Practical uses of the pulse oximeter and capnograph (Proceedings)


Practical uses of the pulse oximeter and capnograph (Proceedings)

Over the past two decades, technologies have developed to allow for rapid and continuous determination of many physiologic parameters in anesthetized and critical care patients. Two of the most important modalities are pulse oximetry and capnometry. With their use, a clinician is better equipped to ensure adequate oxygen delivery at the cellular and microcellular level and ensure a proper pH for optimal physiologic cellular function in patients. This has led to a dramatic improvement in patient safety, care and outcomes. In a study of closed claims of anesthetic-related malpractice cases, it was determined that a combination of pulse oximetry and capnography could have prevented 93% of avoidable mishaps. Another study determined that pulse oximetry provided the first warning of an incident in 27% of situations. Additionally, the number of unanticipated intensive care unit admissions decreased after the introduction of pulse oximetry. Although these findings were from studies involving human patients, similar results can be expected in veterinary patients where pulse oximetry and capnography are routinely used as part of patient care.

Pulse oximetry

Pulse oximetry is a non-invasive method allowing the monitoring of the oxygenation of a patient's hemoglobin. This monitoring is performed using a pulse oximeter, a medical device that indirectly measures the oxygen saturation of the blood of a patient. The device yields a computerized readout and sounds an alarm if the blood saturation becomes less than optimal. Additionally, the pulse oximeter may have a photoplethysmograph showing changes in blood volume during pulsatile blood flow.

The first developed use of pulse oximetry was by Matthes in 1935. The technology was continued to be refined until the 1960's when a commercial unit was made available my Hewlett Packard. However, due to the size and cost, the unit had minimal and limited application. The technology and size were further refined through then 1970's and 1980's. By 1987, the standard of care for the administration of a general anesthetic in the US included pulse oximetry. In 1996, the Masimo Company introduced the first pulse oximeter able to provide accurate measurement during periods of patient motion or low peripheral perfusion expanding the use of the pulse oximeter to intensive care unit setting. Continued advances in technology have allowed for increased accuracy and capability with the adoption of high resolution pulse oximetry.

The principle behind the pulse oximeter is the measurement of the differential absorption between two different wavelengths of light based on the Lambert-Beer law (the empirical relationship that relates the absorption of light to the properties of the material through which the light is travelling). Oxyhemoglobin absorbs more light in the infrared spectrum (850 to 1000 nm) whereas deoxyemoglobin absorbs more light in the red spectrum (600 to 750 nm). The pulse oximeter has two light emitting diodes (LED) of different wavelengths, one red and the other infrared. Typically the red LED has a wavelength of 660 nm and the infrared a wavelength of 940 nm. The LEDs are pulsed on and off hundreds of times per second and a photodetector collects the red and infrared light that passes through the tissue the pulse oximeter is placed on. A ratio of red to infrared light absorption is developed and applied to an internal algorithm in the pulse oximeters software. A number is then displayed on the readout. Newer pulse oximeters can compensate for extraneous light and the rapid sampling rate allows for detection of pulsatile blood flow which is assumed to be arterial.

It is important to understand the principle of the pulse oximeter so that a clinician has an understanding of what is actually being measured by the pulse oximeter and what its limitations are. An understanding of fraction oximetry (SaO2) versus functional oximetry (SpO2) is necessary. SaO2 is defined as the oxyhemoglobin (O2Hb) divided by the total hemoglobin (including all hemoglobin species) in a sample and can be written as:

Where Hb is deoxyhemoglobin, Met Hb is methemoglobin, and COHb is carboxyhemoglobin. SpO2 is defined as the oxyhemoglobin divided by all the functional hemoglobin in a sample and can be written as:

These values are determined by analysis of a blood sample using an in vitro oximeter. In clinical practice, a pulse oximeter is a non-invasive estimate of SpO2. This in turn can be used to estimate a patient PaO2 using the oxyhemoglobin dissociation curve. Under normal physiologic conditions, near maximal Hb saturation is achieved at a PO2 of 75 to 80 mm Hg.

Many pulse oximeters also display a plethysmographic waveform. This waveform can be used to determine if a signal is artifactual and pulse rate. The most beneficial use of a pulse oximeter in an anesthetized or ICU patient is as an early warning device for hypoxemia. During anesthesia, this most commonly happens during induction and recovery. It can also indicate the continued need for oxygen supplementation. Other applications for use of pulse oximetry include controlling oxygen supplementation, monitoring circulation, determining systolic blood pressure, and monitoring vascular volume.

There are many advantages that support the routine use of pulse oximetry in veterinary patients. Some of these include accuracy, dependability, cost, non-invasive, easy application, convenience, and response time. Disadvantages of pulse oximeters include poor function with poor perfusion, difficulty detecting high oxygen partial pressures, skin pigmentation, optical and electrical interference, pressure on vascular beds, and motion artifacts. Additionally, because there only two LEDs in a pulse oximeter, it is not able to accurately measure dyshemoglobins (MetHb and COHb). Newer pulse oximeters have been developed with up to 12 different light waves to detect other hemoglobins. These "Rainbow pulse Co-oximeters" may result in improved veterinary patient care as the technology is developed.