Delivery of adequate oxygen to tissues and cells is the overall goal of the cardiovascular system. Oxygenation is dependant
on cardiac output (heart rate and stroke volume), arterial blood pressure, circulating blood volume, and pulmonary function.
Critically ill patients often have impairments to one or several of these processes resulting in hypoxemia, ischemia and cell
death. Restoring these functions to normal insures oxygenation.
A pulse oximeter provides immediate and continual estimations of oxyhemoglobin saturation (SpO2) by transmission of varying
light through the skin. Light absorption during arterial pulses is differentiated from absorption due to bone and venous
Oxygen saturation is not the same as arterial pO2 (as measured on a blood gas) but is related to the extent that normal SpO2
is thought to imply normal pO2 and decreased SpO2 correlates with a low pO2. Specifically, due to the oxygen dissociation
curve; at arterial pO2 of 60-70 mm Hg, hemoglobin is expected be fully saturated although normal pO2 is>85 mm Hg.
SpO2 measurement gives important information about delivery of oxygen to tissue and may be used to assess the effectiveness
of supplemental oxygen therapy. Oximetry is also used as an indicator for peripheral perfusion in the anesthetized patient
as well as to monitor patients who are being mechanically ventilated. The limitations of pulse oximetry include inability
to obtain readings or inaccurate readings in patients who have serious perfusion problems, severe hypothermia or gross anemia.
Unfortunately these are often the patients who most require oxygenation assessments. Because of these limitations arterial
blood gas measurement remains essential in critical patients although frequency of sampling can be greatly reduced by pulse
Pulse oximetry has the added advantage of being non-invasive and requires less expensive equipment than blood gas analysis.
Expired or end tidal CO2 can be continuously monitored in the intubated patient by attaching a sensor device to the endotracheal
tube. End tidal CO2 correlates well with CO2 measured on a blood gas (PaCO2) and provides an accurate non-invasive measure
of alveolar ventilation. Capnography is useful for assessing and regulating breathing rate particularly in the hypoventilating
patient as is extremely valuable in weaning from mechanical ventilation.
Combined, pulse oximetry and capnography give the most complete picture of respiratory function available by non-invasive
means. They should not be thought of as substitute for arterial blood gas analyses in patients with respiratory disorders,
however their usefulness for continual monitoring and detection of changes in patient status is unquestionable.
Arterial blood gas analysis
Blood gas analysis provides information about respiratory function and acid/base status. This information is vital in determining
and monitoring treatment of patients with primary or secondary respiratory disease and/or metabolic disturbances Acid/base
disturbances can severely impede successful treatment of illness and can be, in and of themselves, life threatening. For
example, "sick" keto-acidotic diabetics require careful monitoring and treatment to restore normal blood pH. Previously,
the costly , large, high maintenance analyzers required to perform blood gas measurement have prohibited use at most veterinary
clinics. The advent of relatively inexpensive, hand-held, user friendly analyzers such as the i-STAT® has allowed for any veterinary practice to perform blood gas analyses.
Blood gas (BG) can be assessed on arterial (ABG) or venous (VBG) samples although oxygenation assessment is made on arterial
blood only. Results are dependent on the specific patient temperature and hematocrit. Both of these parameters are entered
into the blood gas analyzer prior to evaluation. Understanding the basic principles of blood gas analysis allows simple,
rapid interpretation of results. The basic components of BG analysis include:
• oxygen (PaO2)
• carbon dioxide (PaCO2)
• blood pH
• bicarbonate (HCO3)
• base excess (BE-B)
Together, these parameters convey an accurate and complete picture of respiratory function and metabolic state.