How did oximeter manufacturers deal with frequent alarms in patients who were constantly moving, shivering, or had poor peripheral perfusion for whatever the reason? One technique is simply to increase averaging times so that there becomes sufficient time with a detectable signal to provide a reading. Another technique is to have the oximeter freeze its reading when it detects motion; this is another situation in which clinician acuity is important. Some oximeters will read 0 if the motion artifact persists.
Latest generation oximeters
It is 2010. Our wishes have been granted. The latest generation of oximeters does not need to actually find a pulse in order to determine the oxygen saturation. The oxygen saturation is calculated directly; the pulse rate is determined separately. Here is how they work:
The common knowledge is that desired signal portions are proportional to one another through the optical density ratio. If one subtracts the product of the arterial optical density ratio and the physiologic signal due to infrared light from the physiologic signal due to red light, one obtains a noise reference signal. Using an algorithm, the pulse oximeter considers every optical density from those theoretically possible (from 1 percent to 100 percent) and generates a family of noise signals. So when the optical density ratio corresponds to the arterial oxygen saturation, the reference signal contains only noise portions. An adaptive noise canceller generates only the desired signal portions. Pulse oximeters that use this proprietary technology can detect a pulse at perfusion levels significantly below those detected with an arterial line.
Another new feature is the pulse oximeter can signal how confident it is of the displayed data, both pulse and saturation. Even when the "low signal" message begins to flash, the quality of the displayed data still could be good. In these circumstances, the clinician should investigate for the possibility of: improper sensor type, application, or placement; excessive motion (everything has a limit); some obstruction of blood flow; excessive environmental interference; or severely decreased peripheral perfusion (for example, a patient on a drip of phenylephrine or levarterenol, both pure vasoconstrictors used when nothing else will bring a patient's blood pressure up).
Investigating the type of sensor and its placement is of particular importance. Oximetry can be performed by two different techniques. One technique is transmission of the light, as through a finger, foot, or the bridge of the nose. The other technique is reflectance, as with a forehead sensor. Some oximeters are able to use either technique; others use only transmission oximetry. The techniques are not interchangeable, as the algorithm for calculating hemoglobin saturation is different for each one.
A further piece of data provided to the clinician by new oximeters is the perfusion index (PI). This is the ratio of pulsatile blood flow to the static blood in the tissue being used for the measurement of oxyhemoglobin levels. The perfusion index allows the clinician to quickly assess the appropriateness of a monitoring site because a high, stable PI is preferred. A drop in PI (which is displayed constantly) is also an indicator of inadequate analgesia in an anesthetized patient or one who cannot communicate his sensation of pain. In the neonate, a low PI is an indicator of the severity of acute illness. While it does not pinpoint the illness, it informs the clinician of the need to investigate the cause.
Now, what about differentiating among various types of hemoglobin, both normal and abnormal? As you would suspect, differentiating among multiple types of hemoglobin requires multiple wavelengths of light, such as those used by a co-oximeter. The latest generation of pulse oximeters use seven wavelengths of light and are capable of measuring carboxyhemoglobin, methemoglobin, and oxyhemoglobin, as well as pulse. This technology is especially useful in the emergency department in detecting cases of carboxyhemoglobinemia in patients.
Today's pulse oximeters have much to offer clinicians who are looking for sensitive and reliable technology that can facilitate their decision-making process in a variety of care settings.
Robert R. Fluck Jr. is an associate professor emeritus from the department of respiratory therapy education at Upstate Medical University. The author would like to give credit to Masimo Corporation for providing a number of white papers that explain the newest concepts in pulse oximetry.