Page 106 - Basic Monitoring in Canine and Feline Emergency Patients

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(SO , HCO , BE, etc.) are calculated. The pH is gas inside the bottle will increase, leading to a large
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release of gas if the bottle is opened. The total
measured using a Sanz electrode, which consists of
VetBooks.ir two electrodes; a silver–silver chloride ‘measuring’ amount of gas in the sealed container has not
changed, but the proportion of gas in solution ver-
electrode bathed in a solution of constant pH and
sus in the air will change. What is measured in
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surrounded by a H sensitive glass membrane, and
a silver–silver chloride or calomel (mercurous blood gas analysis is the partial pressure (reported
chloride) ‘reference’ electrode in a KCl solution as PaCO or PaO ) of the gas in solution. The par-
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that generates a steady voltage. When the sample tial pressure will increase as temperature increases,
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passes by the membrane, it changes the H concen- and decrease as temperature decreases. Changes in
tration of the measuring electrode solution and the temperature will also shift the oxyhemoglobin dis-
difference in voltage between the two electrodes is association curve (see Chapter 4). Many blood gas
used to calculate the pH. analyzers will allow the user to input the patient’s
A Severinhaus electrode, used to measure PCO , actual body temperature and will perform tempera-
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works in a similar fashion. The reference electrode ture corrections for PCO , PO , and SO . Although
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is identical to that in the Sanz electrode, but the the formulas used to calculate these corrections are
measuring electrode is a platinum electrode within not linear, around normal body temperatures (37–
a pH sensitive glass membrane. The electrode is 38°C), PaO will decrease by about 5 mmHg and
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surrounded by a thin layer of bicarbonate solution PaCO decrease by about 2 mmHg for every 1°C
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contained within an outer membrane that permits drop in body temperature. Therefore, unless body
CO , but not H , to diffuse in from the sample. temperature changes are extreme, it is unlikely that
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When CO diffuses into the bicarbonate solution, it temperature correction will change clinical decision
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creates carbonic acid which in turn disassociates to making based on these small unit changes.
produce H (see Box 5.1) which is detected by the There is considerably controversy in the human
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measuring electrode. The voltage difference literature as to whether using non-temperature-
between the reference and Severinhaus electrodes is corrected values assuming a normal body tempera-
used to determine the change in pH, which is ture (e.g. 37ºC, the ‘alpha-stat’ method) provides
extrapolated to PCO using the Henderson– any different outcomes from using temperature-
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Hasselbalch equation (see Box 5.2). corrected values based on the patient’s actual body
The PO is measured using a Clarke electrode, temperature (the ‘pH-stat’ method). These contro-
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which consists of a platinum cathode and a silver– versies are more applicable to situations such as
silver chloride anode in an electrolyte solution, cardiac bypass or post-arrest–induced hypothermia,
separated from the patient sample by an oxygen- where temperatures are lowered to extreme levels.
permeable membrane. When oxygen contacts the In addition, values considered ‘normal,’ whether
cathode, it is reduced and draws electrons from the temperature corrected or not, may be overshooting
anode to the cathode to complete an electrical cir- or undershooting the actual needs of the body at
cuit. The current generated by this circuit is propor- dysregulated temperatures. This is because when
tional to the amount of oxygen present. hypothermic the body utilizes less O and produces
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Bicarbonate values are calculated using the less CO , and vice versa when hyperthermic. Currently,
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Henderson–Hasselbalch equation (see Box 5.2) from human medical literature seems to be leaning to the
the measured values of pH and PCO . Base excess is use of non-corrected (alpha-stat) values in adults, and
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calculated from the PCO and pH ± [Hb] using pre- corrected (pH-stat) values in pediatric patients, but
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determined formulas. The SaO is calculated on most this is still an area of much debate. There is no evi-
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analyzers from the pH, [Hb], and PaO . Analyzers dence in veterinary medicine as to the preferred
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with spectrophotometric capabilities (e.g. co-oxime- method; each practice should decide whether to use
try) can directly measure the SaO . See Chapter 4 for temperature-corrected values or not to make medical
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more information on co-oximetry. decisions, and be consistent between clinicians.
Blood gas analyzers assess all samples at normal
human body temperature (37ºC). However, the
partial pressure of gasses within the blood change 5.3 Indications
with temperature. Think again of the analogy of a Ideally, venous blood gas assessment to evaluate ini-
soda bottle: when the temperature rises, the gas tial pH and electrolyte status should be performed in
becomes less soluble and the partial pressure of the any truly emergent patient. As many blood gas panels
98 A.C. Brooks