■ Barotrauma
■ Caused by excessive pressure
■ Prevented by lowering inspiratory pressures
■ Ventilator associated pneumonia
■ Risk increases exponentially in relationship to duration of intubation.
■ Decrease risk by sitting patients up in bed by at least 30 degrees if not
contraindicated.
■ Early pneumonias(< 72 hours postintubation): Community acquired
pathogens
■ Late pneumonias(> 72 hours postintubation): Nosocomial pathogens,
more resistant strains
■ Hemodynamic instability
■ High respiratory rate, PEEP, or inverse ratio ventilation may increase
intrathoracic pressure, decreasing venous return →decreased cardiac
output→hypotension.
■ May also increase cerebral venous pressure →cerebral ischemia
Blood Gases
Blood gases are a useful tool in the evaluation of the patient with respiratory
failure. The use of blood gases to evaluate acid-base status is covered separately
(see Chapter 7).
BASICPRINCIPLES OFPULMONARYPHYSIOLOGY
VENTILATION
■ Theclearance of CO 2 from the body is primarily determined by ventila-
tion.
■ Ventilation is primarily a function of the rate and depth (tidal volume) of
breathing.
■ The air that does not participate in gas exchange (approximately 30% in
normal conditions) = dead space.
■ An increase in dead space (picture breathing through a snorkel) requires
increased ventilation to maintain CO 2 clearance.
■ Diseases with ↑dead space include: COPD, PE.
DIFFUSION
■ Diffusion = movement of O 2 (or other gases) across the alveolar-capillary
membrane.
■ ↑Solubility of gases →better diffusion.
■ O 2 is much less soluble than CO 2 , therefore more easily affected by dis-
eases of the alveolar-capillary membrane.
■ Diseases affecting the alveolar-capillary membrane include: Pulmonary
edema, interstitial fibrosis.
■ ↓Diffusion→low PaO 2 and an increased Alveolar-arterial (A-a) gradient
on blood gas, which improve with supplemental oxygen.
SHUNT
■ Physiologic right-to-left shunt= blood flow (perfusion) to alveoli that
cannot participate in gas exchange (therefore no oxygenation).
■ Occurs in setting of nonfunctioning alveoli, such as ARDS, pneumonia
RESUSCITATIONDecreased ventilation OR an
increase in dead space →
increased CO 2 on blood gas.An increase in dead space
(COPD, PE) requires increased
ventilation to maintain CO 2
clearance.Oxygenation is adversely
affected by diseases of the
alveolar-capillary membrane
that decrease diffusion
(eg. interstitial fibrosis).