Abstract
To maintain arterial oxygen saturation (Sa,O2) above 90% in patients with acute respiratory failure, oxygen (O2) is often added to the circuit of two-level noninvasive positive pressure ventilation (NPPV). However, the final inspiratory oxygen fraction (Fi,O2) is not known.
To clarify this issue, the effect of different inspiratory positive airway pressures (IPAP) of the oxygen tubing connection site and the flow rate of O2, on Fi,O2 was assessed. The effects of the tidal volume (VT) and the respiratory rate on the Fi,O2 were then clarified in a model study.
The Fi,O2 varied depending on the point where O2 was added to the circuit. When all other variables were constant, the connection closest to the exhaust port (ventilator side) gave the highest Fi,O2. Increases in IPAP led to decreases in Fi,O2. Finally, Fi,O2 increased with O2 flow, although it was difficult to obtain an Fi,O2 >0.30 unless very high O2 flows were used. Paradoxically, NPPV with low IPAP values and without O2 supplementation led to a Fi,O2 <0.21 at the circuit-patient interface. VT and respiratory rate did not appear to influence Fi,O2.
To conclude, when using noninvasive positive pressure ventilation with two-level respirators, oxygen should be added close to the exhaust port (ventilator side) of the circuit. If inspiratory airway pressure levels are >12 cmH2O, oxygen flows should be at least 4 L·min−1.
Currently, two-level noninvasive positive pressure ventilation (NPPV) is used in the treatment of patients with acute respiratory failure in intensive care units 1–7, general pulmonary wards 8, 9 and emergency depts 10–12. In these settings, supplemental oxygen (O2) is often added to the circuit of the ventilators to maintain an adequate arterial O2 saturation (Sa,O2). The inspired oxygen fraction (Fi,O2) is generally unknown, and could be influenced by a number of factors such as the inspiratory positive airway pressure (IPAP), the expiratory positive airway pressure (EPAP), the O2 flow rate and the site where O2 is added to the circuit etc.
At the present time, there is no published data on the best way to add O2 to the circuit of a two-level NPPV (i.e. what level of flow is required, where should the connection be made). To clarify this matter, a two-part study was conducted. First, in a clinical setting, the effect of different IPAP values of the O2 tubing connection site and the flow rate of O2, on the Fi,O2 was investigated. Second, an experimental study to clarify the effect of the tidal volume (VT) and the respiratory rate on the Fi,O2 was conducted.
Materials and methods
Clinical study
Three normal volunteers, aged 21, 24 and 27 yrs, in good health and having never smoked were investigated. Two-level NPPV was initiated with a barometric ventilator (Bilevel positive airway pressure device (BiPAP® S/T-D30; Respironics Inc., Murrysville, PA, USA), through a face mask (Bird Corporation, CA, USA), with the subject in a semirecumbent position. The tubing connecting the ventilator to the mask had a volume of 565 mL and a length of 191 cm. The volume between the mask and the exhaust port as depicted in figure 1⇓ was 19.4 mL. Initially, the EPAP was set at 2 cmH2O (the minimal pressure level of the machine) and the IPAP was set at 2 cmH2O. The IPAP was then increased to 8, 12, 16 and 20 cmH2O. The machine was used in the assist-control mode with a backup frequency of 12 breaths·min−1 and a back-up inspiratory/expiratory time ratio of 50%. The device was used with a whisper swivel-type of exhaust port. Different levels of O2 flow at several different locations in the patient circuit were added. The O2 was added at the exit of the BiPAP unit (proximal injection), before the exhaust port (middle injection) and finally at the patient connection immediately before the mask (distal injection) as shown in figure 1⇓. For each level of pressure and connection point, 0, 2, 4, 6, 8, 10, 12, 14 and 16 L·min−1 of O2 were added. The additional O2 came from a high-pressure source governed by a flow-meter assembly (Thorpe-tube flow meter) and the connection was made with a T‐connector.
Schematic representation of the three types of circuit. a) proximal injection; b) middle injection; c) distal injection. O2: oxygen. A: location A, at the exit of the Bilevel positive airway pressure device; B: B, before the first connector; C: C, between the two T‐connectors; D: D, at the terminal part of the circuit before the mask.
For each value of IPAP, O2 flow rate and connection point, the FI,O2 was measured with an O2 monitor (Oxygen monitor 5120; Ohmeda, Madison, WI, USA). Calibration was carried out according to the manufacturers' recommended procedure. The response time of the O2 monitor was measured at several different conditions of IPAP and respiratory rate. The time taken for a 90% change was 43 s and the time taken for a two-third change was 21 s. All measurements were performed at a steady state. The Fi,O2 was measured at 4 points along the circuit using a second T‐connector for each circuit, as shown in figure 1⇑: location A: at the exit of the BiPAP unit; location B: before the first T‐connector (exhaust port or O2 addition); location C: between the two T‐connectors; location D: at the terminal part of the circuit just before the mask. The BiPAP system was cycled at each test setting until the reading on the O2 analyser stabilized. The final value was the result of three successive measures.
Model study
The effects of VT and the respiratory rate were investigated with a test lung. The test lung was a bicompartmental model of balloons in a Plexiglas box. Between the lung model and the ventilator, a low and a high linear resistance were added to modify the VT. A number 3 Fleisch pneumotachograph (Fleisch, Lausanne, Switzerland) with a Validyne pressure transducer (range ±5 cmH2O) (Validyne Engineering Corporation, Northridge, CA, USA) was used for airflow measurement and integration, yielding VT. Fi,O2, airflow and VT, with the two resistances and two different rates of ventilation were recorded. The barometric ventilator used in the clinical study was also used in this set of experiments.
Initially, the EPAP was set at 2 cmH2O and the IPAP was set at 2 cmH2O. The IPAP was then increased to 8, 12, 16 and 20 cmH2O. The barometric ventilator was used in the controlled mode with two different frequencies, 14 and 22 breaths·min−1. The inspiratory/expiratory ratio was 50%. The O2 was added before the exhaust port (middle injection in fig. 1⇑). For each level of pressure, 0, 2, 4, 6, 8, 10, 12, 14 and 16 L·min−1 of O2 were added. The additional O2 came from a high-pressure source governed by a flow-meter assembly and the connection was made with a T‐connector. For each value IPAP, O2 flow rate and value of resistance, the Fi,O2 was measured just before the connection to the lung model with an O2 monitor. The BiPAP System was cycled at each test setting until the reading on the O2 analyser stabilized. The final value was the result of three successive measures.
Statistical analysis
In the clinical part of this study, the relationships between Fi,O2 and each of the following determinants: O2 connection point, IPAP, location of measurement and O2 flow were tested. A standard linear regression analysis was used. The different relationships obtained were compared using covariance analysis, in order to test the difference between their slopes and intercepts. The same linear regression and covariance analysis were applied to the variables recorded during the experimental part of the investigation.
Results
Clinical study
Influence of the oxygen connection point
Figure 2⇓ shows the Fi,O2 values for an IPAP of 8 cmH2O and an EPAP of 2 cmH2O, according to the three connection points (proximal, middle, distal injection). It was shown that, the Fi,O2 changed according to which connection point was utilized (p<0.05). For a given O2 flow, the Fi,O2 was greatest when the O2 was connected just before the exhaust port. This applied to all levels of IPAP and O2 flow and to all measurement locations.
Trends for the inspiratory oxygen fraction (Fi,O2) values for an inspiratory positive airway pressure of 8 cmH2O at location C. ♦: proximal injection; ▪: middle injection; ▴: distal injection.
Influence of the inspiratory positive airway pressure
Figure 3⇓ shows the Fi,O2 values measured in location C with the O2 connected at the middle injection point, according to each level of IPAP. In this case, when levels of O2 flow were low, the value of Fi,O2 increased as IPAP increased from 8 to 12 cmH2O (p<0.05), but there was a decrease in Fi,O2 when pressure was increased from 12 to 16 and 20 cmH2O (p<0.05). At higher levels of O2 flow, Fi,O2 decreased when the IPAP was increased from 8 to 12, 16 and 20 cmH2O. There was no significant difference between IPAP of 16 and 20 cmH2O. Therefore in this circuit, the highest Fi,O2 was obtained with an IPAP of 8 cmH2O. These results did not apply to all circuits, connection points or O2 flows. For instance, at the proximal injection point, when measured at location C, the Fi,O2 was significantly lower for an IPAP of 20 cmH2O but there was no difference between the other pressure levels.
Inspiratory oxygen fraction (Fi,O2) values at location C with the oxygen connection at the middle injection for each level of inspiratory positive airway pressure (IPAP). ▪: IPAP 8; ▵: IPAP 12; □: IPAP 16; ○: IPAP 20.
Influence of the location of measurement
Figure 4⇓ shows the Fi,O2 values for an IPAP of 16 cmH2O measured in the four locations, with O2 connected at the middle injection point. The Fi,O2 increased as the measurement point was moved closer to the patient (p<0.01). This was true for all IPAP, O2 flows and connection points.
Inspiratory oxygen fraction (Fi,O2) values for an inspiratory positive airway pressure (IPAP) of 16 cmH2O with the oxygen connection at the middle injection for each four measurement locations. ♦: A; ▪: B; ▴: C; □: D. Table 1⇓: Influence of IPAP on the FI,O2 value in the location C.
Influence of oxygen flow
When measured at location D, with no added O2 and an IPAP <16 cmH2O, the Fi,O2 decreased along the circuit and fell below 21%. With an O2 flow of 2 L·min−1, the Fi,O2 decreased with increasing IPAP levels with measurements performed at location D, but not at the other locations. This applied to all three connection points. For high IPAP levels (16 and 20 cmH2O), it was difficult to obtain an Fi,O2 >0.35 unless very high O2 flows were used (table 1⇓).
Influence of inspiratory positive airway pressure (IPAP) on the inspiratory oxygen fraction value in the location C: middle injection
Model study
For each level of IPAP, VT was lower with a higher resistance (tables 2 and 3⇓⇓). For a given IPAP and resistance, VT decreased with an increase in frequency. Under both conditions, it was observed that there was no effect of VT or respiratory frequency on the Fi,O2 values recorded at different O2 flows, irrespective of IPAP level.
Results from the model study at low resistance with 14 breaths·min−1
Results from the model study at high resistance with 14 breaths·min−1
Discussion
In the clinical setting, O2 is frequently added to the circuit of two-level NPPV, to maintain Sa,O2 above 90%, in the treatment of patients with acute respiratory failure. O2 is added to the circuit of the ventilator at unspecified points and at different flow rates and the exact concentration of O2 delivered cannot be measured.
In this investigation, the site of O2 injection appears to be an important factor influencing the concentration of O2 that the patient receives. The highest values of Fi,O2 were recorded when the O2 was introduced to the circuit just before the expiratory port. Connecting O2 closer to the respirator or closer to the patient resulted in reduced values of Fi,O2 for the same O2 flows and ventilator settings. The mixture of air and O2 is probably more homogeneous when injected in the middle than in the proximal or distal locations. It may be hypothesized that if the O2 flow is added between the mask and the expiratory port, the blending of the expiratory and inspiratory gases could lower the Fi,O2 on the patient's side. O2 was delivered continuously and although it was not studied, there is a possibility that if O2 was supplied just before the whisper valve it may form a reservoir for the next inspiration. If it was supplied closer to the mask, the O2 delivered during expiration might be exhaled through the whisper valve and lost to the patient.
The level of IPAP also had an influence on the Sa,O2. The highest Fi,O2 values into the mask were obtained at IPAPs between 8–16 cmH2O. Furthermore, a drop in the Fi,O2 value was observed in the distal part of the circuit with IPAP pressures of <8 cmH2O. An Fi,O2 <21% without additional O2 and IPAP values <16 cmH2O appeared to be indirect signs of rebreathing and dilution. Rebreathing phenomena have been reported previously with IPAP <8 cmH2O 13, 14. At low levels of pressure (<8 cmH2O) and without supplemental O2 the patients may be submitted to a hypoxic gas mixture. It could be argued, that the long response time of the O2 monitor produces recordings resulting from the mixing of inspiratory and expiratory gases. Thus, the Fi,O2 would be artefactually lowered by the fractional expired O2. However, this study also reports results from the O2 monitor when located at position B, where the influence of expired gas would be minimal. Once O2 was added into the circuit, the Fi,O2 decreased with increasing IPAP. This seems logical, as higher pressures lead to higher flows of air for a fixed flow of O2, probably an unfavourable situation for this mixing.
It is reassuring that in this experimental model, respiratory rate and VT do not affect the Fi,O2. Although for completion of data collection, the Fi,O2 at different locations was measured, it was clear that only location D was of clinical relevance, as it was the location best representing the central inspired O2 concentration at the patient-circuit interface. Measurements performed within the mask (not used in this study) may give a better idea of the real inspired Fi,O2 but the precision and accuracy of the O2-sensor cell could be unfavourably influenced within the mask. Results may also be different with a nasal mask but the direction of the change would probably be the same.
Conventional ventilators provided with conventional expiratory valves and single tubing still allow some mixing of inspired and expired gases between the airway outlet and the expiratory valve location. These results could therefore be extended to the conventional ventilator, although the effect would probably be reduced. By contrast, ventilators provided with separate expiratory and inspiratory tube lines should not have this problem, although this remains to be tested.
A caveat concerning the absence of the influence of VT on Fi,O2 seems necessary. Indeed the potential role of VT on a model lung, where there was neither O2 consumption nor carbon dioxide production, was assessed. One might suppose that VT could behave in a more disturbing way in a patient, by its influence on dead space and, eventually, rebreathing into the distal part of the circuit. Nevertheless, the model data in this study (VT changing two-fold) suggests that, the problem should be slight.
To the best of the authors' knowledge, this is the first study examining the determinants of Fi,O2 during NPPV. This is an incomplete study. Indeed measurements using one model of two-level NPPV, only one connecting tube and a single face mask have been performed. The subjects studied were normal subjects, with normal lung mechanics and normal dead spaces. Moreover, the possible effect of change in EPAP on Fi,O2 has not been investigated. It is clear that these results could be different if measurements were performed with different respirators, different tubing lengths and volumes, and in patients with different lung diseases. Nevertheless, the important point still remains that Fi,O2 depends on several determinants in addition to O2 flow.
To conclude, when using supplemental oxygen during noninvasive positive pressure ventilation, the inspiratory oxygen fraction depended on three major factors: the point where oxygen is added into the circuit, the level of inspiratory positive airway pressure, and the oxygen flow rate. The respiratory rate and the tidal volume did not influence the delivered inspiratory oxygen fration. For inspiratory positive airway pressures >12 cmH2O, the inspiratory oxygen fraction flows should be at least 4 L·min−1. An inspiratory oxygen fraction ≥0.5 requires a very high level of oxygen flow. This was a limited experimental study, and should be considered as a guide rather than a complete predictor for different two-level pressure support ventilators used with various masks or different levels of expiratory positive airway pressures.
Acknowledgments
The authors would like to thank O. Pitance and D. Reychler for their help in collecting the data and N. Stroobant for her constant dedication to this study.
- Received July 24, 2001.
- Accepted September 10, 2001.
- © ERS Journals Ltd
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