- General anesthesia typically reduces both V̇o2 and V̇co2 by about 15%.
Additional reductions are often seen as a result of hypothermia.
The greatest reductions are in cerebral and cardiac O2 consumption.
- At end-expiration, intrapleural pressure normally
averages about –5 cm H2O and because alveolar
pressure is 0 (no flow), transpulmonary pressure is +5
- The lung volume at the end of a normal exhalation
is called functional residual capacity (FRC). At this volume, the
inward elastic recoil of the lung approximates the outward elastic
recoil of the chest (including resting diaphragmatic tone).
- Closing capacity is normally well below FRC,
but it rises steadily with age. This increase is probably responsible
for the normal age-related decline in arterial O2 tension.
- Whereas both forced expiratory volume in 1 s
(FEV1) and forced vital capacity (FVC) are effort dependent,
forced midexpiratory flow (FEF25–75%) is
effort independent and may be a more reliable measure of obstruction.
- Induction of anesthesia consistently produces
an additional 15–20% reduction in FRC (400 mL
in most patients) beyond what occurs with the supine position alone.
- Local factors are more important than the autonomic
system in influencing pulmonary vascular tone. Hypoxia is a powerful
stimulus for pulmonary vasoconstriction (the opposite of its systemic
- Because alveolar ventilation (V̇a) is normally about 4 L/min
and pulmonary capillary perfusion (Q̇) is 5 L/min,
the overall V̇/Q̇ ratio is about 0.8.
- Shunting denotes the process whereby desaturated,
mixed venous blood from the right heart returns to the left heart
without being resaturated with O2 in the lungs. The overall
effect of shunting is to decrease (dilute) arterial O2 content;
this type of shunt is referred to as right-to-left.
- General anesthesia commonly increases venous
admixture to 5–10%, probably as a result of atelectasis
and airway collapse in dependent areas of the lung.
- Note that large increases in Paco2 (> 75 mm Hg) readily produce
hypoxia (Pao2 < 60 mm Hg)
at room air but not at high inspired O2 concentrations.
- The binding of O2 to hemoglobin appears
to be the principal rate-limiting factor in the transfer of O2 from
alveolar gas to blood.
- The greater the shunt, the less likely the possibility
that an increase in the fraction of inspired oxygen (Fio2) will prevent hypoxemia.
- A rightward shift in the oxygen–hemoglobin
dissociation curve lowers O2 affinity, displaces O2 from
hemoglobin, and makes more O2 available to tissues; a leftward
shift increases hemoglobin’s affinity for O2,
reducing its availability to tissues.
- Bicarbonate represents the largest fraction
of CO2 in blood.
- Central chemoreceptors are thought to lie on
the anterolateral surface of the medulla and respond primarily to
changes in cerebrospinal fluid [H+].
This mechanism is effective in regulating Paco2,
because the blood–brain barrier is permeable to dissolved
CO2 but not to bicarbonate ions.
- With increasing depth of anesthesia, the slope
of the Paco...
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