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Thinking of Buying...Pulse Oximeters
What to look for to reliably and accurately monitor oxygen saturation levels.
Barry Friedberg
Publish Date: October 10, 2007   |  Tags:   Anesthesia

Perhaps no other advance in patient monitoring has been as important in recent years as the pulse oximeter. Although it took several years for pulse oximetry to become a standard of care, there's no mistaking its role in decreasing the mortality rates associated with brain damage and death from hypoxemia. Read on if you're thinking of buying a pulse oximeter.

ABCs of pulse oximetry
Pulse oximetry is a non-invasive method that lets you monitor the oxygen saturation level of a patient's blood (SpO2). Simply put, if there isn't enough oxygen in a patient's blood, brain damage can occur. Before pulse oximetry, the only way to diagnose if a patient had enough oxygen in his blood was to perform a blood gas, which took a lab a minimum of 20 to 30 minutes, a problem for hypoxic patients.

Not long ago, cyanosis (a bluish coloration of the skin due to the presence of deoxygenated blood) was the surest way to determine if a patient's oxygen saturation was too low. There's no need to wait this long anymore, as you're alerted visually (on the monitor) and audibly (by alarms) to the slightest drop in blood oxygen saturation level. ORs especially have become much safer with patients under anesthesia being monitored non-invasively and potential problems being diagnosed and solved early on.

How does a pulse oximeter work?
A pulse oximeter includes some form of visual display and a sensor that's usually placed on the finger or toe of a patient. The sensor has a pair of small light-emitting diodes (better known as LEDs) facing a photodiode. Light passes from one LED, which is red (660nm), to another, which is infrared (940 nm). Changing absorbance of each of the two wavelengths is measured, allowing determination of the absorbances due to the pulsing arterial blood and factoring out venous blood, skin, bone, muscle, fat - even fingernail polish.

Based on the ratio of changing absorbances that the red and infrared light cause by the difference in color between oxygen-bound (bright red) and unbound (dark red or, in severe cases, blue) hemoglobin in the blood, a measure of oxygenation can be made. The measure is converted into a percentage that's then displayed on the monitor, usually along with the heart rate. Acceptable normal SpO2 ranges are from 95 to 100 percent. Anything below 90 percent is cause for concern.

Benefits far outweigh the limitations
Pulse oximeters have come a long way since they debuted in the early 1980s. Back then, we tolerated the new technology's performance flaws. Patient motion, low perfusion, tissue distortion and signal interference could make SpO2 estimates less reliable and more prone to generating false alarms. Today pulse oximeters are much more advanced and accurate. Still, they're not without their limitations. For example:

  • Pulse oximeters are only accurate if the oxygen saturation range is between 70 percent and 100 percent; once the oxygen saturation falls below 70 percent, all the numbers are equal. For example, there's no visible difference on the monitor between 40 percent and 68 percent. Once you get to these low levels, you're operating below the machine's confidence level.
  • Severe hypotension, cold, cardiac failure, some cardiac arrhythmias or peripheral vascular disease can result in a signal too weak to analyze. Hypothermia can also lead to shivering, the constant motion of which can affect the pulsatile signal. A badly positioned sensor can also affect readings, as can bright overhead lights.
  • Falsely high or low readings will occur when hemoglobin is bound to something other than oxygen, such as carbon monoxide.

Despite these few limitations, pulse oximeters let us treat patients more promptly and effectively in the OR. To get the most out of your pulse oximeter purchase, you'll want to keep these factors in mind.

  • Tone. The single biggest issue that you need to think about when looking at pulse oximeters is that the tone must decrease whenever the oxygen saturation goes down. Without a change in tone to alert your anesthesia provider that there is a change in oxygen saturation, the device is worthless. There are many times when the anesthesia provider's vision may be off the monitor, such as when he's looking down to write. If you buy a monitor that produces a constant beep no matter what, there's no way to know without looking at the monitor if the patient is losing oxygen. A pulse oximeter that audibly alerts you that oxygen is decreasing lets everyone in the room know trouble is ahead well before the blood color darkens and a cardiac arrest happens.
  • Wave form display. Wave form display on the monitor is an important safety feature to look for in pulse oximeters because it lets you recognize a false alarm versus a real indication of trouble. You want to have an easily readable, clear picture of the wave form to help determine if a sensor is correctly placed. For example, if the alarm sounds and you see no wave form on the pulse oximeter screen, then it's obvious that the sensor isn't picking up a signal and is likely to be no longer effectively placed on the patient. The wave form display lets you weed out false alarms and increase efficiency in the OR. It's also a good idea before you purchase a pulse oximeter to huddle with your anesthesia personnel and focus on evaluating the appropriateness of both the oximeter performance settings (signal averaging time, for example) and alarm settings that match the clinical context of the cases you perform.
  • Motion sensitivity. Although most anesthetized patients don't move around a lot in the OR, motion can still disrupt the function of a pulse oximeter in other areas, especially recovery, where patients might shiver or shake after coming out of surgery. Many newer pulse oximeters are designed to identify arterial-induced pulsations and calculate an accurate SpO2 value, weeding out any external factors that may cause an inaccurate reading.
  • Low perfusion. Hypotension or vasoconstriction from cold exposure can reduce overall blood flow and reduce the strength of the pulsatile signal. This low perfusion will make it harder for the sensor to calculate the pulse ox number. Oximeters vary in their ability to measure these low-amplitude signals; some oximeters incorporate a perfusion index, or PI, which displays the signal strength of the pulse ox signal. The PI helps you select the best sensor site and position to get the strongest signal.
  • Sensor choices. Pulse oximeter sensors are generally placed on a patient's finger or toe, a translucent part of the body, so that the red and infrared wavelengths can be passed from one side to the other and the changing absorbance can be measured. Even fake nails don't normally disrupt the pulse ox reading. However, there are times when it might be more effective to place the sensor on another part of the body, such as the forehead; for example, if the patient is severely burned and the normal sites are unavailable. Many oximeters offer a sensor that is able to be placed on a flat surface and is insensitive to non-pulsate signals. If you deal with cases where this could be useful, it's a good idea to look into alternative sensors.
  • Disposable versus reusable sensors. Even in the era of universal precautions, reusable finger pad sensors pose little risk of cross-contamination. You'll rarely, if ever, need to use a finger with an open wound to monitor O2 saturation. It will most likely be less expensive for you to use reusable sensors than disposable ones.
  • Multi-tasking. Most pulse oximeters come bundled with a monitor and many perform multiple tasks, which can help clinicians better monitor the patient under sedation. Some other functions that a multi-function monitor might perform are an ECG (electrocardiogram, which measures the electrical activity of the heart), NIBP (non-invasive blood pressure), body temperature and EtCO2 (end-tidal carbon dioxide concentration, which measures the carbon dioxide concentration in the expired air). All of these functions are important in monitoring a patient, so purchasing a pulse oximeter that performs these can be a good investment for your facility.
  • Size and style. Today's pulse oximeters come in a wide variety of sizes and styles, including handheld, fingertip and table-top. Depending on the amount of use your pulse oximeter is going to get and what your anesthesiologist is comfortable with, any of these can work in a surgery center. Some oximeters are even wireless, so there's no wire to get tangled up between the patient and the monitor, and many are now even Bluetooth capable.

Canary in a coal mine
In essence, the pulse oximeter acts as a canary in the coal mine in that it alerts you when something is wrong and allows for early intervention if the patient's oxygen saturation is too low.

CAS Medical Systems
750 Vital Signs Monitor
(800) 227-4414
Price: $4,000 to $7,000
FYI: Casmed 750 Vital Signs Monitor is designed for procedural sedation, says the company. The 750 utilizes best-in-class technology such as Masimo SET pulse oximetry and Oridion Microstream EtCO2 technology. A compact, mobile unit, the 750 can operate on emergency vehicle power, direct AC or battery, says Casmed.

ConMed Corporation
PRO2 Pulse Reflectance Oximeter
(800) 765-8375 x-3140
Price: $4,995 to $7,600
FYI: The first monitor designed as a true reflectance oximeter system, the PRO2 sensor employs three light emitters (one red, two infrared) and two light detectors that surround the emitters and are sized to capture from equal surface areas, says ConMed. You can use the PRO2 on the patient's back or forehead, which results in faster response time and more reliable data than standard transmission or reflectance oximetry systems, adds ConMed.

Criticare Systems
504DX Portable
Pulse Oximeter
(800) 458-4615
Price: $1,595 (with printer) $1,195 (without printer)
FYI: The ergonomically designed 504DX Portable Pulse Oximeter is compact, lightweight and offers advanced DOX pulse oximetry and heart rate monitoring, says the company. The device features audible and visual alarms. The optional fully integrated internal printer produces high quality hard copies of graphic trends, tabular trends and sleep summary reports, says Criticare. Unique DOX digital SpO2 technology improves performance, even under the most challenging clinical conditions, adds the company.

Draeger Medical Systems
Infinity Masimo SET SpO2 SmartPod
(800) 437-2437
Price: Not disclosed
FYI: Draeger's Infinity Masimo SET SpO2 SmartPod brings the advantages of Masimo's gold standard pulse oximetry technology and sensors to Infinity patient monitors, says Draeger. Masimo Signal Extraction Technology read-through motion and low perfusion pulse oximetry is accurate during low perfusion and most types of patient movement - helping to reduce false alarms and detect true alarms, adds the company.

DRE Medical Equipment
BCI 3401 FingerPrint Pulse Oximeter
(800) 477-2006
Price: Not disclosed
FYI: DRE Medical's FingerPrint handheld pulse oximeter puts comprehensive spot-checking capability conveniently and comfortably in the palm of your hand, says the company. This low-cost, easy-to-use solution features robust memory and an integrated printer for immediate hard-copy documentation of real-time or trended data for patients ranging from neonate to adult, adds DRE.

Masimo Corporation
Masimo Radical-7
(949) 297-7498
Price: Not disclosed
FYI: The Masimo Radical-7 is the first bedside monitor to feature Masimo Rainbow SET, the first and only technology platform capable of continuously and non-invasively measuring carboxyhemoglobin (SpCO) and methemoglobin (SpMet) in addition to oxyhemoglobin (SpO2), perfusion index, pleth variability index and pulse rate, says the company. The product lets clinicians continually measure levels of SpCO and SpMet and accurately determine patients' true oxygenation status, adds Masimo.

Nellcor/Tyco Healthcare
OxiMax N-600 Pulse Oximeter
Price: $4,500
FYI: Nellcor's OxiMax N-600 pulse oximeter, along with other OxiMax standalone and handheld oximeters, lets you confidently monitor even the most critically ill patients, says the company. Most leading patient monitors come with OxiMax options, making them compatible with the broad spectrum of OxiMax sensors, including the Max-Fast forehead sensor, which offers better performance in low perfusion, adds Nellcor/Tyco.

Omron Colin Medical Corp.
Press-Mate Prodigy II
(800) 829-NIBP
Price: $2,995 to $5,395
FYI: Omron Colin's Press-Mate Prodigy II non-invasive blood pressure (NIBP) monitors offer accuracy and flexibility, according to the company. In addition to Colin's patented NIBP with Smart Inflation and Dynamic Linear Cuff Deflation, the Press-Mate Prodigy II is available with optional SpO2 and 7-Second E-Temp. Some of the product's advanced features include user-designed cuff interval programs, patient ID and memory.

Smiths Medical PM, Inc.
BCI 3301 Hand-Held Pulse Oximeter
(800) 558-2345
Price: $595
FYI: Smith Medical's BCI 3301 Hand Held Pulse Oximeter is a cost-effective, portable pulse oximetry monitoring system compatible with personal computers or commercially available printers, says the company. Designed for simple, accurate spot-checking, it is ideal for use during emergency transport, in emergency rooms, outpatient clinics or for home care. It provides fast, reliable SpO2 and pulse rate measurements on any patient from neonate to adult, adds Smiths Medical.