Abstract
Objective. Volume guarantee (synchronized intermittent mandatory ventilation [SIMV]+VG) is a novel mode of SIMV for automatic adjustment of the peak inspiratory pressure to ensure a minimum set mechanical tidal volume (VT mech). The objective of this study was to compare the effects of SIMV+VG with conventional SIMV on ventilation and gas exchange in a group of very low birth weight infants recovering from acute respiratory failure.
Methods. Nine infants were initially studied during 2 consecutive 60-minute ventilatory modalities of conventional SIMV (ventilator settings by clinical team) and SIMV+VG 4.5 (VT mech set at 4.5 mL/kg) in random order. Eight additional infants were studied during the same ventilatory modalities plus 1 additional epoch consisting of SIMV+VG 3.0 (VT mech set at 3.0 mL/kg).
Results. Peak inspiratory pressure was significantly lower during SIMV+VG 3.0. Mean airway pressure, VT mech, number of large VT mech (>7 mL/kg), and mechanical minute ventilation (V′E) were reduced during SIMV+VG 4.5 compared with SIMV and were further reduced during SIMV+VG 3.0. Spontaneous V′E increased during SIMV+VG 4.5 and was even higher during SIMV+VG 3.0. The resulting total V′E was higher during both SIMV+VG modes compared with SIMV. Arterial oxygen saturation by pulse oximetry, transcutaneous carbon dioxide tension, and fraction of inspired oxygen did not differ significantly, although transcutaneous carbon dioxide tension increased slightly during SIMV+VG 3.0.
Conclusions. The short-term use of SIMV+VG resulted in automatic weaning of the mechanical support and enhancement of the spontaneous respiratory effort while maintaining gas exchange relatively unchanged in comparison to conventional SIMV.
Despite a substantial decrease in mortality from initial respiratory failure in very low birth weight (VLBW) infants during the past 2 decades, significant morbidities associated with mechanical ventilation still occur frequently.1 There is no consensus regarding an optimal ventilatory strategy for the support of the preterm newborn.
The mechanical support required by ventilator-dependent preterm infants varies from minute to minute because of the spontaneous changes in inspiratory effort and sudden changes in respiratory compliance and resistance associated with breath-holding and active expiration.2 Slower changes in lung mechanics can occur with development or resolution of atelectasis, pulmonary edema, and accumulation of secretions in the airway. During synchronized intermittent mandatory ventilation (SIMV), these varying conditions are met with a constant peak inspiratory pressure (PIP) regardless of the tidal volume achieved. The PIP is chosen to provide stability of gas exchange throughout these changing conditions, and, therefore, pressure settings are usually higher than those required. This increases the risks of ventilator-induced lung injury3–7 and may result in hypocarbia, which inhibits the infant’s own inspiratory drive and may be associated with pulmonary and neurologic complications.8, 9
Volume guarantee (SIMV+VG) is a novel mode of synchronized, time-cycled, pressure-limited ventilation, developed to maintain a minimal preset mechanical tidal volume (VT mech) by microprocessor-controlled PIP adjustments. The automatic PIP adjustments during SIMV+VG are determined by the difference between set and exhaled VT mech.
The proposed mechanism by which SIMV+VG may benefit mechanically ventilated preterm infants is by ensuring a tidal volume close to a physiologic level, which would result in a more efficient use of their spontaneous inspiratory effort. Downregulation of PIP when VT mech remains at or above the physiologic level releases the infant’s own respiratory drive from the suppression caused by superimposed ventilation and averts overinflation pressures that increase the risk of baro- and volutrauma. In addition, prevention of excessively low tidal volumes, attributable to sudden deterioration in the mechanical characteristics of the respiratory system, can preserve alveolar gas exchange and prevent atelectasis. 10–12
The objective of this study was to assess the effects of SIMV+VG on ventilator-generated airway pressure, minute ventilation, oxygenation, and ventilation in relation to conventional SIMV in a group of stable preterm VLBW infants in the recovery phase of acute respiratory failure. We hypothesized that guaranteeing a VT mech within the normal range of spontaneously breathing VLBW preterm infants would allow reduction in PIP while maintaining adequate ventilation and oxygenation.
METHODS
Study Population
Mechanically ventilated, clinically stable infants who were appropriate for gestational age, had birth weights between 600 g and 1200 g, and were at least 48 hours of age at the time of the study were considered eligible. Exclusion criteria included severe congenital anomalies, perinatal asphyxia, sepsis, symptomatic patent ductus arteriosus, severe intraventricular hemorrhage (grades 3–4), sedation, and patient deemed to be clinically unstable by the attending neonatologist.
The study was approved by the University of Miami School of Medicine, Subcommittee for the Protection of Human Subjects. Patients were enrolled after written informed consent was obtained from the parents.
Study Protocol
Initially, infants were studied during 2 consecutive 60-minute epochs of conventional SIMV and SIMV+VG at a target VT mech of 4.5 mL/kg (SIMV+VG 4.5) in random order. Ventilator settings of PIP, positive end-expiratory pressure (PEEP), mechanical inspiratory time, and rate during SIMV were those selected by the clinical team before the study. During SIMV+VG, all ventilator settings were the same except for PIP, which was set at 10 cm H2O above the clinical setting and was used to limit the delivered PIP during VG.
An interim evaluation of the results from the first 9 patients showed evidence that, the majority of the time, VT mech was larger than that during spontaneous breaths, preventing the infants from increasing their respiratory contribution. Therefore, to explore further the effect of VT mech on PIP, we studied 8 additional infants while they underwent the same ventilatory modalities of conventional SIMV and SIMV+VG 4.5 plus 1 additional epoch consisting of SIMV+VG 3.0 (VT mech set at 3.0 mL/kg) also in random order. Ventilator settings of mechanical rate, inspiratory time, and PEEP were left unchanged during both SIMV+VG periods.
Randomization was determined using sealed envelopes. The infants were studied in their incubators in the prone position.
Mechanical Ventilator
A Draeger Babylog 8000 plus, a timed-cycled, continuous-flow, pressure-limited, flow-triggered neonatal ventilator, with expiratory tidal volume targeting (Software version 3; Draeger, Inc, Lubeck, Germany), was used for all ventilation modes. The flow sensor, a hot wire anemometer (direction sensitive), was calibrated before each study. According to the manufacturer’s instructions, the ventilator undergoes an internal calibration procedure while both ends of the sensor are occluded. The ventilator airflow and airway pressure output signals were also internally calibrated. The same ventilator unit was used to ventilate all infants during the 3 modes. The reported relative volume error of this device is −5.3 ± 1.1% for volumes <10 mL.13
During SIMV+VG, the ventilator compares the measured exhaled VT mech to the target guaranteed level set by the operator. After each mechanical breath, the necessary pressure for the subsequent mechanical breath is calculated using a proprietary algorithm based on such comparison. For achieving a relatively smooth transition from the initial tidal volume to the target volume, only a portion (±3 cm H2O) of the pressure difference is added to the pressure on the next breath. As a safety feature, if total inspiratory VT mech exceeds 150% of the set tidal volume, then the ventilator expiratory valve opens, ending the mechanical inspiration. The automatic adjustments of PIP are kept within a preset range defined by an upper limit of PIP and the PEEP. In this study, the PIP limit was set 10 cm H2O above the PIP that was used during the conventional SIMV mode.
Measurements
The following parameters were measured throughout the entire study period: Airflow and airway pressure signals were obtained from the ventilator’s analog output, and tidal volume was obtained by digital integration of airflow signal. Fraction of inspired oxygen (Fio2) was measured using an oxygen analyzer (VTI oxygen analyzer; Vascular Technology, Inc, Chelmsford, MA). Arterial oxygen saturation was monitored continuously by pulse oximetry (Spo2; Novametrix 520 A; Novametrix Medical Systems Inc, Wallingford, CT). Transcutaneous carbon dioxide tension (TcPco2) was measured with a Transcend Shuttle (Sensormedics, CA) or TCM3 (Radiometer, Copenhagen, Denmark). All signals were digitized at a rate of 100 Hz and recorded on disk using data acquisition software (DATAQ Instruments, Inc, Akron, OH).
Data Processing and Statistical Analysis
Computerized analysis was performed during the last 45 minutes of each 60-minute epoch of conventional SIMV, SIMV+VG 4.5, and SIMV+VG 3.0. The first 15 minutes of each epoch was considered a stabilization period.
PIP, mean airway pressure (MAP), and the number of ventilator-generated breaths were obtained from the airway pressure signal. The tidal volume resulting from the combined ventilator positive pressure plus the infant’s inspiratory effort during synchronous breaths was counted as a mechanical breath. An effective mechanical breath was defined as any mechanical breath delivered with positive inspiratory pressure above the PEEP level. The mechanical rate refers to the number of effective mechanical breaths occurring every minute.
The number of effective mechanical breaths and VT mech coefficient of variation were calculated for each epoch. The number of effective mechanical breaths with VT mech >7 mL/kg was calculated for each mode and reported as a percentage of the total number of mechanical breaths. Spontaneous breaths were defined as those generated by the infant’s spontaneous respiratory effort without any positive pressure generated by the ventilator above the PEEP level.
The mean and standard deviation (SD) of all individual breaths were obtained for each infant during each ventilatory mode. The reported data represent the mean and SD of all infants’ mean values.
For each infant, the spontaneous (V′E spont) and mechanical (V′E mech) components of the total expiratory minute ventilation (V′E tot) were derived from the number and tidal volumes of spontaneous and mechanical breaths for every minute, and the mean and SDs were calculated for each epoch. The average Fio2, Spo2, and TcPco2 were obtained for each 45-minute epoch.
Within-subjects comparisons were done using 1-way repeated measures analysis of variance. P < .05 was considered statistically significant.
RESULTS
Seventeen mechanically ventilated, clinically stable, VLBW preterm infants were included in the study. These infants weighed 854 g (655–1140 g) at birth (mean [range]), were born at 27 weeks of gestation (24–31 weeks), and were 5 days old (2–9 days) at the time of the study. Of the 17 infants, 8 had perinatal respiratory depression without significant lung disease. Nine infants were recovering from respiratory distress syndrome, and they all received surfactant therapy within the first 6 hours of life. Seven infants were boys, 15 were exposed to antenatal steroids; 12 mothers received magnesium sulfate before delivery. Their mechanical ventilatory support consisted of an SIMV rate of 16 breaths/min (10–20 breaths/min), PIP of 15 cm H2O (13–20 cm H2O), PEEP of 4 cm H2O (3–5 cm H2O), and Fio2 of 0.22 (0.21–0.30).
There was a significant reduction in PIP during SIMV+VG 3.0 compared with both SIMV+VG 4.5 and SIMV periods. There was a small but not significant reduction in PIP during SIMV+VG 4.5 in comparison with conventional SIMV (Table 1).
Mechanical Support, Ventilation, and Oxygenation
Downregulation of PIP to PEEP level when spontaneously generated tidal volumes were sufficiently large (at or above the VT mech set) during SIMV+VG resulted in a lower number of effective mechanical breaths delivered when compared with conventional SIMV. Therefore, during SIMV+VG, the effective mechanical rate was often lower than the one set at the ventilator. A significant reduction of the effective mechanical rate was observed with SIMV+VG 3.0 compared with SIMV and SIMV+VG 4.5 (Table 1).
The combined reduction in PIP and number of breaths generated by the ventilator resulted in a lower MAP during SIMV+VG modes compared with SIMV and was lower during SIMV+VG 3.0 in comparison with SIMV+VG 4.5 (Table 1).
As expected, the reduction in the mechanical support affected the ventilation in these infants. V′E tot was higher during both SIMV+VG modes compared with conventional SIMV, but its mechanical component was lower during SIMV+VG. This reduction in V′E mech was more pronounced with SIMV+VG 3.0. The reduction in V′E mech during SIMV+VG was accompanied by a proportional increase in VE spont, thus increasing the contribution of the spontaneous component on V′E tot(Fig 1).
V′E tot, V′E mech, and V′E spont, during conventional SIMV, SIMV+VG 4.5, and SIMV+VG 3.0 (mean ± SD). V′E tot was higher, whereas its mechanical component was lower during SIMV+VG when compared with conventional SIMV. *P < .05 versus SIMV; #P < .05 versus SIMV+VG 4.5.
The increase in V′E spont observed at both levels of SIMV+VG with respect to SIMV resulted from a combined increase in spontaneous tidal volume (VT spont) and spontaneous respiratory rate (ie, unsupported breaths generated by the infant). Furthermore, there was a greater increase in spontaneous respiratory rate during SIMV+VG 3.0 than during SIMV+VG 4.5. This increase in the spontaneous respiratory rate resulted in a significant increase in the total respiratory rate (Table 1).
The driving pressures generated by the ventilator alone or combined with the spontaneous inspiratory effort during synchronous breaths resulted in a smaller VT mech delivered during SIMV+VG than during conventional SIMV. This was more evident during SIMV+VG 3.0. This reduction in VT mech during SIMV+VG resulted in a lower incidence of mechanical breaths with excessive tidal volumes (>7 mL/kg) during both SIMV+VG modes when compared with SIMV (Table 1). There was a significantly greater coefficient of variation (%) in VT mech during SIMV+VG 3.0 (41 ± 7) when compared with conventional SIMV (27 ± 7) and SIMV+VG 4.5 (31 ± 7). Fio2, Spo2, and TcPco2 did not differ significantly among the 3 different ventilatory modalities (Table 1).
DISCUSSION
Microprocessor technology present in the new generation of neonatal ventilators allows for precise control of pressure, volume, rate, and inspiratory time. This technology renders itself useful for the automatic control of these parameters, thereby providing gentler mechanical ventilatory support to preterm infants by preventing the delivery of excessive pressure or tidal volume while providing the support needed to maintain adequate lung volume and gas exchange.
In this group of preterm newborns, using SIMV+VG to guarantee the delivery of VT mech in the range of that spontaneously generated by the infant was associated with a reduction in mechanical support to a level significantly lower than that chosen by the clinical team for conventional SIMV. Despite the reduction in the mechanical ventilatory support, Fio2, Spo2, and TcPco2 were not significantly different between SIMV and SIMV+VG.
The reduction in PIP observed during SIMV+VG 4.5 was similar to that reported in a recent publication,14 but in the present study, a more striking reduction was observed only during SIMV+VG 3.0. In that study, PIP limit during VG was set at the same level used for conventional ventilation, whereas in the present study, PIP was limited to 10 cm H2O above the clinical setting. This higher PIP limit did not have a significant influence on the results of the present study, which included infants in relatively stable condition. However, it may have significant effects in a population of infants who present with dynamic changes in lung compliance and resistance.2
No adverse effects were observed during SIMV+VG when tidal volume target values in the physiologic range and a pressure limit of 10 cm H2O above the PIP used clinically were used. No excessive pressures or tidal volumes were recorded, and episodes of low arterial saturation and bradycardia did not increase in number.
Although this group of infants was already receiving a relatively low level of mechanical support during SIMV, automatic weaning further stimulated the infants’ respiratory drive, resulting in an increase in VT spont and spontaneous respiratory rate during SIMV+VG. The enhanced respiratory drive is reflected in the increased spontaneous component of V′E tot.
These results suggest that VLBW infants frequently require less ventilatory support than that provided clinically and that they are able to increase their inspiratory effort when challenged. A lower level of mechanical support should reduce the risks of baro- and volutrauma, as shown by a reduction in the incidence of breaths with excessive tidal volumes during SIMV+VG.
A similar reduction in mechanical support could be obtained by decreasing PIP during conventional SIMV. This maneuver, however, may result in a VT mech that fluctuates over time, particularly in the lower ranges of PIP, where VT mech has a greater spontaneous inspiratory effort component that is characterized by its variability in preterm infants. Therefore, a reduction in PIP during SIMV may require more frequent monitoring to prevent delivery of a VT mech less than or close to the anatomic dead space. This could result in insufficient gas exchange and progressive lung collapse.
The respiratory drive of VLBW infants has an intrinsic instability2, 15; thus, shifting the ventilation to a larger V′E spont component during SIMV+VG led to increased variability in VT mech. Most of the variability in VT mech during SIMV+VG resulted from the variability of the infant inspiratory effort during synchronized mechanical breaths, but it may also be secondary to a delayed response by the ventilator’s algorithm that adjusts PIP in subsequent breaths after a change in VT mech. The delay in the response is larger at lower SIMV rates, which in the face of rapidly changing conditions could augment the variability in VT mech.
The most appropriate setting for VT mech during SIMV+VG or even SIMV for different clinical conditions and its long-term implications still remain to be determined. In this study, a significant decrease in PIP was obtained only with SIMV+VG when the VT mech was set at approximately 50% of the VT mech delivered during SIMV. However, the smaller VT mech may result in lower alveolar ventilation in infants with a relatively large anatomic dead space, thus requiring them to increase V′E tot beyond the levels observed during SIMV to compensate for increased dead space ventilation. Conversely, setting too high a guaranteed VT mech during SIMV+VG may override the infant’s inspiratory effort, allowing the ventilator to take over ventilation and thereby inhibiting the infant’s own respiratory drive.
In the present study, the increase in V′E tot during VG in comparison with SIMV was generated by an increase in both VT spont and respiratory rate. In contrast to the results reported by Cheema and Ahluwalia, 14 the observed compensatory increase in V′E spont exceeded the reduction in V′E mech.
Our results show that it was necessary to guarantee a relatively low VT mech during SIMV+VG 3.0 to reduce significantly the mechanical support. This, however, was accompanied by an upward trend in TcPco2, suggesting that not all infants were able to maintain adequate ventilation when the support was reduced to those levels. The increasing values of TcPco2 observed during the SIMV+VG modes, especially with SIMV+VG 3.0, although not statistically different, could reach significance with a larger sample size. Furthermore, our findings are based on a relatively short study period, which may change when the support is reduced for longer periods of time.
In the present study, we sought to investigate the short-term effects of SIMV+VG in infants requiring relatively low levels of mechanical support. These results suggest that SIMV+VG may be a useful tool for the optimization of the mechanical support, but additional investigation is required to determine the effects of this modality when used for longer periods of time and in different clinical conditions. Whether SIMV+VG facilitates weaning and reduces the risk of baro- and volutrauma and associated morbidities during long-term use could be determined only with a randomized clinical trial.
Acknowledgments
This study was supported by the University of Miami Project: New Born.
REFERENCES
- Copyright © 2002 by the American Academy of Pediatrics
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