ORIGINAL ARTICLE | https://doi.org/10.5005/jp-journals-10030-1252 |
Minimal Leak Test vs Manometry for Endotracheal Cuff Pressure Monitoring: A Pilot Study
1–3,6Department of Surgery, Tulane University, New Orleans, Louisiana, USA
4Department of Pharmacology, Xavier University, New Orleans, Louisiana, USA
5Department of Cardiopulmonary Sciences, Louisiana State University Health Sciences Center, New Orleans, Louisiana, USA
7–10Department of Surgery, Louisiana State University Health Sciences Center, New Orleans, Louisiana, USA
Corresponding Author: Alison A Smith, Department of Surgery, Tulane University, New Orleans, Louisiana, USA, Phone: +1 5049882306, e-mail: alison.annette.smith@gmail.com
How to cite this article Schroll R, Smith AA, Farrulla ML, et al. Minimal Leak Test vs Manometry for Endotracheal Cuff Pressure Monitoring: A Pilot Study. Panam J Trauma Crit Care Emerg Surg 2019;8(3):139–142.
Source of support: Nil
Conflict of interest: None
ABSTRACT
Introduction: Optimal cuff pressure for intubated patients is 20–30 cm H2O as routinely measured by manometry. This methodology is associated with elevated costs due to equipment requirements. The objective of this study was to evaluate another methodology, i.e., the minimal leak testing (MLT).
Materials and methods: Initial cuff pressures were measured by manometry for all mechanically ventilated patients in a surgical intensive care unit (ICU). Two critical care physicians separately performed an MLT for each subject and cuff pressure was then remeasured by manometry. The rate of ventilator-associated pneumonia (VAP) was determined.
Results: Thirty subjects with 100 patient events were evaluated. The post-MLT measured cuff pressures were highly consistent between physicians, with a Pearson correlation coefficient of 0.770 (p = 0.01). Average initial cuff pressures were not significantly different between manometry and MLT (25 cm H2O vs 14 cm H2O, p = 0.1894). Manometry had a higher incidence of elevated cuff pressures (n = 13/50 vs 2/100, p %3C; 0.0001), while MLT had higher incidences low cuff pressures (n = 72/100 vs 17/50, p < 0.0001). No difference was observed in the VAP rate (2.8 vs 3.0 per 1,000 ventilator days, p = 0.96).
Conclusion: Minimal leak testing is a known method of cuff pressure monitoring that was demonstrated in this study to provide a reproducible technique.
Keywords: Critical care, Intubation, Monitoring.
ABSTRACTO
Introducción: la presión óptima del manguito para pacientes intubados es de 20–30 cm H2O, medida de forma rutinaria por manometría. Esta metodología está asociada con costos elevados debido a los requisitos del equipo. El objetivo de este estudio fue evaluar otra metodología, es decir, la prueba de fugas mínimas (MLT).
Materiales y Métodos: las presiones iniciales del manguito se midieron por manometría para todos los pacientes con ventilación mecánica en una unidad de cuidados intensivos quirúrgicos (UCI). Dos médicos de cuidados críticos realizaron por separado un MLT para cada sujeto y la presión del manguito se volvió a medir por manometría. Se determinó la tasa de neumonía asociada al ventilador (VAP).
Resultados: Treinta sujetos con 100 eventos de pacientes fueron evaluados. Las presiones del manguito medidas después de MLT fueron altamente consistentes entre los médicos, con un coeficiente de correlación de Pearson de 0.770 (p = 0.01). Las presiones promedio iniciales del manguito no fueron significativamente diferentes entre la manometría y el MLT (25 cm H2O frente a 14 cm H2O, p = 0.1894). La manometría tuvo una mayor incidencia de presiones elevadas del manguito (n = 13/50 frente a 2/100, p <0.0001), mientras que MLT tuvo una mayor incidencia de presiones bajas del manguito (n = 72/100 frente a 17/50, p <0.0001). No se observó diferencia en la tasa de VAP (2.8 vs 3.0 por 1,000 días de ventilador, p = 0.96).
Conclusión: La prueba de fuga mínima es un método conocido de monitoreo de la presión del manguito que se demostró en este estudio para proporcionar una técnica reproducible.
Keywords: Critical care, Intubation, Monitoring.
Introduction
Intubation of the trachea with endotracheal or nasotracheal tubes (ETTs or NTTs) or tracheostomy tubes is a commonly used medical intervention in the United States, with over 20 million intubations performed annually.1 To ensure proper functioning during mechanical ventilation, the balloon cuff of the breathing tube must be inflated to occlude the patient’s airway. The inflated cuff maintains a seal permitting positive pressure ventilation with low risk of aspirating upper airway secretions that could cause pneumonia.2–4 However, there is a narrow acceptable range of cuff inflation pressures between 20 and 30 cm H2O that lead to an acceptable seal with minimal side effects.3–15
Several methods to monitor cuff pressure are routinely utilized. Three of the most common are manometry, MLT, and minimal occlusive volume (MOV). Manometry involves direct measurement of the pressure in the ETT cuff via the pilot balloon, while the MLT involves removing air from the cuff until an air leak is heard. The MOV testing involves reinflating the cuff to a small amount to provide the minimum volume necessary to occlude the airway.16,17
Despite the widespread usage of these methods in ICUs, there is a paucity of data comparing these two methods.6,16,18,19 Therefore, the objective of this pilot study was to compare the efficacy and complication rates of these two methods in ventilated ICU patients. The primary outcome of this study was to evaluate the differences in cuff pressure measurement between these two methods. Secondary end points measured included the rate of VAP and aspiration.
Materials and Methods
A prospective study of all positive pressure mechanically ventilated patients over a 2-month period from April to May 2013 in a 20-bed surgical ICU was conducted after obtaining appropriate institutional review board approval. Patients requiring positive end-expiratory pressure (PEEP) %3E;16 were excluded to prevent provoking desaturation and hypoxemia from loss of PEEP during MLT. Flowchart 1 shows the study design.
Data collected for each patient included ventilator mode, amount of PEEP, pressure support, peak inspiratory pressure (PIP), mean airway pressure (MAP), fraction of inspired oxygen (FiO2), respiratory rate (RR), and tidal volume (TV). Breathing tube type and size were also recorded.
Cuff pressure measurements were obtained with a manometer (8199 Cufflator Endotracheal Tube Inflator and Manometer; Posey Products, LLC) per the standard protocol. Minimal leak test was performed separately for each of the subjects by two different critical care physicians by following a standardized protocol. For the purpose of this study, the MLT technique performed included measuring MOV. First, secretions were suctioned from the patient’s airway and oropharynx above the cuff to minimize aspiration risk. The cuff was slowly deflated using a 10-cc syringe until an air leak was audible at PIP, then the cuff was reinflated slowly with small amounts of air until the air leak was no longer audible. Manometry was then used to measure the corresponding cuff pressure at the end of each physician’s MLT. Cuff pressures were adjusted twice a day by respiratory therapy staff after initial evaluation and measurement by the physicians per the standard protocol.
Flowchart 1: Study design of cuff pressure measurement MLT, minimal leak test; VAP, ventilator associated pneumonia
The cuff pressures corresponding to each MLT measured by the two physicians were compared by calculating a Pearson’s correlation coefficient (SPSS, version 23). Study groups were compared using a Student’s unpaired two-tailed t test for continuous variables and a Chi-square test for categorical variables (GraphPad Prism, version 5.0). The VAP rate per 1,000 ventilator days was calculated for the study population and compared to the rate for the same ICU population over the 2 months prior to the study using a Chi-square test. Ventilator-associated pneumonia was defined using standard CDC criteria.20 A p value <0.05 was considered to be statistically significant.
RESULTS
Demographics
A total of 30 subjects met the inclusion criteria. A total of 50 events were identified, as some patients met the study inclusion criteria on multiple days. Average age of the patients was 53.4 years (range, 15–96 years). Males represented 63% (n = 19/30) of the study population. Average body mass index was 29.2 kg/m2 (range, 18.4–78.8 kg/m2). Average initial cuff pressures for all study patients were 24.9 ± 3.3 cm H2O. Endotracheal tubes were used in 78% of patients (n = 39/50), 20% via tracheostomy (n = 10/50) and 2% with an NTT (n = 1/50). The most common breathing tube sizes were 8 (40%, n = 20/50) and 7.5 (38%, n = 19/50). Table 1 shows the demographic data for the study population.
Ventilator Settings
Different types of ventilator modes included in this study are continuous mandatory ventilation (n = 32/50, 64%), pressure support ventilation (n = 8/50, 16%), pressure-controlled continuous mandatory ventilation, synchronized intermittent mechanical ventilation (n = 3/50, 6%), airway pressure release ventilation (n = 1/50, 2%), and proportional assist ventilation (n = 1/50, 2%). Ventilator settings were recorded at the onset of the study and did not change significantly during the course of the investigation. The average parameters for the ventilator settings are presented in Table 2.
| Demographics (n = 30) | |
| Avg age, year (range) | 53.4 (15–96) |
| Male gender, n (%) | 19 (63) |
| Avg BMI kg/m2 (range) | 29.2 (18.4–78.8) |
| ETTs* (n = 39) | |
| Tube distance measured at lip, mean cm (SEM) | 24 (0.2) |
| Initial cuff pressures, cm H2O, mean (SEM) | 24.9 (3.3) |
| ETT size, n (%) | |
| Size 8 | 15 (38) |
| Size 7.5 | 17 (44) |
| Size 7 | 7 (18) |
* One patient had a nasotracheal tube and 10 patients had tracheostomy tubes
BMI, body mass index; SEM, standard error of the mean.
| Initial ventilator settings, mean (range) (n = 100 events) | |
| Fraction of inspired oxygen, % | 45 (28–70) |
| Respiratory rate, breaths/minute | 21 (14–42) |
| Positive end-expiratory pressure, cm H2O | 6 (0–16) |
| Tidal volume, mL | 540 (300–850) |
| Mean airway pressure, cm H2O | 12.5 (7–25) |
| Peak inspiratory pressure, cm H2O | 25.6 (10–40) |
| Ventilator mode, n (%) (n = 100 events) | |
| Volume control | 32 (64) |
| Pressure support | 8 (16) |
| Pressure control continuous mandatory ventilation | 5 (10) |
| Synchronized intermittent mandatory ventilation | 3 (6) |
| Airway pressure release ventilation | 1 (2) |
| Proportional assist ventilation | 1 (2) |
Fig. 1: Correlation of measurement of minimal leak test cuff pressures between physician evaluators. MLT, minimal leak test
Correlation between Physician Measurements
Post-MLT cuff pressures measured by manometry were compared between the two physician evaluators. Correlation between evaluators was measured using the Pearson correlation coefficient, which was measured to be 0.770 (p = 0.01). This result demonstrates that the post-MLT manometry measurements were consistent between the two physicians (Fig. 1).
Cuff Pressures Measurements
Initial measured cuff pressures ranged from 0 to 56 cm H2O, with an average of 24.9 ± 3.3 cm H2O for all patients. A total of 100 MLT events were recorded as there were 50 subject events with two measured MLT pressures (one per physician). No difference was found between the average cuff pressures when comparing conventional manometry vs MLT (25 cm H2O, range 0–56 cm H2O vs 14 cm H2O, range 0–32 cm H2O, p = 0.1894). The MLT group had a higher incidence of aberrant cuff pressures with 74% of measurements not falling within the acceptable range of 20–30 cm H2O. The manometry group had an increased amount of high measured cuff pressures (n = 13/50, 26% vs 2/100, 2%, p %3C; 0.0001). The incidence of initial cuff pressure %3E;30 cm H2O was not significantly different between the first and second months of the study (19% vs 31%, p = 0.38). However, of the subjects with unacceptably high cuff pressures, the mean was significantly higher during the first month of our study compared to the second (47.5 ± 7.7 vs 38.9 ± 5.3 cm H2O, p = 0.038) as was the mean elevation above the normal range (17.5 ± 7.7 vs 8.9 ± 5.3 cm H2O, p = 0.038). The MLT group had a higher incidence of low cuff pressures compared to manometry measurements (n = 72/100, 72% vs 17/50, 34%, p < 0.0001). Of the MLT patients with cuff pressure <20 cm H2O, 18/72 (25%) had a post-MLT cuff pressure of 0 cm H2O. This signified that no air leak was identified in spite of the complete cuff deflation.
| Outcome | Manometry (n = 50) | Minimal leak test (n = 100) | p value |
|---|---|---|---|
| Cuff pressure, mean cm H2O (range) | 25 (0–56) | 14 (0–32) | 0.19 |
| Cuff pressure, <20 cm H2O (range) | 17 (34) | 72 (72) | <0.001 |
| Cuff pressure, >30 cm H2O (range) | 13 (26) | 2 (2) | <0.001 |
| Ventilator associated pneumonia per 1,000 ventilator days, n | 2.8 | 3.0 | 0.96 |
Incidence of VAP
The incidence of VAP was measured during the study period and compared to a control population. No difference in VAP rate using MLT compared to manometry was found during this study period (2.8 vs 3.0 VAP per 1,000 ventilator days, p = 0.96). Results are shown in Table 3.
DISCUSSION
Minimal leak test has been previously proposed as a safe and effective way to monitor the cuff pressure of breathing tubes. However, few studies compare it directly to manometry.6,16,18,19 Given the high incidence of ICU patients who require mechanical ventilation, it is imperative to define the optimal methods to monitor cuff pressures. This study demonstrated that MLT was a reproducible indicator of endotracheal cuff pressures with good interobserver reliability between the two physician evaluators conducting the test.
A recent study by Harvie and colleagues found similar results with high interoperator reliability between nurses and physician investigators.18 Thus, it can likely be concluded that MLT has high reproducibility for maintaining cuff pressures. Overall, the average cuff pressures were found to be similar by manometry and MLT measurements. However, one of the most important observations from this study was a significant discrepancy in cuff pressures not falling within the acceptable range of 20–30 cm H2O. Pressures below 20 cm H2O provide an inadequate barrier against secretions and thus increase the risk of aspiration and pneumonia,3–7 while pressures above 30 cm H2O have been found to impede tracheal capillary blood flow and increase the risk of ischemia and necrosis.8–13 Pressures >50 cm H2O completely occluded capillary blood flow.10–15 This study demonstrated that while MLT provided a higher incidence of cuff pressures below 20 cm H2O, while manometry showed an increase in cuff pressures above 30 cm H2O. Interestingly, a similar evaluation by Harvie and colleagues found that MLT resulted in unacceptably high cuff pressures more frequently than low cuff pressures, with 14 (31%) of 45 subjects having cuff pressures >30 cm H2O and 11 (24%) of 45 patients having cuff pressures <20 cm H2O.18 Future studies from a larger sample size are needed to evaluate the ramifications of this discrepancy between measurement techniques.
No significant difference in the rate of VAP was found between manometry and MLT despite the discrepancies in cuff pressures, with MLT producing lower values. A study by Rello et al. previously demonstrated that ETT cuff pressures lower than 20 cm H2O are an independent risk factor for VAP development in a subset of mechanically ventilated patients.8 However, this study demonstrated no significant increase in pneumonia events over the course of the study.
This study has several limitations that need to be discussed. First, it represents a pilot study at a single institution with a small sample size. The number of VAP events was low, given the small sample size. It would be important to determine whether similar results are observed with a larger study population. In addition, ventilator modes varied among patients, which could have influenced the cuff pressure and the volume necessary to seal a leak after cuff deflation.
In conclusion, this study showed that MLT is a reliable indicator of cuff pressure shown to consistently prevent unacceptably high cuff pressures for surgical ICU patients. However, it could result in pressures that are lower than the accepted safe range. The incidence of VAP did not significantly increased with additional MLT during our pilot study, suggesting MLT is a safe adjunct for cuff pressure monitoring. Further prospective studies are warranted to continue to evaluate MLT in this patient population and should focus on the influence of different types of ventilator modes on the cuff pressures.
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