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Oxygen Toxicity in the Preterm: The Role of Pulse Oximetry
Initiatives in Safe Patient Care asked our panelists to discuss the features of a pulse oximetry device that important in the care of neonates and what would be the ideal features to reduce the risks of oxygen toxicity.
1. How important is real time recognition of saturation values on neonates?
Kimura: The answer to this question depends on the clinical situation. For example, in a growing premature infant without lung disease, short desaturation episodes have no clinical significance. These desaturations can be caused by many nonclinically significant events. However, patients with acute pulmonary and/or heart disease, abrupt changes in oxygen saturation may have great clinical significance. For example, in a baby with a tension pneumothorax, a sudden increase in oxygen saturation following chest tube placement is an important marker of decompression.
Klein: This is a critically important factor in using a pulse oximeter in neonatal patients. It is important that the device correlates with the healthcare provider’s physical exam. It is not reassuring when a cyanotic infant's oximeter is still reading saturations above 90% as the infant declines, or when an intervention is made and the infant is pink again, but the oximeter still lags behind reading saturations below 80%. It is reassuring to healthcare workers when the device is in rapid agreement with their physical exams.
Merritt: It is very important to have real time recognition of SpO2 status in neonates that is not subject to motion or other artifacts. "Real time" always involves a time delay between the sensor and the display. This is especially true when attempting to determine SpO2 status soon after delivery and as a part of neonatal resuscitation. The exact millisecond delay may be 20 or 200 milliseconds, but it is important that the display reflects an actual measurement. Displays that lack a good waveform or a waveform that is not in synchrony with the heart rate shown by auscultation or electronic monitoring are often considered invalid. It would be ideal to know when the value displayed is accurate or not, and why if it is not. When a real time SpO2 is difficult to obtain, one is led to question sensor positioning, ambient light, infant movement, or poor perfusion.
Syke: For the lack of anything better, real time recognition of oxygen saturation levels in the neonate is important. It is the best, noninvasive tracking method we have available, so at least we have an idea about how the baby is doing. Our ability as bedside nurses to respond appropriately to the highs and lows of saturation levels is a huge concern.
2. What is an acceptable error range for pulse oximeters when they are the only noninvasive methodology used to continuously monitor neonatal oxygenation?
Kimura: Five percent at the high end and 10% at the low end.
Klein: An error range of 2% to 3% for saturations from 70% to 100% would help to minimize inadvertent hyperoxic exposure if saturations were always kept <95% when on supplemental oxygen. For saturations below 65% to 70%, an error range of 4% to 5% could be acceptable, since at these lower levels one is usually intervening to improve the patient’s status. Even with cardiac patients with cyanotic congenital heart disease, it is rare to spend considerable periods of time with stable saturations <60%.
Merritt: The acceptable error range depends on the precision of the instrument in determining the true SpO2. Because physician ordered SpO2 ranges may vary, it is important to know that the precision of the measurement is the same across all ranges. If precision varies, then "acceptable error range" is a meaningless term. However, if the precision is the same across all ranges, then a measurement error of ±2% would probably be acceptable. Thus, if an ordered range was 90% to 94%, and the error range is 88% to 96%, the infant would experience values that are 2.27% to 2.02% of the ordered values. Both of these values overlap the low and high range used in the SUPPORT trial. However, these values are constantly changing. Thus the time spend at the lower 2.27% and the upper 2.02% would be helpful.
In my interpretation, the SUPPORT trial tells us that maintenance with the prescribed range is not always possible, and going too low and too high are where the problems arise. Thus a 2% error range from the ordered desired SpO2 range makes the most sense to me, although this needs to be tested in a randomized trial. We do look at histograms of the SpO2 range in our NICU. These histograms generally depict SpO2 time within or outside the range in blocks of 5%. I think that these need to be narrowed to 2% to 3%, but this would need to be tested in a trial. It also needs to be tested whether or not the wide range of 85% to 89% or 90% to 95% used in the SUPPORT trial are the best ranges to test for reduction of ROP, increase in death/BPD, and other outcomes. In our NICU, we have selected 88% to 94% for infants <30 weeks gestation from 1 hour after birth until they go home. This was the best guess of 14 neonatologists. In some term infants with PPHN we will prescribe higher ranges, usually 95% to 98%. Obviously, in infants with congenital heart disease with surgical correction, the pediatric cardiologists recommend lower saturations of 75% to 85%. Different situations may define different ideal SpO2 ranges. We do not choose an SpO2 of 70% for infants, and yet on occasion that is all we can achieve. Also, we do not choose an SpO2 of 100%, but many preterm infants in room air are 100% saturated. Nature plays these tricks on us! SpO2 is only one value. It is actually the oxygen content and the perfusion that is important, with oxygen content being dependent on instantaneous measures of hemoglobin, SpO2, and the amount of oxygen dissolved in the plasma (which requires a blood gas). Without perfusion, 100% SpO2 is not of much help.
Syke: With histogram technology being implemented in ever more units and becoming a part of the permanent medical record, and with nurses being required to sign contracts stating they will maintain patients within set O2 saturation parameters, the acceptable error range should be very minimal, 1% to 2% at most. We also must make certain that our sick VLBW infants are given the very best opportunity to avoid hyperoxia.
3. What would be the benefit of a clinician-controlled alarm feature that differentiates between serious hypoxemia/hyperoxemia events versus minor transient events?
Kimura: This would be very helpful. Many transient events are physiologic and responding to transient physiologic events can result in “oversteering,” which can lead to significant variations of O2 saturations. These swings in O2 saturation can cause harm such as ROP. However, in patients with reactive pulmonary hypertension, sudden decreases in O2 saturation can indicate significant right to left shunt requiring an immediate response.
Klein: The value of this type of alarm system is that it could help minimize overreactions to minor artifactual events (causing excess oxygen exposure) or underreactions to serious life threatening events. It would be of value to have a different alarm, depending on the severity and degree of the desaturation episode.
Merritt: A clinician-controlled alarm feature that differentiates between serious hypoxemia/hyperoxemia events versus minor events would be helpful. There could be an alarm for minor range violations that gives an audible but not horribly annoying alarm, however, when the ranges are seriously outside the prescribed ranges, the alarm could use a different sound or intensity. We must guard against alarm fatigue, but we must recognize when the alarm is well outside our prescribed limits. Further, I would recommend that when the duration of the serious alarm exceeds a certain duration (e.g. 20 seconds), that the volume increases incrementally and gets really annoying, as long as there is a manual silencing for situations such as intubation or resuscitation when lower oxygen levels are anticipated. The capacity to silence an alarm for more than 30 seconds without having to manually re-silence it must also be built into any new monitor.
Syke: Most CRM's have features that allow units to identify minor or “yellow” alarms versus major or “red" alarms. I would not think this would be a very helpful feature.
4. What features would an ideal pulse oximeter have in order to help clinicians reduce risks of oxygen toxicity?
Kimura: These include: (1) Rapid response time on placement of the probe, (2) A clinician-controlled alarm feature that differentiates between serious hypoxemia/hyperoxemia events versus minor transient events, (3) Associated with control of FiO2 in a closed loop ventilator system, and (4) Short and long term data collection for evaluation of desaturation events and trends.
Klein: The ideal pulse oximeter would have an error range of just +1% to avoid risk of the infant having PaO2 >80 when saturating in the 90's. It would also never detect a venous motion artifact as a pulse and inadvertently give a falsely low saturation reading, which would lead to unnecessary exposure to increased oxygen.
Merritt: The desired features of a new monitor are multiple: (1) It should have the capabilities described above, (2) It should be servo-connected to a ventilator or other oxygen blender to adjust fraction inspired oxygen based on measured SpO2 maintained for a specific time, (3) It should give a display indicating the reason for a poor signal (e.g. perfusion, ambient light interference), (4) It should continuously measure lactate and VEGF levels in the blood (sensed continuous variables) as a determinant of hypoxia, and it could measure one of several indices of oxidant stress as a continuous variable, (5) It would have different alarms based on ranges outside of ideal, (6) The monitor might also intermittently print a histogram of “time outside of prescribed range,” (7) The monitor should operate on batteries for at least 4 hours, (8) Costs for these added features should not be excessive.
Syke: Smaller, lighter and more flexible patient oximeter probes would be helpful. The babies we worry most about as far as oxygen toxicity is concerned are the tiny and fragile VLBW babies. Many probes still in use are just not the right for tiny babies with gelatinous skin. Probes that provide reliable readings which could be positioned on an ear or forehead would be tremendous. Also, an oximeter tied into the ventilator, thus allowing for automatic dialing of FiO2 to adjust for hyperoxia or hypoxia would be very useful in today's busy NICU's.