Page 170 - Basic Monitoring in Canine and Feline Emergency Patients

followed by an inability to depolarize, leading to then ‘meet’ in the center portion of the testing slide,
bradycardia and/or other bradyarrhythmias forming a concentration cell. The resulting electri-
VetBooks.ir (although rarely tachyarrythmias may occur). cal potential of this junction between reference
solution and sample solution is measured (i.e. the
Hyperkalemic animals may also be acidemic. Often
hyperkalemia occurs concurrently with decreased
trations and types. Specifically, a volt meter is used
renal excretion of acids, but acidemia may also ‘junction cell’) and correlated to the ionic concen-
occur when hydrogen ions translocate out of the to determine the electrical potential and a micro-
cells into the bloodstream in exchange for potas- processor mathematically converts this voltage into
sium ions shifting into the cells to improve the the concentration of the ion in the blood sample.
hyperkalemia (see Fig. 8.4). Typically, there is either an ion-specific membrane
through the test electrode so that only the electro-
lyte of interest can move into the junction cell and
8.2 How the Monitor Works
be measured or separate ISE for each electrolyte to
When measuring electrolytes, clinicians are limited be measured.
to either measuring them on a bench-top analyzer Direct and indirect ISE techniques exist. With
as part of a full chemistry panel (either in-house or direct ISE, the blood sample is presented to the
sent out to a laboratory) or via a point-of-care electrode and the electrolyte levels are directly
analyzer. There are two main ways that laboratory measured. With indirect ISE, the blood sample is
machines measure electrolytes – flame photometry first diluted in a buffer by the machine before being
(flame atomic emission spectrometry) or ion-selective presented to the electrode; this allows for measure-
electrodes (ISE). Flame photometry is an older ment of the electrolyte activity versus the concen-
technology where the sample is heated via a flame tration of the buffer solution. This yields values
or burner until the liquid portion of the sample most closely correlated to flame photometry which
evaporates, leaving the ions. These ions are excited is the historic gold standard. Most laboratories and
when they absorb energy from the flame and, after point-of-care instrumentation use indirect ISE.
the heat is removed, they return to their normal Most bedside point-of-care analyzers use a minia-
(non-excited state) while giving off a characteristic turized version of ion-specific electrode technology
wavelength of radiation. This radiation signature to determine electrolyte concentrations which may
and its magnitude is measured by the analyzer and be direct or indirect ISE.
in turn is proportional to the concentration of the Laboratory machines also use colorimetry to
electrolyte in the blood. measure total calcium and phosphorus in labora-
The downside of flame photometry is that for tory analyzers. The complexed and ionized calcium
electrolytes other than sodium and potassium, the in the blood reacts with a dye such as orthocresol-
absorption and resulting release of radiation from phthalein to form a colored complex; this complex
the electrolytes is too variable to be reliable and/or is measured spectrophotometrically and the amount
the analyzers cannot detect electrolytes at a low (density) of the color is indicative of the volume of
enough level to be clinically useful. In addition, calcium. A similar technique is used for free or
some electrolytes can alter the flame temperature complexed inorganic phosphorus, except a differ-
or react with the flame or environment to make ent dye (molybdate) is used.
new compounds (e.g. calcium combines with oxy-
gen from the flame to form CaO), nullifying their
ability to be measured. Therefore, if flame atomic 8.3 Indications
emission spectrometry is used, it is limited to deter-
mining potassium and sodium levels. Sodium
The other major technology used in laboratory When handling emergency or critical care patients,
machines is ISE to measure electrolyte concentra- sodium is generally a marker of the volume of
tions. This technology is used for sodium, chloride, water in the plasma relative to the amount of
ionized calcium, and potassium. With ISE, one sodium ions. Monitoring sodium is most impor-
electrode is in contact with or contains a reference tant in animals with kidney disease (or other non-
solution with a known concentration of an electro- renal causes of diuresis such as hyperglycemia or
lyte to be measured and the other electrode is in mannitol administration), polyuria/polydipsia,
contact with the blood sample. The two solutions vomiting, and/or diarrhea. Clinicians can alter

162 E.J. Thomovsky