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.

Capnometry

Capnometry is the measurement of the percentage or partial pressure of carbon dioxide (CO2) in the respiratory gases. A capnometer is the device that performs the measurement and displays the reading in numerical form. Capnography is the recording of CO2 concentration versus time. A capnogram is the graphical waveform of the CO2 concentration versus time displayed on the monitors screen or print out. Knowledge of normal waveform morphology can be very helpful and waveform analysis can often explain abnormal CO2 readings.

Capnometry developed at a later date than pulse oximetry. The first infra-red CO2 measurement and recording device was introduced in 1943 by Luft. In 1978, Holland was the first country to adopt capnography as a standard on monitoring during anesthesia. Since then, capnometry and capnography have become standard and essential tools in anesthesia monitoring in humans worldwide. In the ICU setting, capnometry use is becoming more frequent in mechanically ventilated and spontaneously breathing awake patients. Its use in the veterinary profession has grown steadily as knowledge of its use expands and less expensive new and refurbished equipment becomes available.

The technology by which the CO2 is measured can vary depending on the equipment. The most commonly used method is analysis of infrared absorption. Similar to pulse oximetry, an infrared light from an LED passes through the gas sample and the amount of absorption is determined and computed with an algorithm to determine the concentration of CO2 in the sample. Both side stream and main stream methods are available and have advantages and disadvantages of each. Side stream analysis is probably the most versatile and can be used in both intubated and non-intubated patients making it valuable in both anesthesia and ICU settings. Two other methods of analysis are mass spectrometry and colorimetric detection. These two methods are not commonly used in the smaller and portable monitors that are available for veterinary patients.

Capnometry has proven to be a valuable tool to assess ventilator status in a patient and as an indirect monitor of arterial CO2 and the respiratory component of pH. End-tidal CO2 (ETCO2) usually follows arterial CO2 with a gradient of less than 5 mm Hg. In patients with respiratory compromise, after an initial blood gas, ETCO2 may be used to reduce that number of subsequent blood gases needed and can allow for monitoring of breath to breath changes that may require therapeutic intervention. The normal expected range for ETCO2, in most patients, is similar to that of an arterial blood gas. The range is usually 38 to 48 mm Hg. Elevated ETCO2 can be determined to fall in one of four categories: alveolar ventilation, CO2 output, pulmonary perfusion, and technical errors. Within the category of alveolar ventilation is hypoventilation which is the most common cause of elevated ETCO2, bronchial intubation, rebreathing of previously exhaled CO2, and partial airway obstruction. An increase in cardiac output and/or blood pressure can increase delivery of blood and secondarily CO2 to the lungs increasing ETCO2 in the pulmonary perfusion category. Increased production and output of CO2 through condition such as fever, malignant hyperthermia, bicarbonate administration, or surgical insufflations of CO2 can result in elevated ETCO2. Finally, technical errors with the capnometer, mechanical ventilator, or breathing circuit of an anesthesia machine can result in elevated ETCO2. Decreased ETCO2 can be an indicator of problems as serious as causes of elevated ETCO2. These can fall into the same categories as elevated levels and include causes such as hypothermia, reduced cardiac output, hypotension, hypovolemia, cardiac arrest, hyperventilation, apnea, airway obstruction, and technical errors.

In addition to the numerical readout provided by the capnometer, analysis of the waveforms produced by a capnograph can assist in rapid recognition of the condition behind the increased or decreased ETCO2. Knowledge of the shapes and patterns of normal and common abnormal capnograms is essential for optimal use of the equipment. Examples and descriptions will be given during this presentation but can also be found in many texts and the website www.capnography.com.

Capnography and capnometry use are not limited to monitoring changes in patients but can also be used for assessment of therapeutics. These devices can aid in assessing response to cardiovascular inotrops and pressor agents in critical patients as well as a guide to the effectiveness of cardio-pulmonary-cerebral resuscitation and may result in improved outcomes.

Pulse oximetry and capnometry have come a long way since their initial discovery and introduction into clinical practice. Although not recognized as standard of care yet as in the human medical field, use in veterinary medicine is increasing. The expansion of the use of this technology outside of the traditional uses in anesthesia continues to be discovered. This can only result in improved patient care and outcomes.