3. Amount of air in the lungs after normal
expiration.
Dependent on sex, age, height, and weight. FRC
goes up with height and age and down with
weight and is smaller in women than in men.
With increased ventilation, as during exercise,
VT is raised by increasing both inspiration and
expiration so that FRC is lowered by approx
0.5 L.
4. However, in the presence of airway obstruction,
as in asthma, for example, expiration is slowed
down so that the end-expiratory level is elevated
instead of being lowered. This is termed air
trapping.
FRC increases with old age (loss of elastic tissue)
and also in COPD (due to chronic air trapping
and more severe loss of elastic tissue).
FRC decreases in idiopathic fibrosis,
pneumoconiosis, and different forms of
granulomatosis and vasculitis.
5. PEEP
Defined as that positive pressure which is
applied during the end of expiration – which
increases the end expiratory pressure to more
than atmospheric pressure.
Often used to improve patient’s oxygen
status esp in patients with refractory
hypoxemia.
Not a stand alone mode.
6.
7. Indications of PEEP :
1. Refractory hypoxemia to any cause
2. Intra pulmonary shunts
3. Decreased FRC & lung compliance
8. Refractory hypoxemia:
Hypoxemia which responds poorly to moderate
to high levels of oxygen (FiO2 = 1)
Useful clinical guide is a PaO2 -60mm Hg or
less at FiO2 = > 0.5
9. Intrapulmonary Shunts & hypoxemia :
Caused by decreased FRC, atelectasis, V/Q
mismatch.
Decreased FRC & lung compliance:
PEEP, here increases FRC and decreases the
work of breathing, hence improves the
symptoms.
10. PHYSIOLOGY OF PEEP:
Positive pressure at the end of expiration ->
reinflates the collapsed alveoli -> maintains
the alveolar inflation even during expiration ->
increases FRC -> improves ventilation.
Once the recruitment of these alveoli has
occurred, PEEP lowers the alveolar distending
pressure and facilitates the gas diffusion and
oxygenation.
11. Complications of PEEP
1. Decreases venous return
2. Decreased cardiac output
3. Barotrauma
4. Increased intra cranial pressure
5. Alters renal metabolism and water balance
12. CPAP :
Is actually PEEP applied to a spontaneously
breathing person.
The indications are those similar to PEEP.
Additional requirement being that the patient
must have an adequate lung function to sustain
the eucapnic ventilation.
Methods include face mask, nasal mask, ET
tube.
Neonates – nasal CPAP is the method of choice
13. Classically used in obstructive sleep apnoea.
Here it prevents the airway from collapsing.
Because of the risk of gastric distension and
regurgitation, CPAP masks must be used only
in patients with intact airway reflexes and with
CPAP levels less than 15 cm H2O.
14. Auto PEEP:
Incomplete expiration prior to the initiation
of the next breath causes progressive air
trapping (hyperinflation). This accumulation
of air increases alveolar pressure at the end of
expiration, which is referred to as auto-PEEP
or intrinsic PEEP.
15. Auto-PEEP develops commonly in high minute
ventilation (hyperventilation), expiratory flow
limitation (obstructed airway) and expiratory
resistance (narrow airway).
The PEEP value range commonly used ranges
from 5-10 cm H2O, though the upper limit has
been posted at 20 cm H2O. Levels above that is
associated with increased incidence of
complications especially barotrauma. PEEP is
increased at the rate of 2-3cm H2O depending
on the clinical condition of the patient.
16. Contra indications to the use of PEEP/CPAP:
1. Absolute :
Tension pneumothorax
2. Relative :
Barotrauma
Hypovolemia
Increased ICP
Preexisiting bullae
Recent lung surgeries
Unilateral lung disorders
