Panic value < 7.2 or > 7.6 < 7.2 or > 7.6
Panic value < 10 mEq/L < 10 mmol/L
O2 saturation 60–80% 0.60–0.80
Partial Pressure of Carbon Dioxide (PCO2)
PaCO2 (arterial blood) 35–45 torr
PvCO2 (venous blood) 38–50 torr.
Carried in blood in two ways: 10% carried in plasma,
This test is a measurement of the pressure or tension
exerted by dissolved CO2 in the blood and is proportional
to the partial pressure of CO2 in the alveolar air. The test
is commonly used to detect a respiratory abnormality
Blood Gases and Electrolytes 551
and to determine the alkalinity or acidity of the blood. In
order to maintain CO2 within normal limits, the rate and
depth of respiration vary automatically with changes in
metabolism. It is an index of alveolar ventilation and is
reflects how well air is exchanging with blood in the
CO2 tension in the blood and cerebrospinal fluid (CSF)
is the major chemical factor regulating alveolar ventilation.
When the CO2 of arterial blood rises from 40 to 45 torr, it
causes a three-fold increase in alveolar ventilation. A CO2
of 63 torr in arterial blood increases alveolar ventilation
tenfold. When the CO2 concentration of breathed air
exceeds 5%, the lungs can no longer be ventilated fast
enough to present a dangerous rise of CO2 concentration in
tissue fluids. Any further increase in CO2 begins to depress
the respiratory center, causing a progressive decline in
respiratory activity rather than an increase.
1. Obtain an arterial blood sample.
2. Do not expose sample to air.
3. A small amount of blood is then introduced into a
blood gas analyzer and the CO2 tension is measured
with a silver-silver chloride electrode (Severinghaus
in PCO2 through its effect on plasma bicarbonate
concentration decreases renal bicarbonate reabsorption. For each mEq/L fall in HCO3, the PCO2 falls
by 1 to 1.3 mm of Hg. Because HCO3 and PCO2 bear this
close mathematical relationship, and this ratio in turn
defends the hydrogen ion concentration, the outcome
is that the steady state PCO2 in simple metabolic
acidosis is equal to the last two digits of the pH. Also,
addition of 15 to the bicarbonate level also equals the
last two digits of the pH. Failure of the PCO2 to achieve
predicted levels defines the presence of superimposed
respiratory acidosis on alkalosis.
2. The causes of decreased PCO2 include:
• Other cause of hyperventilation.
3. The causes of increased PCO2 are:
b. Reduced function of respiratory center
c. Other more rare causes of hypoventilation, such as
Increased PCO2 may occur even with normal lungs if the
respiratory center is depressed. Always check laboratory
reports for abnormal values. In interpreting laboratory
reports remember that PCO2 is a gas and is regulated by
Arterial blood saturation SaO2 = 95% or higher mixed
venous blood saturation SvO2 = 75%.
This measurement is a ratio of the actual oxygen (O2)
content of the hemoglobin compared to the potential
maximum O2 carrying capacity of the hemoglobin.
The percentage of SO2 saturation is a measure of the
relationship between O2 and hemoglobin. The percentage
of saturation does not indicate the O2 content of arterial
blood. The maximum amount of O2 that can be combined
with hemoglobin is called the O2 capacity. The combined
measurements of O2 saturation, partial pressure of O2, and
of hemoglobin will indicate the amount of O2 available to
the tissue (tissue oxygenation).
Obtain an arterial blood sample. Two methods for
determining oxygen saturation are used.
1. The blood sample is introduced into the oximeter,
which is a photoelectric device for determining the
oxygen saturation of the blood. The value is measured
directly with an oximeter (i.e. spectrometry).
2. Oxygen saturation is calculated from the oxygen
content and oxygen capacity determinations.
Percentage saturation = O2 capacity volume%
100 × volume of O2 actually combined with Hb
volume of O2 with which Hb is capable of combination
The O2 content of blood sample is measured before and
(Volume% = volume percentage = mL/100 mL of blood).
The actual amount of oxygen (O2) in the blood is termed the
oxygen content. Blood can contain less O2 than it is capable
of carrying. About 98% of all O2 delivered to the tissues is
transported in chemical combination with hemoglobin.
One gram of hemoglobin can carry or is capable of
combining with 1.34 mL of O2, whereas 100 mL of blood
plasma can carry upto 0.3 mL of O2. This measurement is
determined mathematically by multiplying the number of
grams of hemoglobin in 100 mL of blood by 1.34 times the
partial pressure of oxygen in the blood.
1. An arterial or venous blood sample is obtained
O2 content = SaO2 × Hb × 1.34 + PaO2 × 0.003.
Decreased arterial blood O2 associated with increased
arterial blood CO2 can be due to:
1. Chronic obstructive lung disease
2. Patients with respiratory complications postoperatively
Partial Pressure of Oxygen (PO2)
PaO2 80 torr or greater: arterial sample
PvO2 30–40 torr: venous or peripheral blood sample.
Oxygen (O2) is carried in the blood in two forms dissolved
and in combination with hemoglobin. Most of the O2 in the
blood is carried by hemoglobin. It is the partial pressure
of a gas that determines the force it exerts in attempting
to diffuse through the pulmonary membrane. The partial
pressure reflects the amount of O2 passing from the
pulmonary alveoli into the blood and is directly influenced
by the amount of O2 being inhaled.
This is a measure of the pressure exerted by the amount
of O2 dissolved in the plasma. It is a test that measures
the effectiveness of the lungs to oxygenate the blood. The
severity of impairment of the ability of the lungs to diffuse
O2 across the alveolar membrane into the circulating blood
is indicated by the level of partial pressure of oxygen (PO2).
1. An arterial blood sample is obtained.
2. A small amount of blood is then introduced into a blood
gas analyzing machine and the O2 tension is measured
with a polarographic electrode (Clark electrode).
1. Increased levels are associated with:
b. Hyperventilation in an arterial blood sample.
2. Decreased levels are associated with:
c. Insufficient atmospheric O2
e. Chronic obstructive disease
f. Restrictive pulmonary disease
g. Hypoventilation due to neuromuscular disease.
3. Decreased arterial PO2 with normal or decreased
arterial blood PCO2 tension is associated with:
a. Diffuse interstitial pulmonary infiltration
d. Postoperative extracorporeal circulation.
Carbon Dioxide (CO2) Content or Total
In normal blood plasma, more than 95% of the total CO2
content is contributed by bicarbonate (HCO3), which
is regulated by the kidneys. The other 5% of the CO2 is
contributed by the dissolved CO2 gas and carbonic acid
(H2CO3). Dissolved CO2 gas, which is regulated by the
Blood Gases and Electrolytes 553
lungs, therefore, contributes little to the total CO2 content.
Total CO2 content gives little information about the lungs.
in the extracellular spaces exists first as CO2 then
as H2CO3, and thereafter, much of it is changed to sodium
bicarbonate (NaHCO2) by the buffers of the plasma and
This test is a general measure of the alkalinity or acidity of
the venous, arterial or capillary blood. This test measures
Total CO2 = HCO3¯ + 0.03 × PCO2
1. A venous or arterial blood sample of 6 mL is collected
2. If the collected blood sample cannot be studied
immediately, the syringe should be placed in an iced
1. Elevated CO2 content levels occur in:
d. Use of mercurial diuretics.
2. Decreased CO2 content levels occur in:
f. Use of chlorothiazide diuretics.
(Note: In diabetic acidosis the supply of ketoacids exceeds
the demands of the cell. Blood plasma acids rise. Blood
plasma HCO3 decreases because, it is used in neutralizing
A double use of the CO2 is one of the main reasons why
understanding acid-base problems may be difficult. Use
the terms CO2 content and CO2 gas to avoid confusion.
1. CO2 content is mainly bicarbonate and a base. It is a
solution and is regulated by kidneys.
2. CO2 gas is mainly acid. It is regulated by the lungs.
1. Drugs that may cause increased or decreased levels
2. Drugs that may cause decreased levels include:
The pH is the negative logarithm of the hydrogen ion
concentration in the blood. The sources of hydrogen ions
are: (i) volatile acids, which can vary between a liquid and
a gaseous state, and (ii) non-volatile acids that cannot be
volatilized and are fixed (e.g. dietary acids, lactic acids and
This is a measurement of the chemical balance in the
body and is a ratio of acids to bases. A determination of
the blood pH is one of the best ways to tell if the body is
too acid or too alkaline. Low pH numbers (< 7.35) indicate
an acid stage, and higher pH numbers (> 7.45) indicate an
alkaline state. This balance is extremely intricate and must
be kept within the very slight margin of 7.35 to 7.45 pH
(alkaline) in the extracellular fluid. pH limits compatible
The respiratory response to changes in blood pH is
almost instantaneous. Acidosis (CO2 retained; pH falls)
stimulates ventilation; alkalosis (CO2 blown off; pH
rises) depresses ventilation. The respiratory center in the
medulla appears to respond to a pH intermediate between
those of the blood and CSF (7.35–7.40).
1. An arterial blood sample is obtained.
2. Two methods of determining the pH are used, the
direct method and the indirect method:
a. Direct method: A small amount of blood is introduced into a blood gas machine and the pH is
b. Indirect method: The Henderson-Hasselbalch
Depression of respiratory centers
Interference with mechanical function of
(a) Deformity of thoracic cage
acidemia (acute ventilatory failure)
acidemia (chronic respiratory failure)
(c) Excessive artificial respiration
(a) Uncompensated respiratory alkalemia
(acute alveolar hyperventilation)
(b) Compensated respiratory alkalemia
(Chronic alveolar hyperventilation)
Hyperventilation through stimulation of
(a) Uncompensated nonrespiratory
(b) Compensated respiratory acidemia
Potassium or chloride depletaion base
(a) Uncompensated nonrespiratory alkalemia
(b) Compensated respiratory alkalemia
antagonistic or a synergistic effect upon each ones.
2. Compensation is most efficient is respiratory and nonrespiratory acidemia.
defines the presence of superimposed respiratory acidosis or alkalosis.
TABLE 21.1: Four basic forms of acid-base imbalance and their compensatory mechanism
Blood Gases and Electrolytes 555
major blood base pH = pk + log major blood acid
1. Generally speaking, the pH is decreased in acidemia
because of increased formation of acids. pH is increased in alkalemia because of a loss of acids.
2. When attempting to interpret an acid-base abnormality,
a. Check the pH to see if there is an alkalemia or an
No comments:
Post a Comment
اكتب تعليق حول الموضوع