As described in the letter, after initial communication from Massachusetts General Hospital (MGH) on this topic, we provided MGH with a software update that allows the user to select either SmarTone (Masimo Corporation, Irvine, CA), which uses Masimo SET's sophisticated signal processing to enable saturation tones even during low signal-to-noise conditions, or a more traditional pulse tone algorithm, which uses signal morphology only, causing breaks in the tone in the presence of low perfusion or signal interference. We have been offering SmarTone as a user-selectable feature on our monitors for some time now. However, we believe it is important that users understand the value of SmarTone so that they can decide which pulse tone to choose for their particular needs.

The American Society of Anesthesiologists standard that Mr. Forde et al.  at MGH cite as being diminished by the Masimo SmarTone algorithm is exactly the standard that many other clinicians believe we have enhanced. This American Society of Anesthesiologists standard was established in the context of oxygenation, which implies that tracking the pitch of the tone and thereby indicating changing saturation values was the true intent of the standard. With this understanding and the knowledge that interruptions in the pulse tone caused by low perfusion, motion, and electrical or other interference are common, causing frequent “false alarms” and decreasing the amount of time that the tone was available, we concluded that it would be of clinical value to decrease these interruptions in the pulse tone.

The pulse tone feature on many pulse oximeters is related to the morphology of the plethysmographic waveform, emitting a tone only when a clean pulse signal is recognized. In the presence of low signal-to-noise conditions, the arterial pulse waveform can be virtually impossible to distinguish when looking at the raw plethysmograph. In these instances, most other pulse oximeters either discontinue the tone until a valid pulse signal is recognized or sound a tone based on the noise frequency, one that is not indicative of the patient's pulse. This leads to frequent, long periods without audible information on oxygenation status or false low-saturation indications.

Masimo SET's unique signal processing algorithms, which include five distinct signal processing engines working in parallel, enable identification of the arterial pulse wave under far more of these difficult clinical conditions. This allows Masimo to provide a variable pitch saturation tone during periods of signal disturbance that would cause loss of signal or false saturation tones in conventional pulse oximeters. The SmarTone feature uses real-time signal morphology, similar to conventional pulse oximetry, to create the tone during periods in which the pulse can be clearly recognized. During periods wherein the pulse signal becomes obscured, Masimo uses its advanced signal processing algorithms to identify the pulse and provides a tone indicative of true oxygen saturation. Feedback from current and prospective customers has been overwhelmingly positive toward this feature.

The case reported in the MGH letter focuses narrowly on one aspect of the SmarTone feature. Because of the sophisticated signal processing involved, during periods of signal disturbance or low perfusion there can be up to an 8-s delay before the cessation of the pulse tone. This short delay is well within internationally recognized performance requirements for heart rate meter response to asystolic events.*

The letter makes reference to a bradycardic period preceding the asystole, during which the pulse tone continued. It is not clear that the surgical team was able to observe whether the pace of the tone was consistent with the rate of the bradycardia. It is likely that the Masimo device tracked the bradycardia, as evidenced by several independent and objective studies that have demonstrated the superior ability of Masimo SET to track sudden changes in heart rate.1–3†‡

The letter also suggests that a simulation using a blood pressure cuff reproduces this clinical scenario. Although these two scenarios may seem similar, they are actually very different because in one case the heart has stopped, whereas in the other a pressure cuff is occluding the flow from a beating heart. Our own pressure cuff testing, which is supported by the literature,4–6has proven that it is very difficult to occlude 100% of the blood pulsations on certain patients. Because Masimo SET has been shown to have superior low perfusion performance,7–12it may be the only pulse oximeter to continue to read during cuff inflation when weak arterial pulsations are present. We recently performed more than 100 cuff inflations on various subjects, using a dual bladder tourniquet inflated to 250 mmHg with a Masimo Radical pulse oximeter and a Nellcor N-600 (Nellcor-Covidien, Mansfield, MA) attached to the test arm. In every case, the Radical would display a reading during cuff inflation only if there was a visible arterial pulse wave in the plethysmograph, and it would zero out after the plethysmograph became flat. Overall, the results were inconsistent, with no clear pattern of either monitor (Nellcor or Masimo) zeroing out before the other, and neither monitor consistently zeroed out within 8 s of cuff inflation. We have captured a number of these cases on video and are pleased to share these results upon request.

As discussed above, after initial communication from MGH on this topic, we provided MGH with a software update that allows the user to select either SmarTone or a more traditional pulse tone algorithm. Initial feedback regarding this new software has been positive. Since early this year, SmarTone has been offered as a user-selectable feature on all of our products, allowing clinicians to decide for themselves which pulse tone to use, based on their clinical scenario.

We value the feedback and suggestions of our customers in helping us make our products the safest and best pulse oximeters in the world, and we appreciate the useful comments of the MGH team. We trust that giving clinicians the option of choosing whether to activate the SmarTone feature will provide them greater flexibility in making informed decisions regarding the clinical management of their patients.

Masimo Corporation, Irvine, California.

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