202
U N I T 2
Integrative Body Functions
hyperventilation syndrome, which is characterized
by recurring episodes of overbreathing often associ-
ated with anxiety. Persons experiencing panic attacks
frequently present in the emergency room with mani-
festations of acute respiratory alkalosis. Progesterone
increases ventilation in women; during the progesterone
phase of the menstrual cycle, normal women decrease
their PCO
2
values by 2 to 4 mm Hg and increase their
pH by 0.01 to 0.02.
74
Women can also develop sub-
stantial decrease in PCO
2
during pregnancy, most nota-
bly during the last trimester.
72
Respiratory alkalosis has long been recognized as
a common acid–base disorder in critically ill patients,
and is a consistent finding in both septic shock and
the systemic inflammatory response syndrome (see
Chapter 20). Hypoxemia exerts its effect on pH
through the peripheral chemoreceptors in the carotid
bodies. Stimulation of peripheral chemoreceptors
occurs in conditions that cause hypoxemia with rela-
tively unimpaired CO
2
transport such as exposure to
high altitudes.
Mechanical ventilation may produce respiratory
alkalosis if the rate and tidal volume are set so that
CO
2
elimination exceeds CO
2
production. Because car-
bon dioxide crosses the alveolar capillary membrane
20 times more rapidly than oxygen, the increased min-
ute ventilation may be necessary to maintain adequate
oxygen levels while producing a concomitant decrease
in CO
2
levels. In some cases, respiratory alkalosis may
be induced through mechanical ventilation as a means
of controlling disorders such as severe intracranial
hypertension.
Manifestations.
Respiratory alkalosis manifests with a
decrease in PCO
2
and a H
2
CO
3
deficit. The pH is above
7.45, arterial PCO
2
is below 35 mm Hg, and serum
HCO
3
–
levels usually are below 24 mEq/L (24 mmol/L).
The signs and symptoms of respiratory alkalosis are
associated with hyperexcitability of the nervous sys-
tem and a decrease in cerebral blood flow
75
(see Table
8-11). A decrease in the CO
2
content of the blood causes
constriction of cerebral blood vessels. CO
2
crosses the
blood–brain barrier rather quickly; thus, the manifesta-
tions of acute respiratory alkalosis are usually of sudden
onset. The person often experiences light-headedness,
dizziness, tingling, and numbness of the fingers and toes.
These manifestations may be accompanied by sweating,
palpitations, panic, air hunger, and dyspnea. Chvostek
and Trousseau signs may be positive, and tetany and
convulsions may occur. Because CO
2
provides the stimu-
lus for short-term regulation of respiration, short peri-
ods of apnea may occur in persons with acute episodes
of hyperventilation.
Treatment.
The treatment of respiratory alkalosis focuses
on measures to correct the underlying cause. Hypoxia
may be corrected by administration of supplemental oxy-
gen. Changing ventilator settings may be used to prevent
or treat respiratory alkalosis in persons who are being
mechanically ventilated. Persons with hyperventilation
syndrome may benefit from reassurance, rebreathing
from a paper bag during symptomatic attacks, and
attention to the psychological stress associated with the
disorder.
SUMMARY CONCEPTS
■■
Normal body function depends on the precise
regulation of acid–base balance. Metabolic
processes produce the volatile carbonic acid
(H
2
CO
3
) in equilibrium with dissolved carbon
dioxide (PCO
2
), which is eliminated through the
lungs, and nonvolatile acids, which are excreted
by the kidneys.
■■
Because of its low concentration in body fluids,
the hydrogen (H
+
) concentration is expressed as
pH, or the negative log of the H
+
ion concentration.
It is the ratio of the bicarbonate (HCO
3
–
)
concentration to H
2
CO
3
(PCO
2
), normally 20:1, that
determines body pH.
■■
The ability of the body to maintain pH within
the normal range depends on intracellular and
extracellular buffers, as well as respiratory
and renal compensatory mechanisms. The
respiratory regulation of pH, which relies
on pulmonary ventilation for release of CO
2
into the environment, is rapid but does not
return the pH completely to normal. Renal
mechanisms, which rely on the elimination of
H
+
ions and conservation of HCO
3
–
ions, take
longer but return pH to normal or near-normal
levels.
■■
Metabolic acid and base disorders reflect an
decrease or increase in HCO
3
–
. Metabolic acidosis,
which reflects a decrease in pH due to a decrease
in HCO
3
–
, is caused by conditions that prompt
an excessive production and accumulation of
metabolic acids or excessive loss of HCO
3
–
.
Metabolic alkalosis, which reflects an increase
in pH due to an increase in HCO
3
–
, is caused by
conditions that produce a gain in HCO
3
–
or a
decrease in H
+
.
■■
Respiratory acid–base disorders reflect an
increase or decrease in PCO
2
levels due to altered
pulmonary ventilation. Respiratory acidosis,
which reflects a decrease in pH due an increase in
PCO
2
levels, is caused by conditions that produce
hypoventilation. Respiratory alkalosis, which
reflects an increase in pH due to a decrease in
PCO
2
levels, is caused by conditions that produce
hyperventilation.
