PEDIATRICS Vol. 107 No. 5 May 2001, pp. 1120-1124
Closed-Loop Controlled Inspired Oxygen Concentration for Mechanically Ventilated Very Low Birth Weight Infants With Frequent Episodes of Hypoxemia
From the Division of Neonatology, Department of Pediatrics, University of Miami School of Medicine, Miami, Florida.
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ABSTRACT |
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Top
Abstract
Methods
Results
Discussion
Conclusion
References
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Background. Mechanically ventilated very low birth weight infants often present with frequent episodes of hypoxemia, and maintaining arterial oxygen saturation by pulse oximetry (SpO2) within a normal range by manual fraction of inspired oxygen (FIO2) adjustments is difficult and time consuming.
Objectives. An algorithm for closed-loop FIO2 control (cFIO2) to maintain SpO2 within a target range was compared with continuous manual FIO2 (mFIO2) adjustments by a nurse in a group of ventilated infants who presented with frequent episodes of hypoxemia.
Results. Fourteen infants (birth weight: 712 ± 142 g; gestational age: 25 ± 1.6 weeks; age: 26 ± 11 days; synchronized intermittent mandatory ventilation rate: 24 ± 10 b/m; peak inspiratory pressure: 17.5 ± 2.0 cmH2O; positive end-expiratory pressure: 4.3 ± 0.5 cmH2O) were studied for 2 hours on each mode in random sequence. Both modes aimed to maintain SpO2 between 88% and 96%. There were 15 ± 7 and 16 ± 6 hypoxemic episodes/hour (SpO2 <88%, >5 s) during mFIO2 and cFIO2, respectively; episode duration was 41 ± 23 and 32 ± 15 s, totaling 19 ± 16% and 17 ± 12% of recording time. There were 13 ± 10 and 10 ± 8 hyperoxemic episodes/hour (SpO2>96%, >5 s) during mFIO2 and cFIO2, respectively; episode duration was 27 ± 15 and 24 ± 19 s, totaling 15 ± 14% and 10 ± 9% of recording time. Mean SpO2 and FIO2 levels were similar during both modes. The nurse made 29 ± 17 adjustments/hour during mFIO2. There was a significant increase in the duration of normoxemia (SpO2 between 88%-96%) during cFIO2 (75 ± 13 vs 66 ± 14% of recording time).
Conclusion. In this group of infants, cFIO2 was at least as effective as a fully dedicated nurse in maintaining SpO2 within the target range, and it may be more effective than a nurse working under routine conditions. We speculate that during long-term use, cFIO2 may save nursing time and reduce the risks of morbidity associated with supplemental oxygen and episodes of hypo- and hyperoxemia. Key words: hypoxemic episodes, preterm infant, mechanical ventilation, FIO2, automatic FIO2 adjustment, closed-loop FIO2 control.
Very low birth weight (VLBW) infants who require extended
mechanical ventilatory support often present with episodes of
hypoxemia.1-5 These episodes are detected by arterial
oxygen saturation monitoring by pulse oximetry
(SpO2) and are usually assisted with
a transient increase in the fraction of inspired oxygen
(FIO2).
Given the rapid onset and the frequency at which most of these episodes
of hypoxemia occur, maintaining SpO2
within a normal range by manual FIO2
adjustment during each episode is a difficult and time-consuming task.
Newborn intensive care unit personnel respond to high/low
SpO2 alarms but because of their
responsibilities under routine clinical conditions, their response time
is not always consistent and optimal. This exposes these infants to
periods of hypo- and hyperoxemia that may increase the risk of
retinopathy of prematurity6-10 and neonatal chronic lung
disease.11-14
These limitations make the use of a system for automatic
FIO2 adjustment a desirable
alternative. We have recently developed an algorithm that aims to
maintain SpO2 within a target range by closed-loop FIO2 control
(cFIO2). The primary objective of this study was to evaluate the efficacy of the
cFIO2 algorithm and secondarily, to
quantify the nursing time involved in maintaining SpO2 by manual
FIO2 adjustments.
We hypothesized that the cFIO2
algorithm will be more effective than a neonatal nurse stationed at the
infant's bedside in maintaining SpO2
within a specific range of normoxemia in a group of mechanically
ventilated preterm infants who presented with frequent episodes of
hypoxemia.
Patients
Preterm infants admitted to the University of Miami Jackson
Memorial Medical Center's newborn intensive care unit weighing <1500
g at birth, who required supplemental oxygen and presented with
frequent episodes of hypoxemia while undergoing mechanical ventilation,
were enrolled in the study. Infants were excluded if judged clinically
unstable by the clinical team. The study was approved by the University
of Miami Subcommittee for the Protection of Human Subjects, and written
informed consent was obtained from the parents or legal guardian of
each infant.
FIO2 Control Modes
cFIO2 Mode
The cFIO2 system was based on
a hybrid algorithm of differential feedback and rule-based control. The
algorithm continuously acquired the infant's
SpO2 information and adjusted the
FIO2 delivered by a mechanical
ventilator to maintain SpO2 within a
specific range set by the user. The
cFIO2 algorithm was set to calculate and adjust the set FIO2 once per
second on a closed-loop basis by means of a direct electronic
interface between the cFIO2 system and the ventilator's air-oxygen blender control.
METHODS
Top
Abstract
Methods
Results
Discussion
Conclusion
References
Manual FIO2 Control Mode (mFIO2) The reference mode to which the cFIO2 algorithm was compared consisted of manual adjustments of the FIO2 made continuously by a neonatal research nurse stationed at bedside, fully dedicated to maintain SpO2 within the same target range of normoxemia.
In the occurrence of a hypoxemic episode (SpO2 <88%), FIO2 was increased by 0.1 and by 0.2 if SpO2 dropped below 80%. Successive increments of the same magnitude were performed every 20 seconds if SpO2 did not return to the desired range or at shorter intervals if SpO2 continued to decline. Additional increments in FIO2 were halted when SpO2 improved, and FIO2 was eventually weaned down after SpO2 had returned to the target range. When SpO2 exceeded the upper limit of the target range, FIO2 was reduced by smaller adjustments until SpO2 returned to the target range. This mode was used to eliminate personnel-dependent variability in the response time and the FIO2 adjustments. This idealized mode of FIO2 control is not feasible in the routine clinical setting because it requires a neonatal nurse dedicated exclusively to monitor SpO2 and adjust FIO2.Study Protocol
Infants were studied for 2 continuous hours on each mode of FIO2 control (cFIO2 and mFIO2) for a total study time of 4 hours. The sequence of FIO2 modes was determined at random (tossed coin). Both modes aimed to maintain SpO2 within 88% to 96%.
The infants were studied in their incubators with their skin temperature servo-controlled at 36.5°C. They were kept in the same body position as they were before the study period. The infant's endotracheal tube was suctioned before the study. Ventilator parameters of peak inspiratory pressure, positive end-expiratory pressure, and synchronized intermittent mandatory ventilation rate remained unchanged. No nursing procedures were performed during the recording time, and the infants were left undisturbed. The research team was standing by at the infant's bedside at all times.
Measurements and Instrumentation
A neonatal mechanical ventilator (Babylog 8000, Draeger, Lubeck, Germany) with an electronic interface for remote FIO2 adjustment by the cFIO2 system was used for both modes of FIO2 control. During mFIO2, the nurse adjusted the FIO2 at the ventilator's front panel.
The added time delay during a change in FIO2 until such concentration is reached at the airway opening is determined by the air-oxygen blender, the combined volumes of the inspiratory limb of the ventilator's circuit plus humidifier, and the continuous bias flow set at the ventilator during time cycled-pressure limited ventilation. In the present study setup, the added time delay for a step change in FIO2 (manual or automatic) from 0.21 to 1.0 at a continuous bias flow of 6 L/min was ~4 seconds to reach a level of FIO2 of 0.8, and 6 seconds until a concentration of 1.0 was reached.
SpO2 was measured by a neonatal pulse oximeter (Oxypleth 520A, Novametrix Medical Systems Inc, Wallingford, CT). Changes in SpO2 displayed by a pulse oximeter are delayed in relation to changes in patient's arterial oxygen saturation by the duration of the SpO2-averaging interval. The user selectable averaging interval of the oximeter used in the present study was set at 2 seconds.
Figure 1 shows a diagram of the study setup. SpO2 and FIO2 signals were recorded into a personal computer for analysis.
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Data Analysis
Computerized analysis was used to calculate mean SpO2, frequency and duration of episodes of hypoxemia (SpO2 below 88%, 85%, and 75% for >5 seconds) and hyperoxemia (SpO2 above 96% for >5 seconds). Episodes with SpO2 below 88% included all episodes with SpO2 below 85%, and those in turn, included all episodes with SpO2 below 75%.
The percentage of time each infant spent with SpO2 within the range of normoxemia (88%-96%), hyperoxemia with SpO2 above 96%, and hypoxemia with SpO2 below 88%, 85%, and 75% was calculated for each infant.
The frequency and mean duration of episodes of missing SpO2 information (dropout) longer than 5 seconds was calculated during each mode, and the resulting cumulative total time is reported as percentage of the total recording time. To characterize the conditions after the occurrence of these episodes, the number of missing SpO2 episodes that resulted in readings within the normoxemic, hyperoxemic, or hypoxemic ranges immediately after the SpO2 signal was recovered (after 5 seconds) is reported as percentage of the total number of missing SpO2 episodes.
To evaluate SpO2 over- and undershoot resulting from FIO2 adjustments made during episodes of missing SpO2 information, the number of episodes followed by SpO2 readings within the hyper- and hypoxemic ranges for 60 or more seconds during the 2 minutes after the recovery of the SpO2 signal is reported as percentage of the total number missing SpO2 episodes.
Overshoot resulting from FIO2 adjustments made in response to episodes of hypoxemia (SpO2 <88%) was quantified as percentage of hypoxemic episodes followed by SpO2 readings within the hyperoxemic range for 60 or more seconds during the 2 minutes after the resolution of the episode.
Mean FIO2 levels and the percentage of time spent with FIO2 above, below, or at each infant's basal FIO2 requirement were calculated for each mode of FIO2 control. The number of manual FIO2 adjustments made by the neonatal nurse during the mFIO2 mode was counted for each infant. FIO2 adjustments during the cFIO2 mode were not quantified because they occurred on a continuous basis.
Within-participant comparisons using a 2-tailed paired t test or Wilcoxon Signed Rank Test were done where appropriate. Results are reported as mean ± standard deviation or median (range) respectively. A P value of <0.05 was considered statistically significant.
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RESULTS |
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Top
Abstract
Methods
Results
Discussion
Conclusion
References
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Fourteen VLBW infants undergoing mechanical ventilation were included in the study. Their mean birth weight was 712 ± 142 g and their gestational age was 25 ± 1.6 weeks. At the time of the study, infants were 26 ± 11 days old. Six infants were studied on supine and 8 in the prone position. Position was not changed during the study. All infants tolerated the 2 periods of FIO2 control well and there were no adverse events.
The ventilatory support consisted of a synchronized intermittent mandatory ventilation rate of 24 ± 10 breaths/minute, a peak inspiratory pressure of 17.5 ± 2.0 cmH2O, and a positive end-expiratory pressure of 4.3 ± 0.5 cmH2O. The ventilator's continuous bias flow was set at 6 L/min. Episodes of apnea were not frequent and did not require intervention. Ventilator settings provided enough ventilatory support during periods of absent spontaneous breathing activity.
Episodes of Missing SpO2 Information
Excessive motion artifact resulted in dropout episodes of missing SpO2 information (>5 s) during both modes. There were 11.8 ± 7.4 and 12.8 ± 7.4 episodes per hour of missing SpO2 information with a mean episode duration of 14.3 ± 7.0 and 16.9 ± 8.2 seconds, for a total of 6.0 ± 5.1% and 6.3 ± 4.4% of the recording time during mFIO2 and cFIO2, respectively.
On resolution of the episodes of missing SpO2 information, SpO2 displayed hypoxemic readings (SpO2 <88%) within 5 seconds after signal recovery in 55.3 ± 25.8% and 51.6 ± 25.5% during mFIO2 and cFIO2, respectively. Normoxemic readings (SpO2 88%-96%) were displayed after 41.2 ± 23.2% and 45.0 ± 24.3% of the episodes, while hyperoxemic readings (SpO2 >96%) were observed after 3.3 ± 4.5% and 3.2 ± 4.9% of the episodes during mFIO2 and cFIO2, respectively.
A few of the FIO2 adjustments made during episodes of missing SpO2 information resulted in overshoot. Hyperoxemic readings (SpO2 >96%) for 60 or more seconds during the 2 minutes after SpO2 signal recovery were observed in 4.3 ± 6.0% and 3.5 ± 6.1% of the episodes during mFIO2 and cFIO2, respectively. Undershoot was more frequently observed with hypoxemic readings (SpO2 <88%) occurring in 24.1 ± 23.8% and 21.4 ± 19.4% of the episodes after SpO2 signal recovery.
Episodes of Hypoxemia and Hyperoxemia
Both modes of FIO2 control resulted in a similar frequency and mean duration of episodes with SpO2 below 88%. Episodes of more severe hypoxemia, defined as SpO2 below 85% and below 75%, also occurred at similar frequencies and mean duration during both modes of FIO2 control. The similarities observed in the frequency and duration of episodes with SpO2 below 88%, 85%, and 75% led to similar total duration of these levels of hypoxemia during both modes (Table 1).
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Most of the FIO2 adjustments made during both modes in response to episodes of hypoxemia (SpO2 <88%) resulted in the return of SpO2 to the normoxemic range. Only a small percentage was followed by an overshoot to hyperoxemic ranges. SpO2 displayed hyperoxemic readings (SpO2 >96%) for 60 or more seconds during the 2 minutes after the resolution of a hypoxemic episode in 5.8 ± 7.8% and 1.5 ± 1.9% of the cases during mFIO2 and cFIO2, respectively.
The frequency and mean duration of episodes of hyperoxemia defined as SpO2 above 96% were not significantly different between both modes of FIO2 control. This resulted in a small but not significant reduction in the duration of hyperoxemia during cFIO2 (Table 1).
Normoxemia and Mean Arterial Oxygen Saturation
Despite frequent episodes of hypoxemia observed in the infants studied, both modes of FIO2 control were able to maintain the infant's SpO2 within the target range of 88% to 96% for most of the time. The infants spent 66.3% and 74.9% of the total recording time within the normoxemic range during mFIO2 and cFIO2, respectively (P < .05). This significantly longer time spent within the target range during the cFIO2 mode resulted from a combined reduction in the duration of hypo- and hyperoxemia (Table 1).
Mean SpO2 levels during the 2 hours of recording were similar for both modes of FIO2 control and fell within the range of normoxemia (Table 1).
Supplemental Oxygen
The mean FIO2 delivered to these infants during the 2-hour period on each mode of FIO2 control was similar. The total duration of periods when infants received supplemental oxygen at a concentration either above, below, or at their basal FIO2 level was similar during both modes of FIO2 control (Table 2).
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During the mFIO2 mode, the neonatal research nurse made a mean of 29 ± 17 adjustments per hour (Table 2), either to increase or decrease FIO2, to maintain SpO2 within the normoxemic range.
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DISCUSSION |
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Top
Abstract
Methods
Results
Discussion
Conclusion
References
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Closed-loop or semiclosed-loop FIO2 control systems consisting of various combinations of oxygen delivery mode, feedback signal, and type of controller have been used before in preterm infants 15-19 with positive results in feasibility and effectiveness. Beddis et al15 and Dugdale et al16 used PaO2 measured by indwelling umbilical electrodes as feedback signal, and FIO2 was delivered by a ventilator and hood, respectively. Bhutani et al17 used SpO2 as feedback signal and FIO2 was delivered by hood to nonventilated infants. Sun et al18 also used SpO2, but the ventilator's FIO2 was manually adjusted. Only Morozoff and Evans19 used SpO2 as feedback and adjusted the FIO2 delivered to ventilated infants by means of a closed-loop system. They used a differential-feedback control algorithm that provided a reasonable degree of control, but it failed to respond to rapid SpO2 changes and required manual intervention. They expressed the need for additional development.
The present study evaluated the efficacy of an algorithm developed to maintain SpO2 within a target range by continuous closed-loop FIO2 adjustment in VLBW infants who present with frequent episodes of hypoxemia while undergoing mechanical ventilation. Furthermore, it quantified the personnel time spent in making manual FIO2 adjustments to assist these infants during each episode.
Most comparisons in the present study showed little or no difference in the frequency and the duration of hypo- and hyperoxemic episodes and in the FIO2 delivered to these infants between the 2 modes of FIO2 control. However, the time spent within the target range of normoxemia was significantly longer during cFIO2 than during mFIO2. These results are relevant because the effectiveness of the cFIO2 algorithm was compared with an ideal mode of FIO2 control consisting of a neonatal nurse whose only task was to maintain SpO2 within the target range at all times, instead of the routine clinical care.
The number of FIO2 adjustments made by the nurse to maintain SpO2 within the target range suggests that full dedication is necessary to perform such task. This requirement is difficult to achieve in the clinical setting where nurses have multiple responsibilities. Therefore, the results suggest potential savings in nursing time and/or improvement in patient care if a system of closed-loop FIO2 control was used for a similar group of infants under less optimal routine clinical conditions.
Mild episodes of hypoxemia may resolve spontaneously without the need of FIO2 adjustments by the clinical personnel. In such cases, the use of a system that will assist every episode of hypoxemia with a transient increase in FIO2 may result in periods of unnecessary oxygen exposure. To minimize this unnecessary exposure, milder episodes are assisted by the cFIO2 algorithm with smaller FIO2 increments than those episodes of more severe hypoxemia, and the FIO2 is gradually weaned as soon as the episode begins to resolve. On the other hand, a small FIO2 increase soon after the onset of a mild hypoxemic episode may avert alveolar hypoxia and thus prevent the episode from becoming more severe.20-25 In this study, SpO2 overshoot rate after an episode of hypoxemia was relatively low, with <6% of the hypoxemic episodes eventually developing into hyperoxemia.
Open- or closed-loop systems that use SpO2 as feedback are limited by the oximeter's ability to reflect true arterial oxygen saturation in the presence of motion artifact. Most signal processing algorithms built into pulse oximeters perform validation tasks to identify periods when SpO2 is not reliable. Currently, the cFIO2 algorithm relies on SpO2 data filtered and processed by the pulse oximeter, the same information used by clinicians for SpO2 monitoring and FIO2 adjustment. Excessive motion artifact results in missing SpO2 information, leaving clinicians, or in this case the closed-loop system, without the necessary feedback to respond accordingly.
To maintain patient safety whenever SpO2 information is missing, the cFIO2 algorithm locks the FIO2 at a user-determined backup level or at the current level, whichever is higher, until SpO2 information is available again. Although this approach may result in periods of unnecessary oxygen exposure, lack of response when hypoxemia is accompanied or caused by infant's motion may allow the hypoxemia to worsen. In the present study, the majority of the episodes of missing SpO2 signal were immediately followed by hypoxemic readings, therefore justifying a step up in FIO2 during such episodes. Also, SpO2 overshoot rate after episodes of missing SpO2 signal was relatively low. Less than 5% of them eventually developed into hyperoxemia.
Despite its limitations, SpO2 still is the most reliable and common method for continuous noninvasive monitoring of arterial oxygen saturation as an indicator of arterial oxygen content in VLBW infants over extended periods of time and, therefore, a suitable feedback signal for closed-loop FIO2 control. Transcutaneous electrodes are limited by the site application time and by the infant's skin condition. Indwelling umbilical artery electrodes are invasive and limited to the availability of an umbilical artery line.
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CONCLUSION |
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Top
Abstract
Methods
Results
Discussion
Conclusion
References
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In this particular group of premature infants who presented with frequent episodes of hypoxemia, the cFIO2 algorithm was at least as effective as a fully dedicated research nurse (an idealized mode of FIO2 control) in maintaining SpO2 within the target range. We speculate that the algorithm may be more effective in maintaining SpO2 within a desired range than a nurse working under less favorable routine clinical conditions.
Because this is a preliminary study, the feasibility and effectiveness of the cFIO2 algorithm should be investigated in a larger number of patients under routine clinical conditions. Its application in a population of more acutely ill patients as well as in nonventilated infants who require supplemental oxygen therapy should also be evaluated.
Although it remains to be proven, we speculate that long-term closed-loop FIO2 control may reduce nursing time spent to maintain adequate oxygenation and reduce the risks of morbidity associated with supplemental oxygen and frequent episodes of hypoxemia and hyperoxemia.
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ACKNOWLEDGMENT |
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This study was supported by the University of Miami Project Newborn.
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FOOTNOTES |
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Received for publication Apr 18, 2000; accepted Nov 22, 2000.
Address correspondence to Nelson Claure, MS, Division of Neonatology, Department of Pediatrics, University of Miami School of Medicine, Box 016960 R-131, Miami, FL 33101. E-mail: nclaure{at}miami.edu
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ABBREVIATIONS |
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VLBW, very low birth weight; SpO2, arterial oxygen saturation by pulse oximetry; FIO2, fraction of inspired oxygen; cFIO2, closed-loop FIO2 control mode; mFIO2, manual FIO2 control mode.
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REFERENCES |
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References
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