due to reduced compliance - FRC
This can be prevented by
Periodic large mechanical inspiration
Spontaneous sigh
Peep

10. Tracheal tube (low or high pressure cuffs any place in trachea)
. Decreased removal of secretion:
Increasing viscosity & slowing mucocilliary clearance
Tracheal tube (low or high pressure cuffs any place in trachea)
High FiO2
Low moisture
Low temperature
Halogenated anesthetics

11. Compliance and Resistance of the Respiratory System
Static compliance(lungs and chest wall) is reduced – from 95 to 60 mL/cm H2O during anesthesia
static lung compliance- 187 mL/cm H2O awake to 149 mL/cm H2O during anesthesia
Resistance( total respiratory system and lungs)increases both spontaneous breathing and mechanical ventilation
increased lung resistance reflects reduced FRC during anesthesia

12. Causes of decreased lung compliance
Atelectasis
15% to 20% of lung is collapsed at the base of lung during uneventful anesthesia.
thoracic surgery and cardiopulmonary bypass > 50% of the lung can be collapsed.
decreases towards apex of lung
increases with BMI but is independent of age
COPD patients show less atelectasis
Risk factors:
High FiO2
Low V/Q ratio
Longer time exposure of high FiO2 to low V/Q

13. ZONE A – ventilation > perfusion resulting in dead space like effect
ZONE B – perfusion > ventilation leading to low Va/Q and caused impaired oxygenation of blood due to intermittent airway closure
ZONE C – there is complete cessation of ventilation (atelectasis) but still perfusion is there (shunt)

14. Prevention of atelectasis
Positive end expiratory pressure (PEEP)
Application of 10 cm water PEEP can open collapsed lung but it recollapses on cessation of peep
Gen PEEP of 10 cm H2O squeezes perfusion to lower lung
Selective application of PEEP to lower lung might lead to redistribution to upper lung

15. Maintenance of muscle tone
Anesthetic that allows maintaince of respiratory muscle tone will prevent atelectasis e.g ketamine
Pacing of diaphragm through phrenic nerve stimulation prevents atelectasis ,but is too complicated

16. Recruitment maneuvers
Sigh maneuver
Double VT
airway pressure of 30 cm of H2O decrease atelectasis by 50 % of initial size
for complete reopening 40 cm of H2O is req.

17. Prevention of atelectasis
VC maneuver
Vital capacity maneuver is the volume inflated to the maximum breath by the awake subject before anesthesia.
Inflation of lungs to +40 cm H2O maintained for no more then 7 to 8 sec re expand all previously collapsed lung tissue

18. Prevention of atelectasis
Minimising gas resorption
100% O2 - collapse reappears faster but using 40% O2 in nitrogen, atelectasis appears slowly
Avoidance of preoxygenation procedure (ventilation with 30% O2) eliminates atelectasis formation during induction and subsequent anesthesia
CPAP of 10 cm H2O can prevent atelectasis even with 100 % O2

19. Prevention of atelectasis
Postanesthetic oxygenation
Postanesthetic oxygenation (100% O2) 10 minutes before termination of anesthesia together with a VC maneuver at the end of anesthesia will not protect against atelectasis at the end of anesthesia
VC maneuver followed by a low O2 concentration, 40% keeps the lung open after recruitment until end of anesthesia.

20. Airway Resistance Increase airway resistance, leads to airway collapse
Factors:
Decreases in FRC
ETT
Upper and lower airway passages
External anesthesia apparatus

21. Uneven distribution of ventilation
Right > left
Nondependent > dependent
PEEP increases dependent lung ventilation

22. Distribution of Lung Blood Flow(Perfusion)
Uneven distribution
Base> apex
successive increase in perfusion down the lung, from the ventral to the dorsal aspect.
PEEP impede venous return to the right heart and therefore reduce cardiac output.
PEEP causes a redistribution of blood flow toward dependent lung regions.By this upper lung regions may be poorly perfused,causing a dead space–like effect.

23. V/Q ratios V/Q ratio: 0.8 Shunt: V/Q ratio =0, perfusion only
Dead space: V/Q ratio =infinity, ventilation only
Perfusion increases at a greater rate than ventilation
Apical area: higher V/Q ratio
Basal area: lower V/Q ratio (shunt)

25. during anesthesia
increased VA /Q mismatch
increased Venous admixture (approx 10% cardiac output).
increased alveolar dead space

26. Hypoxic Pulmonary Vasoconstriction
Normally PaO2 decrease will cause HPV
inhaled anesthetics inhibit HPV . Aggravate an existing V/Q mismatch
no such effect seen with intravenous anesthetics (barbiturates)
isoflurane and halothane depress the HPV response by 50% at 2 MAC
Direct: nitroprusside ,NTG, Isoproterenol ,inhaled anesthetics, hypocapnia
Indirect: MS , fluid overload, high fio2 , hypothermia ,emboli, vasoactive drugs, lung disease

27. Effect of depth of anesthesia on respiratory drive
Inhaled anaesthetics and barbiturates reduce sensitivity to CO2 and the effect is dose dependent.
due to impeded function of intercoastal muscles
Anaesthesia also reduces response to hypoxia due to effect on carotid body receptors

28. Effect of depth of anesthesia on respiratory pattern
Less than MAC
vary from excessive hyperventilation to breath holding
1 MAC (light anesthesia)
regular pattern with larger VT than normal
More deep
end inspiration pause (hitch) – active and prolong expiration

29. Effect of depth of anesthesia on respiratory pattern
More deep (moderate)
faster and more regular – shallow –no pause – Inspiration = Expiration
Deep
Narcotic- N2O : Deep and slow
Volatiles : rapid & shallow (panting)
Very deep
all inhaled drugs : gasping-jerky respiration – paradoxical movement of chest-abdomen (only diaphragmatic respiration) just like airway semi obstruction or partial paralysis

30. Effect of depth of anesthesia on spontaneous minute ventilation
Minute ventilation decreases progressively as depth of anesthesia increases
ET CO2 increases as depth of anesthesia increases
Increase of CO2 caused by halogenated anesthetics
(<1.24 MAC) enflurane > desflurane =isoflurane > sevoflurane > halothane
(>1.24 MAC) enflurane = desflurane > isoflurane > sevoflurane
Ventilation response to CO2 increase is decreased
Apnea threshold is increased

31. Factors That Influence Respiratory Function During Anesthesia
Spontaneous Breathing
FRC is reduced to the same extent during anesthesia
atelectasis occurs to almost the same extent in anesthetized spontaneously breathing subjects as during muscle paralysis.

32. Increased Oxygen Fraction
As Fio2 is increased, shunt is also increased
explained by attenuation of HPV response with increasing Fio2 or further development of atelectasis and shunt in lung units with low VA /Q ratios
Body Position
FRC is reduced in supine position
Lateral position causes severe impairment in arterial oxygenation in some patients.
ventilation distribution was more uniform in anesthetized subjects who were in the prone position

33. Age arterial oxygenation is impeded with increasing age of the patient
shunt is independent of age 23 to 69 years
There is increasing VA /Q mismatch with age
major cause of impaired gas exchange during anesthesia at ages younger than 50 years is shunt, whereas at higher ages mismatch becomes increasingly important.

34. Obesity worsens the oxygenation of blood
markedly reduced FRC, which promotes airway closure to a greater extent than in a normal subject
PEEP , CPAP or near-VC inflations followed by PEEP ventilation

35. Preexisting Lung Disease
Smokers and patients with lung disease have severe impairment of gas exchange in the awake state as well as during anesthesia
smokers with moderate airflow limitation have less shunt, however, considerable Va /Q mismatch with a large perfusion fraction to low Va /Q regions
Reason - chronic hyperinflation which changes the mechanical behavior of the lungs and their interaction with the chest wall such that the tendency to collapse is reduced
these low Va /Q ratios can be converted over time to resorption atelectasis.

36. Regional Anesthesia
extensive blocks (thoracic and lumbar segments)- inspiratory capacity is reduced by 20% and expiratory reserve volume approaches zero.
Diaphragmatic function is often spared, even in sensory block up to the cervical segments.
Arterial oxygenation and carbon dioxide elimination are well maintained

37. Thank
you