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*Alfred Hospital/La Trobe University, Melbourne, Prince of Wales Hospital, Hong Kong, University of Queensland, Australia
Address correspondence and reprint requests to C. Savian, Cardiopulmonary Research Unit, 4th Floor, Philip Block, Alfred Hospital, Commercial Rd, Prahran 3181, Victoria, Australia. Address e-mail to Camila_Savian{at}hotmail.com.
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Abstract |
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Introduction |
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One of the main uses is for the application of an "artificial cough," through the generation of a large tidal volume (Vt) and a peak expiratory flow (PEF) high enough to mobilize secretion, to prevent sputum plugging and subsequent nosocomial pneumonia. Removal of secretions in tracheally intubated patients has been demonstrated to occur via a process of annular two-phase gas-liquid flow (6). The criteria for this has been suggested to be an inspiratory flow rate slower than expiratory flow rate and a larger Vt (7), producing a decrease of pressure between the alveolus and the mouth during the gas liquid interaction. One of the factors that are hypothesized to influence this decrease is the level of positive end-expiratory pressure (PEEP).
PEEP is currently used in mechanically ventilated intensive care patients to re-expand atelectasis (8), improve oxygenation (9), reduce intrapulmonary shunt fraction, and protect the lungs against shear stress (10). "Protective lung" and "open lung" ventilation, i.e., small Vt and increased levels of PEEP, are currently accepted as the most effective ventilatory strategies (10), and there is controversy as to whether the patient should be disconnected from high levels of PEEP to provide MHI, as disconnection may result in a decrease in functional residual capacity (FRC) (11), decrease in oxygenation (12), and potential shear stress of distal lung units (13).
To overcome these adverse effects, a valve to maintain PEEP may be connected to the manual resuscitation bag (MRB) (14). However, high levels of PEEP may decrease the alveolus-to-mouth pressure gradient and consequently decrease the PEF to a level where MHI may not be effective as a secretion maneuver technique. It is therefore important to investigate whether different levels of PEEP alter the effectiveness of MHI as a secretion removal technique and whether it is justified in disconnecting the patient from high levels of PEEP.
The aim of this study was to compare the PEF and Vt generated in a lung model at 6 different levels of PEEP and 2 levels of lung compliance, 0.05 L/cm H2O to represent normal lung compliance (15) and 0.02 L/cm H2O to simulate low compliant lungs such as those of acute respiratory distress syndrome (ARDS) patients (16). Two types of MRB were investigated: a Mapleson-C circuit and the Laerdal self-inflating resuscitator.
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Methods |
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The 2-L Mapleson-C antistatic re-breathing circuit (BS 3352) and the 1.6-L Laerdal self-inflating resuscitation bag were connected via a Pneumotach to a test-training lung (MI Instruments Inc). The test-training lung was connected to a respiratory mechanics monitor (Cosmo Model 8000; Novametrix Medical Systems, Inc. Wallingford, CT). Data were downloaded to a personal computer and analyzed by the Analysis Plus PC program (Novametrix Medical Systems, Inc.) (Fig. 1).
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A portable spring-loaded PEEP valve (AMBU®) was connected to the MRB and a constant oxygen flow rate of 15 L/min was delivered. Lung compliance of 0.05 L/cm H2O and 0.02 L/cm H2O were chosen to simulate normal (15) and low (16) compliance lung conditions, respectively. An airway resistance of 2.3 ± 0.5 cm H2O/s was standardized for all measures in this study.
All subjects were instructed to perform MHI as if trying to promote secretion clearance. A 2-s deep inspiration, an inspiratory plateau of 2 s, and a fast release (1 s) were requested. The respiratory wave forms were examined and only the breaths that fulfilled this pattern were included in the analysis. Peak inspiratory pressure (PIP) was standardized to maximum of 35 cm H2O using a pressure manometer. The expiratory valve of the Mapleson-C circuit was adjusted to the fully open position but manually held closed during inspiration, then released to the fully open position during expiration.
A trial for 60 s was allowed on each circuit before testing. Five breaths were given in random order (using sealed envelopes) for both bagging circuit and each level of PEEP (0, 5, 7.5, 10, 12.5, and 15 cm H2O). For PEEP levels of 0, 5, and 7.5 cm H2O, compliance of 0.05 L/cm H2O was standardized, whereas for PEEP levels of 10, 12.5, and 15 cm H2O, MHI was performed at both compliance settings of 0.05 and 0.02 L/cm H2O. The physiotherapist performing MHI was blinded as to the level of PEEP and compliance in the circuit. The PEF, Vt, PIP, and PEEP level were recorded by the respiratory mechanics monitor. The cross-sectional area of the tube (CSA) was 2.2 cm2.
Data were extracted from each waveform using the Analysis Plus software, copied into an Excel spreadsheet and analyzed by the SPSS program (version 11.5 for Windows; SPSS, Chicago, IL). As the scores did not generate a normal distribution, a base-10 logarithm was used to transform all the dependent variables.
Using the log-transformed variables, a general linear model, which is an extension of the multivariate analysis of variance, was performed for the dependent variables PEF and Vt, and for the independent variables PEEP level, compliance, and type of circuit. The analysis included all the main effects and also second level interactions. If this proved significant, a post hoc multiple comparisons for observed means was performed using the least significant difference comparison. In addition, a correlation analysis matched PEF and both PIP and Vt.
Sample size calculation was based on PEF using the mean difference and standard deviation from a previous study (17). For the given effect size, 
= 0.05 (two-tailed) and power of 90%, the sample size estimated was 10 subjects. Data are presented as mean values and standard deviation unless otherwise stated.
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Results |
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When compliance was set at 0.05 L/cm H2O for the Laerdal circuit, there was a significant decrease in PEF between levels 0 and 15 cm H2O of PEEP, with effect size (d) of 1.2 and P = 0.032. For the Mapleson-C circuit, a significant decrease in PEF was observed between 0 and every PEEP level equal to or more than 10 cm H2O (d = 1.45, P = 0 < 0.01)(Table 2).
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At a compliance of 0.02 L/cm H2O, no significant decrease in PEF was observed between 10, 12.5, and 15 cm H2O of PEEP for either circuit; however, the Mapleson-C circuit generated significantly higher PEF (d = 2.21, P = 0.03) when compared with the Laerdal (Table 2).
The PEF was significantly higher with a compliance of 0.02 L/cm H2O for both circuits at all levels of PEEP (d = 0.88 and P < 0.01). In addition, the peak inspiratory flow to PEF ratio (PIF/PEF) generated by the Mapleson-C circuit (1.14 ± 0.43) was significantly lower when compared with the Laerdal resuscitation bag (1.70 ± 0.91; d = 0.8, P < 0.01).
The Mapleson-C circuit produced significant larger Vt (1.560.3 ± 637.5) when compared with the Laerdal circuit (1.031 ± 187; d = 1.28, P < 0.01). For both resuscitation bags, Vt was significantly larger when compliance was set at 0.05 L/cm H2O (d = 0.89, P < 0.01).
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Discussion |
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This study has shown that, in a lung model, the Mapleson-C circuit generates a PEF that is theoretically capable of annular 2-phase gas-liquid flow at all PEEP levels up to 15 cm H2O. The Laerdal circuit, according to this theoretical model, may not be effective when using PEEP levels more than 10 cm H2O of PEEP. This study is the first to investigate the effect of PEEP levels on PEF, and a significant reduction in PEF was observed in both circuits as the level of PEEP increased.
Increasing numbers of intensive care patients are mechanically ventilated on high levels of PEEP with small Vts. If the critical care practitioner considers that MHI is clinically indicated, a PEEP valve is usually connected to the MRB to prevent shear stress and decreased oxygenation and FRC during disconnection from the ventilator. A survey conducted by Hodgson et al. (18) in 32 Australian intensive care units showed that 66% of physiotherapists would choose to include PEEP valve in the bagging circuit if the patient was ventilated with PEEP. However, high levels of PEEP may decrease alveolus-to-mouth pressure gradient and consequently decrease the expiratory flow rate, rendering MHI ineffective as a secretion maneuver technique.
Maxwell and Ellis (7), in a review of effective secretion mobilization during mechanical ventilation and MHI, have stated that one of the conditions necessary to remove secretion during manual hyperinflation is PEF 
0.41 L/s (24.6 L/min). In the current study, the PEF generated did not meet this condition using the Laerdal circuit at a PEEP level of 10 and 15 cm H2O with the normal compliance setting (0.05 L/cm H2O), suggesting that the use of the Laerdal MRB may not be the best option in patients on high levels of PEEP when the aim of the therapy is secretion mobilization.
Another important factor responsible for promoting secretion clearance is to generate PIF slower than PEF (7). The Mapleson-C circuit generated significantly lower PIF to PEF ratio than the Laerdal circuit. These results confirm the findings of Maxwell and Ellis (19).
The Mapleson-C circuit applied significantly higher PEF, Vt, and PIP when compared with the Laerdal circuit. There are important differences between these two bag types that may explain these results. The Laerdal circuit has a 1.6-L reservoir bag, whereas the Mapleson-C circuit has a 2-L bag. The Mapleson-C circuit's rubber bag is more compliant than the silicone Laerdal bag. Also, when using the Laerdal circuit, the inspiratory fishmouth valve results in leak of volume and airway pressure during the end of the inspiratory plateau (20), whereas when using the Mapleson-C circuit, the therapists need to hold the expiratory valve during inspiration and also during the inspiratory plateau to fill the bag, allowing the airway pressure and volume to increase until the end of inspiration.
Setting the test lung to a low compliance (0.02 L/cm H2O) resulted in increased airway pressure and a decrease in Vt during MHI, concurring with previous results (20,21). The PEF was demonstrated to be significantly correlated with PIP, and in the current study, PEF was greater in lungs of poor compliance than normal compliance, although PIP was limited to 35cm H2O. This suggests that in patients with poor compliance (e.g., ARDS), MHI is effective as a secretion maneuver technique.
Volume restoration has been considered to play an important part in secretion clearance, and impairment in mucociliary clearance has been observed in reduced lung volumes (22). In the current study, significantly smaller Vts were generated when low compliance was used, in accordance with the study by Rusterholz and Ellis (23).
Kim et al. (24) demonstrated that gas velocity slower than a cough may remove secretion via annular two-phase gas-liquid flow. As the linear velocity is determined by the expiratory flow rate divided by the total CSA of the airways, the effective velocity should decrease with increasing airway generations as the total CSA increases. The more distal the airway generation, the slower the linear velocity generated, but also the slower the critical airflow rate required to produce upward movement in a viscoelastic liquid (7). In the current study, the CSA of the tube was constant at 2.2 cm and it is difficult to draw conclusions regarding the effect of MHI in distal airway generations.
Patients receive high PEEP for various reasons, including intrapulmonary and extrapulmonary ARDS, pulmonary edema, and impaired gas exchange. MHI would not be advantageous in all these situations. There is also controversy over the disconnection of patients on high levels of PEEP, as it may cause a decrease in FRC and shear stress. Inline suctioning devices are in common use and one of the advantages of them is that disconnection of ventilation is not needed for suction. This study was a bench study only, but it does suggest that if mucous plugging was considered the cause of poor gas exchange, MHI with a Mapleson -C circuit and PEEP valve would generate a PEF capable of mobilizing secretions. However, this same circuit may potentially cause complications, including barotrauma or volutrauma, resulting from high inspiratory pressure and volume generated.
The current study demonstrated that the Laerdal resuscitation bag did not generate PEF that theoretically would clear pulmonary secretions when PEEP levels of 10 and 15 cm H2O were used. Clinical studies are necessary to define if the MHI technique is effective in more distal airway generations when a PEEP valve is used and also to confirm these laboratory findings in clinical settings, especially considering the heterogeneity of lung disease in critically ill patients.
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Footnotes |
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