Download respiratory physiology in annaesthesia

RESPIRATORY
PHYSIOLOGY DURING
ANESTHESIA
Presenter – Hitesh Gupta
Moderater – Dr Anil Ohri
• 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)
• GA produces
1.
Fall in FRC
2.
Fall in lung compliance
3.
Uneven distribution of ventilation
4.
Increased physiological dead space
5.
Increased P(A-a)O2
• 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
• 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).
5
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 0.8-1.0L
• Diaphragm cephalad displacement
. 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
. Surgical position:
1.
Supine : FRC
2.
Trendelenburg: FRC
3.
Steep trendelenburg: FRC
4.
Lateral decubitus : FRC in dependent lung and FRC in un
dependent lung (overall FRC )
5.
Lithotomy : FRC more than supine
6.
Prone : FRC
Prone> lateral decubitus > supine > lithotomy> trendelenburg>
steep trendelenburg
Ventilation pattern:
• Rapid shallow breathing occurs due to reduced
compliance - FRC
• This can be prevented by
• Periodic large mechanical inspiration
• Spontaneous sigh
• Peep
. Decreased removal of secretion:
Increasing viscosity & slowing mucocilliary clearance
1. Tracheal tube (low or high pressure
2.
3.
4.
5.
cuffs any place in trachea)
High FiO2
Low moisture
Low temperature
Halogenated anesthetics
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
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
• 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)
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
• 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
• 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.
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
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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
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.
Airway Resistance
• Increase airway resistance,
leads to airway collapse
• Factors:
• Decreases in FRC
• ETT
• Upper and lower airway passages
• External anesthesia apparatus
Uneven distribution of ventilation
• Uneven distribution
• Right > left
• Nondependent > dependent
• PEEP increases dependent lung ventilation
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.
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)
• during anesthesia
increased VA /Q mismatch
increased Venous admixture (approx 10% cardiac
output).
increased alveolar dead space
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
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
27
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
28
Effect of depth of anesthesia on
respiratory pattern
•
More deep (moderate)
faster and more regular – shallow –no pause – Inspiration =
Expiration
•
Deep
1. Narcotic- N2O : Deep and slow
2. 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
29
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
30
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.
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
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.
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
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.
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