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Quantitative assessment of pulmonary aspiration: A novel porcine model
International Journal of Pediatric Otorhinolaryngology 77 (2013) 2014–2018
Contents lists available at ScienceDirect
International Journal of Pediatric Otorhinolaryngology journal homepage: www.elsevier.com/locate/ijporl
Quantitative assessment of pulmonary aspiration: A novel porcine model§ Sohit P. Kanotra a,*, Evan J. Propst a,b, Paolo Campisi a,b, Joseph A. Fisher c, Vito Forte a,b a
Department of Otolaryngology – Head and Neck Surgery, The Hospital for Sick Children, Toronto, Canada Department of Otolaryngology – Head and Neck Surgery, University of Toronto, Toronto, Canada c Department of Anaesthesiology, University Health Network, University of Toronto, Toronto, Canada b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 2 June 2013 Received in revised form 23 September 2013 Accepted 24 September 2013 Available online 3 October 2013
Objectives: Pulmonary aspiration is a common cause of ventilator-associated pneumonia in the intensive care setting. Current bench and animal models of aspiration are based on the qualitative assessments. The purpose of the present study was to develop a porcine model for the real-time quantitative assessment of aspiration. Methods: Five sus scrofa piglets were anaesthetized and underwent placement of a pH probe through the endotracheal tube so that the distal tip of the probe resided at the carina. The pH probe was sutured to the posterior tracheal wall via an open approach and the position of the probe tip was verified by flexible endoscopy. 10 mL of acidic solution (pH = 2.7) was delivered through a catheter attached to the outside of the endotracheal tube so that the solution remained between the endotracheal tube and trachea proximal to the inflated endotracheal tube cuff. The pH probe was connected to a pH metre, a multifunctional data acquisition device with an analogue output signal measuring the voltage generated, and a computer for analysis. Leakage of fluid past the endotracheal tube cuff (aspiration) was therefore continuously assessed quantitatively by detecting voltage changes over a period of time. Results: The mean voltage of the tracheal mucosa at the beginning of the experiment (maximum voltage) was 916.6 mV 24.5 mV (range 891.0–945.7 mV). There was a slight drop in voltage at the end of the 2 h period to 840.8 22.6 mV (range = 812.3–867.3 mV). After deflation of the endotracheal tube cuff, the mean voltage dropped to 497.3 mV 24.8 mV (range 435.7–567.1 mV) with a mean drop in voltage of 419.3 mV 32.6 mV (range 368.9–455.3 mV). Conclusions: This porcine model allows for the continuous quantitative assessment of aspiration over time. Such a model may be of value for the evaluation of techniques for reducing aspiration. Crown Copyright ß 2013 Published by Elsevier Ireland Ltd. All rights reserved.
Keywords: Pulmonary Aspiration Animal model pH Voltage Endotracheal intubation
1. Introduction Aspiration is the misdirection of oropharyngeal secretions or gastric contents past the larynx into the trachea or lower respiratory tract. Ventilator associated pneumonia (VAP) in the intensive care setting occurs when the oropharynx becomes colonized with aerobic Gram negative bacteria after illness, antibiotic treatment, or hospital admission, with the resulting contaminated oropharyngeal secretions pooling above and then leaking around the cuff of an endotracheal (ETT) or tracheotomy tube [1,2]. The incidence of (VAP) in the intensive care setting ranges from 9% to 27% with a mortality rate as high as 9% [3,4].
§ Presented as a poster at the American Academy of Otolaryngology Annual Meeting, Washington, DC, USA, September 9–12, 2012. * Corresponding author at: Department of Otolaryngology – Head and Neck Surgery, 6th Floor Burton Wing, The Hospital for Sick Children, 555 University Avenue, Toronto, Canada M5G 1X8. Tel.: +1 416 813 4938; fax: +1 416 813 5036. E-mail addresses: [email protected], [email protected] (S.P. Kanotra), [email protected] (V. Forte).
Proposed techniques for preventing aspiration and VAP include nursing in a semi-recumbent position, changing ventilator settings, preventing subglottic pooling of secretions and modifying endotracheal tube characteristics [4–8]. Unfortunately, currently available models for studying the efficacy of these interventions are limited. Available bench models provide a quantitative assessment of aspiration but do not account for tracheal compliance and its interaction with an ETT cuff [9–11]. Available animal models that look for methylene blue dye in the distal airway or inflammatory markers following bronchioalveolar lavage only provide indirect evidence of aspiration [12–16]. The purpose of the present study was to create an animal model that would allow for a real-time continuous quantitative assessment of aspiration.
2. Methods This study was approved by the Research Ethics Board and the Animal Care Committee at the Hospital for Sick Children, Toronto, Canada.
0165-5876/$ – see front matter . Crown Copyright ß 2013 Published by Elsevier Ireland Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijporl.2013.09.025
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S.P. Kanotra et al. / International Journal of Pediatric Otorhinolaryngology 77 (2013) 2014–2018
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Fig. 1. Schematic representation of the animal model of aspiration. (See text for details.)
Table 1 Nernst equation.
2.1. Animal preparation A schematic of the animal model is provided in Fig. 1. Five Sus Scrofa piglets (mean weight = 18.6 kg 1.5 kg, range 16.4–20.3 kg) were premedicated with a mixture (0.15 mL/kg) of Ketamine (58.82 mg/mL), Acepromazine (1.18 mg/mL) and Atropine (0.009 mg/mL). Animals were intubated with a cuffed ETT (size 6.5 Sheridan1, Hudson RCI, USA) modified by attaching a catheter to the outside of the tube with the tip of the catheter residing proximal to the ETT cuff (Fig. 2). The ETT cuff was inflated and maintained at a constant pressure of 25 cm H2O using a manometer. The ETT was secured at the pig’s snout and connected to an Air Shields Ventimeter Volume Cycled Ventilator (Narco Health Company, Hatboro, PA). The right auricular vein was cannulated for administration of fluid and medication. 2.2. Insertion of pH probe and setup of recording device The power of hydrogen (pH) scale measures the acidity or alkalinity of an aqueous solution on a scale of 0–14, with 0 being the most acidic, 14 being the most basic and 7 being neutral. pH can be determined quantitatively using potentiometric sensing
[(Fig._2)TD$IG]
Fig. 2. Modified endotracheal tube with catheter (solid arrow) attached outside the lumen for delivery of acidic solution. Note that the tip of the catheter (dashed black arrow) located proximal to the cuff of the tube.
þ EðTÞ ¼ E ðTÞ þ 2:3 RT nF log ½H E(T) = voltage difference between sensing electrode and reference electrode (V) at temperature T (K) E8(T) = constant, voltage difference in a solution with pH = 7, at temperature T (K) R = gas constant (8.314 J/K mol) T = temperature in Kelvin (K) n = number of valence electrons per mole (1 for H+) F = Faraday’s constant (96,500 J/V mol e) The equation can be simplified as E(T) = E8(T) 0.1984T pH
and reference electrodes. The voltage difference between the two electrodes is used to determine the pH of the unknown solution using the Nernst equation (Table 1). Assuming that the temperature remains constant, changes in cell voltage are proportional to changes in the pH of the solution with a slope of 59.16 mV/pH. In the present study, a pH probe (Synectics Medical1, UK) was passed through the endotracheal tube to the carina. The trachea was divided vertically in the midline via an external surgical approach to confirm that pH probe electrode was exposed and
[(Fig._3)TD$IG]
Fig. 3. Endoscopic view of the position of the pH probe in trachea.
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S.P. Kanotra et al. / International Journal of Pediatric Otorhinolaryngology 77 (2013) 2014–2018
distal to the tip of the endotracheal tube and to suture the probe to the posterior tracheal wall with 4–0 silk sutures. The anterior tracheotomy was closed with a 4–0 silk and an airtight closure was tested by putting normal saline on the surgical field and increasing the airway pressure to detect for any air leakage. The position of the electrode was again verified by flexible bronchoscopy (Fig. 3). The pH probe was attached to a pH metre (Digitrapper MKIII1 Gold, Synectics Medical, UK) and the probe and pH metre were calibrated using standard buffer solutions of pH = 1 and pH = 7. The pH metre was connected to a multifunctional data acquisition
[(Fig._4)TD$IG]
device (DAQ, NI USB 6009, National Instruments, USA) that converts analogue waveforms to digital values for processing. Digital signals were processed using NI LabVIEWSignalExpress (National Instruments, USA) which measures the electrode potential in millivolts (mV) between two pH probe electrodes over time (Fig. 4a). Aspiration of acidic solution past the endotracheal tube cuff into the trachea is recorded as a drop in voltage (Fig. 4b). Voltage characteristics (mean, standard deviation, maximum, minimum) were recorded over time and an event log was created to enable the analysis for changes in voltage.
Fig. 4. (a) Continuous recording of voltage as seen in NI LabVIEWSignalExpress. Voltage (millivolts) on the Y axis and time (milliseconds) on the X axis. (b) Drop in voltage when the endotracheal tube cuff is deflated at end of experiment signifying aspiration.
S.P. Kanotra et al. / International Journal of Pediatric Otorhinolaryngology 77 (2013) 2014–2018
2.3. Introduction of acidic solution and evaluation 10 mL of white vinegar (pH = 2.7) were injected through the catheter attached to the ETT to fill the space between the endotracheal tube and the trachea proximal to the endotracheal tube cuff. Animals were placed in reverse Trendelenburg (208) and the voltage of the trachea distal to the endotracheal tube cuff was recorded for 2 h. The initial pH of the trachea is recorded as the maximum pH. Aspiration of acidic solution past the endotracheal tube cuff into the trachea is recorded as a drop in voltage. Because the initial voltage and pH of the trachea varied across animals, it is more accurate to compare drops in pH rather than absolute values. After 2 h of recording voltages with the ETT cuff inflated, the cuff was deflated completely to simulate significant aspiration in order to validate the model. 3. Results All 5 pigs survived the study. The mean voltage of tracheal mucosa at the beginning of the experiment (maximum voltage) was 916.6 mV 24.5 mV (range 891.0–945.7 mV); pH = 7.6 0.1 (range 7.5–7.8). After addition of acidic solution above the ETT cuff (acetic acid, pH = 2.7), the voltage remained fairly constant with only a small decrease in mean voltage at the end of the 2 h observation period to 840.8 22.6 mV (range = 812.3–867.3 mV); pH = 7.02 0.16 (range = 6.8–7.2). After deflation of the ETT cuff, the mean voltage dropped by 419.3 mV 32.6 mV (range 368.9– 455.3 mV) to a mean final voltage of 497.3 mV 24.8 mV (range 435.7–567.1 mV). 4. Discussion In this study we configure and validate an animal model for quantitative real time testing of liquid aspiration past an endotracheal tube cuff during mechanical ventilation. The benefits of using live anaesthetized animals rather than a bench model are that (1) the complex dynamics of the human trachea can be better simulated in live animals; (2) increased tracheal compliance allows for a tighter ETT cuff seal; (3) horizontally oriented ETTs more accurately represent a recumbent patient; (4) ventilator settings can be accounted for; (5) the effects of positive end expiratory pressure (PEEP) on intratracheal pressure, diameter and leakage around the ETT cuff can be evaluated [11]. We evaluated aspiration directly using a pH monitor rather than indirectly using secondary measures of aspiration. Berra investigated tracheobronchial bacterial colonization in animals as evidence of aspiration of infected oropharyngeal secretions [12]. Unfortunately, animal airways can be heavily colonized by pathogenic and non-pathogenic bacteria, making this model inaccurate [12]. Petring and later Mercer filled the subglottis of anaesthetized animals with methylene blue dye and evaluated the trachea for evidence of leakage around the ETT [16,17]. Unfortunately, this provided only a qualitative rather than quantitative assessment of aspiration. Several other researchers have looked for secondary markers of aspiration such as interleukin-6, lipid-laden macrophages and milk-laden macrophages, but unfortunately, interleukin-6 can be falsely elevated due to pre-existing inflammation, detection of lipid-laden alveolar macrophages is nonspecific and can be elevated in pneumonia [13], and detection of milk laden macrophages via immunostaining with anti-human a lactalbumin antibody is expensive and only reliable during the first four days following an aspiration event [14]. The mean voltage of the tracheal mucosa at the beginning of the experiment (maximum voltage) was 916.6 mV 24.5 mV (range 891.0–945.7 mV) corresponding to a mean pH of 7.6 0.1 (range 7.5– 7.8). Comparable values have been reported for respiratory mucosa in
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rabbits (pH = 6.7) and rats (pH = 7.4–7.6) [18]. In humans, the pH of tracheal mucus ranges from 6.1 to 7.9, depending on the measurement technique employed [18]. This small variability in pH may be due in part to technical differences between studies. After the addition of acidic solution (pH = 2.7) above the ETT cuff, the mean pH remained fairly constant with only a small drop at the end of the 2 h observation period. This change in the mucosal pH may represent micro aspiration due to inadequate sealing of high volume, low pressure cuffs which has been demonstrated in various studies [17,19,20]. ETTs with high volume cuffs have cuff diameters ranging from 1.5 to 2 times the tracheal diameter, causing them to develop folds that eventually lead to microaspiration [19]. Seegobin placed dye above large volume ETT cuffs and using bronchoscopy, found evidence of dye tracking past the cuff in 100% of cases [19]. A bench model assessing aspiration past various high volume low pressure ETTs found that leakage (>20 mL water in 5 min) occurred even at the highest intracuff pressure (50 cm of H2O) in the majority of ETTs. Our model is consistent with this previous evidence for aspiration across ETT cuffs in a continuous and quantitative fashion. To establish a positive control, we deflated the ETT cuff at the end of our experiment which led to a mean voltage drop of 419.3 mV 32.6 mV (range 368.9–455.3 mV) to a mean voltage of 497.3 mV 24.8 mV (range 435.7–567.1 mV). This confirmed that the probe was capable of detecting and registering an increase in acidity as the acidic solution entered the trachea. The software recorded the drop in voltage accurately. It was difficult to assess the drop in pH because the pH begins to rise immediately after the acidic solution passes the electrodes. Evaluating for changes in voltage appears to be a more sensitive method of detecting microaspiration because small changes in pH convert to large changes in voltage making it easier to detect. A limitation of this study is that the experiment lasted for only 2 h. We arbitrarily selected 2 h for the study, which was tolerated well by the animals. This suggests that the experiment could be run for longer, which could have shown a greater degree of microaspiration prior to deflation of the ETT cuff. Nevertheless, having a dynamic experimental model of aspiration that is possible to run for long periods of time will likely prove useful in future studies aimed at establishing the safety of interventions that could prevent aspiration. 5. Conclusion This porcine model allows for the continuous quantitative assessment of aspiration over time. Such a model could allow the evaluation of techniques for reducing aspiration in the future.
Funding No external funding was received in association with this study, and no financial interests are held by any of the authors. References [1] P.E. Marik, Aspiration pneumonitis pneumonia: a clinical review, N. Engl. J. Med. 344 (2001) 665–672. [2] J.D. Hunter, Ventilator associated pneumonia, BMJ 344 (May) (2012) e3325. [3] J. Rello, D.A. Ollendorf, G. Oster, M. Vera-Llonch, L. Bellm, R. Redman, et al., Epidemiology and outcomes of ventilator-associated pneumonia in a large US database, Chest 122 (2002) 2115–2121. [4] W.G. Melsen, M.M. Rovers, M. Koeman, M.J. Bonten, Estimating the attributable mortality of ventilator-associated pneumonia from randomized prevention studies, Crit. Care Med. 39 (2011) 1–7. [5] M.B. Drakulovic, A. Torres, T.T. Bauer, J.M. Nicolas, S. Nogue, S.M. Ferrer, Supine body position as a risk factor for nosocomial pneumonia in mechanically ventilated patients: a randomised trial, Lancet 354 (1999) 1851–1858.
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[6] C. Dezfulian, K. Shojania, H.R. Collard, H.M. Kim, M.A. Matthay, S. Saint, Subglottic secretion drainage for preventing ventilator-associated pneumonia: a meta-analysis, Am. J. Med. 118 (2005) 11–18. [7] A. Dullenkopf, A. Gerber, M. Weiss, Fluid leakage past tracheal tube cuffs: evaluation of the new Microcuff endotracheal tube, Intensive Care Med. 29 (2003) 1849–1855. [8] H.M. Coffman, C.J. Rees, A.E. Sievers, P.C. Belafsky, Proximal suction tracheotomy tube reduces aspiration volume, Otolaryngol. Head Neck Surg. 138 (2008) 441–445. [9] J. Hodd, A. Doyle, J. Carter, J. Albarran, P. Young, Increasing positive end expiratory pressure at extubation reduces subglottic secretion aspiration in a bench-top model, Nurs. Crit. Care 15 (2010) 257–261. [10] R. Pitts, D. Fisher, D. Sulemanji, J. Kratohvil, Y. Jiang, R. Kacmarek, Variables affecting leakage past endotracheal tube cuffs: a bench study, Intensive Care Med. 36 (December) (2010) 2066–2073. [11] I. Ouanes, A. Lyazidi, P.E. Danin, N. Rana, A. Di Bari, F. Abroug, et al., Mechanical influences on fluid leakage past the tracheal tube cuff in a benchtop model, Intensive Care Med. 37 (2011) 695–700. [12] L. Berra, L. De Marchi, M. Panigada, Z.X. Yu, A. Baccarelli, T. Kolobow, Evaluation of continuous aspiration of subglottic secretion in an in vivo study, Crit. Care Med. 32 (2004) 2071–2078.
[13] R.W. Corwin, R.S. Irwin, The lipid-laden alveolar macrophage as a marker of aspiration in parenchymal lung disease, Am. Rev. Respir. Dis. 132 (1985) 576–581. [14] O. Elidemir, L.L. Fan, G.N. Colasurdo, A novel diagnostic method for pulmonary aspiration in a murine model. Immunocytochemical staining of milk proteins in alveolar macrophages, Am. J. Respir. Crit. Care Med. 161 (2000) 622–626. [15] C. Springer, A. Avital, Simple and specific test for the diagnosis of aspiration into the airways using a corn flourmilk mixture in a hamster model, Pediatr. Pulmonol. 35 (2003) 400–404. [16] Mercer, H. Michael, An assessment of protection of the airway from aspiration of oropharyngeal contents using the Combitube airway, Resuscitation 51 (2001) 135–138. [17] O.U. Petring, B. Adelhøj, B.N. Jensen, N.O. Pedersen, N. Lomholt, Prevention of silent aspiration due to leaks around cuffs of endotracheal tubes, Anesth. Analg. 65 (1986) 777–780. [18] H. Fischer, J.H. Widdicombe, Mechanisms of acid and base secretion by the airway epithelium, J. Membr. Biol. 211 (2006) 139–150. [19] R.D. Seegobin, G.L. van Hasselt, Aspiration beyond endotracheal cuffs, Can. J. Anesth./J. can. d’anesth. 33 (1986) 273–279. [20] P.J. Young, M. Rollinson, G. Downward, S. Henderson, Leakage of fluid past the tracheal tube cuff in a benchtop model, Br. J. Anaesth. 78 (1997) 557–562.
Contents lists available at ScienceDirect
International Journal of Pediatric Otorhinolaryngology journal homepage: www.elsevier.com/locate/ijporl
Quantitative assessment of pulmonary aspiration: A novel porcine model§ Sohit P. Kanotra a,*, Evan J. Propst a,b, Paolo Campisi a,b, Joseph A. Fisher c, Vito Forte a,b a
Department of Otolaryngology – Head and Neck Surgery, The Hospital for Sick Children, Toronto, Canada Department of Otolaryngology – Head and Neck Surgery, University of Toronto, Toronto, Canada c Department of Anaesthesiology, University Health Network, University of Toronto, Toronto, Canada b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 2 June 2013 Received in revised form 23 September 2013 Accepted 24 September 2013 Available online 3 October 2013
Objectives: Pulmonary aspiration is a common cause of ventilator-associated pneumonia in the intensive care setting. Current bench and animal models of aspiration are based on the qualitative assessments. The purpose of the present study was to develop a porcine model for the real-time quantitative assessment of aspiration. Methods: Five sus scrofa piglets were anaesthetized and underwent placement of a pH probe through the endotracheal tube so that the distal tip of the probe resided at the carina. The pH probe was sutured to the posterior tracheal wall via an open approach and the position of the probe tip was verified by flexible endoscopy. 10 mL of acidic solution (pH = 2.7) was delivered through a catheter attached to the outside of the endotracheal tube so that the solution remained between the endotracheal tube and trachea proximal to the inflated endotracheal tube cuff. The pH probe was connected to a pH metre, a multifunctional data acquisition device with an analogue output signal measuring the voltage generated, and a computer for analysis. Leakage of fluid past the endotracheal tube cuff (aspiration) was therefore continuously assessed quantitatively by detecting voltage changes over a period of time. Results: The mean voltage of the tracheal mucosa at the beginning of the experiment (maximum voltage) was 916.6 mV 24.5 mV (range 891.0–945.7 mV). There was a slight drop in voltage at the end of the 2 h period to 840.8 22.6 mV (range = 812.3–867.3 mV). After deflation of the endotracheal tube cuff, the mean voltage dropped to 497.3 mV 24.8 mV (range 435.7–567.1 mV) with a mean drop in voltage of 419.3 mV 32.6 mV (range 368.9–455.3 mV). Conclusions: This porcine model allows for the continuous quantitative assessment of aspiration over time. Such a model may be of value for the evaluation of techniques for reducing aspiration. Crown Copyright ß 2013 Published by Elsevier Ireland Ltd. All rights reserved.
Keywords: Pulmonary Aspiration Animal model pH Voltage Endotracheal intubation
1. Introduction Aspiration is the misdirection of oropharyngeal secretions or gastric contents past the larynx into the trachea or lower respiratory tract. Ventilator associated pneumonia (VAP) in the intensive care setting occurs when the oropharynx becomes colonized with aerobic Gram negative bacteria after illness, antibiotic treatment, or hospital admission, with the resulting contaminated oropharyngeal secretions pooling above and then leaking around the cuff of an endotracheal (ETT) or tracheotomy tube [1,2]. The incidence of (VAP) in the intensive care setting ranges from 9% to 27% with a mortality rate as high as 9% [3,4].
§ Presented as a poster at the American Academy of Otolaryngology Annual Meeting, Washington, DC, USA, September 9–12, 2012. * Corresponding author at: Department of Otolaryngology – Head and Neck Surgery, 6th Floor Burton Wing, The Hospital for Sick Children, 555 University Avenue, Toronto, Canada M5G 1X8. Tel.: +1 416 813 4938; fax: +1 416 813 5036. E-mail addresses: [email protected], [email protected] (S.P. Kanotra), [email protected] (V. Forte).
Proposed techniques for preventing aspiration and VAP include nursing in a semi-recumbent position, changing ventilator settings, preventing subglottic pooling of secretions and modifying endotracheal tube characteristics [4–8]. Unfortunately, currently available models for studying the efficacy of these interventions are limited. Available bench models provide a quantitative assessment of aspiration but do not account for tracheal compliance and its interaction with an ETT cuff [9–11]. Available animal models that look for methylene blue dye in the distal airway or inflammatory markers following bronchioalveolar lavage only provide indirect evidence of aspiration [12–16]. The purpose of the present study was to create an animal model that would allow for a real-time continuous quantitative assessment of aspiration.
2. Methods This study was approved by the Research Ethics Board and the Animal Care Committee at the Hospital for Sick Children, Toronto, Canada.
0165-5876/$ – see front matter . Crown Copyright ß 2013 Published by Elsevier Ireland Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijporl.2013.09.025
[(Fig._1)TD$IG]
S.P. Kanotra et al. / International Journal of Pediatric Otorhinolaryngology 77 (2013) 2014–2018
2015
Fig. 1. Schematic representation of the animal model of aspiration. (See text for details.)
Table 1 Nernst equation.
2.1. Animal preparation A schematic of the animal model is provided in Fig. 1. Five Sus Scrofa piglets (mean weight = 18.6 kg 1.5 kg, range 16.4–20.3 kg) were premedicated with a mixture (0.15 mL/kg) of Ketamine (58.82 mg/mL), Acepromazine (1.18 mg/mL) and Atropine (0.009 mg/mL). Animals were intubated with a cuffed ETT (size 6.5 Sheridan1, Hudson RCI, USA) modified by attaching a catheter to the outside of the tube with the tip of the catheter residing proximal to the ETT cuff (Fig. 2). The ETT cuff was inflated and maintained at a constant pressure of 25 cm H2O using a manometer. The ETT was secured at the pig’s snout and connected to an Air Shields Ventimeter Volume Cycled Ventilator (Narco Health Company, Hatboro, PA). The right auricular vein was cannulated for administration of fluid and medication. 2.2. Insertion of pH probe and setup of recording device The power of hydrogen (pH) scale measures the acidity or alkalinity of an aqueous solution on a scale of 0–14, with 0 being the most acidic, 14 being the most basic and 7 being neutral. pH can be determined quantitatively using potentiometric sensing
[(Fig._2)TD$IG]
Fig. 2. Modified endotracheal tube with catheter (solid arrow) attached outside the lumen for delivery of acidic solution. Note that the tip of the catheter (dashed black arrow) located proximal to the cuff of the tube.
þ EðTÞ ¼ E ðTÞ þ 2:3 RT nF log ½H E(T) = voltage difference between sensing electrode and reference electrode (V) at temperature T (K) E8(T) = constant, voltage difference in a solution with pH = 7, at temperature T (K) R = gas constant (8.314 J/K mol) T = temperature in Kelvin (K) n = number of valence electrons per mole (1 for H+) F = Faraday’s constant (96,500 J/V mol e) The equation can be simplified as E(T) = E8(T) 0.1984T pH
and reference electrodes. The voltage difference between the two electrodes is used to determine the pH of the unknown solution using the Nernst equation (Table 1). Assuming that the temperature remains constant, changes in cell voltage are proportional to changes in the pH of the solution with a slope of 59.16 mV/pH. In the present study, a pH probe (Synectics Medical1, UK) was passed through the endotracheal tube to the carina. The trachea was divided vertically in the midline via an external surgical approach to confirm that pH probe electrode was exposed and
[(Fig._3)TD$IG]
Fig. 3. Endoscopic view of the position of the pH probe in trachea.
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distal to the tip of the endotracheal tube and to suture the probe to the posterior tracheal wall with 4–0 silk sutures. The anterior tracheotomy was closed with a 4–0 silk and an airtight closure was tested by putting normal saline on the surgical field and increasing the airway pressure to detect for any air leakage. The position of the electrode was again verified by flexible bronchoscopy (Fig. 3). The pH probe was attached to a pH metre (Digitrapper MKIII1 Gold, Synectics Medical, UK) and the probe and pH metre were calibrated using standard buffer solutions of pH = 1 and pH = 7. The pH metre was connected to a multifunctional data acquisition
[(Fig._4)TD$IG]
device (DAQ, NI USB 6009, National Instruments, USA) that converts analogue waveforms to digital values for processing. Digital signals were processed using NI LabVIEWSignalExpress (National Instruments, USA) which measures the electrode potential in millivolts (mV) between two pH probe electrodes over time (Fig. 4a). Aspiration of acidic solution past the endotracheal tube cuff into the trachea is recorded as a drop in voltage (Fig. 4b). Voltage characteristics (mean, standard deviation, maximum, minimum) were recorded over time and an event log was created to enable the analysis for changes in voltage.
Fig. 4. (a) Continuous recording of voltage as seen in NI LabVIEWSignalExpress. Voltage (millivolts) on the Y axis and time (milliseconds) on the X axis. (b) Drop in voltage when the endotracheal tube cuff is deflated at end of experiment signifying aspiration.
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2.3. Introduction of acidic solution and evaluation 10 mL of white vinegar (pH = 2.7) were injected through the catheter attached to the ETT to fill the space between the endotracheal tube and the trachea proximal to the endotracheal tube cuff. Animals were placed in reverse Trendelenburg (208) and the voltage of the trachea distal to the endotracheal tube cuff was recorded for 2 h. The initial pH of the trachea is recorded as the maximum pH. Aspiration of acidic solution past the endotracheal tube cuff into the trachea is recorded as a drop in voltage. Because the initial voltage and pH of the trachea varied across animals, it is more accurate to compare drops in pH rather than absolute values. After 2 h of recording voltages with the ETT cuff inflated, the cuff was deflated completely to simulate significant aspiration in order to validate the model. 3. Results All 5 pigs survived the study. The mean voltage of tracheal mucosa at the beginning of the experiment (maximum voltage) was 916.6 mV 24.5 mV (range 891.0–945.7 mV); pH = 7.6 0.1 (range 7.5–7.8). After addition of acidic solution above the ETT cuff (acetic acid, pH = 2.7), the voltage remained fairly constant with only a small decrease in mean voltage at the end of the 2 h observation period to 840.8 22.6 mV (range = 812.3–867.3 mV); pH = 7.02 0.16 (range = 6.8–7.2). After deflation of the ETT cuff, the mean voltage dropped by 419.3 mV 32.6 mV (range 368.9– 455.3 mV) to a mean final voltage of 497.3 mV 24.8 mV (range 435.7–567.1 mV). 4. Discussion In this study we configure and validate an animal model for quantitative real time testing of liquid aspiration past an endotracheal tube cuff during mechanical ventilation. The benefits of using live anaesthetized animals rather than a bench model are that (1) the complex dynamics of the human trachea can be better simulated in live animals; (2) increased tracheal compliance allows for a tighter ETT cuff seal; (3) horizontally oriented ETTs more accurately represent a recumbent patient; (4) ventilator settings can be accounted for; (5) the effects of positive end expiratory pressure (PEEP) on intratracheal pressure, diameter and leakage around the ETT cuff can be evaluated [11]. We evaluated aspiration directly using a pH monitor rather than indirectly using secondary measures of aspiration. Berra investigated tracheobronchial bacterial colonization in animals as evidence of aspiration of infected oropharyngeal secretions [12]. Unfortunately, animal airways can be heavily colonized by pathogenic and non-pathogenic bacteria, making this model inaccurate [12]. Petring and later Mercer filled the subglottis of anaesthetized animals with methylene blue dye and evaluated the trachea for evidence of leakage around the ETT [16,17]. Unfortunately, this provided only a qualitative rather than quantitative assessment of aspiration. Several other researchers have looked for secondary markers of aspiration such as interleukin-6, lipid-laden macrophages and milk-laden macrophages, but unfortunately, interleukin-6 can be falsely elevated due to pre-existing inflammation, detection of lipid-laden alveolar macrophages is nonspecific and can be elevated in pneumonia [13], and detection of milk laden macrophages via immunostaining with anti-human a lactalbumin antibody is expensive and only reliable during the first four days following an aspiration event [14]. The mean voltage of the tracheal mucosa at the beginning of the experiment (maximum voltage) was 916.6 mV 24.5 mV (range 891.0–945.7 mV) corresponding to a mean pH of 7.6 0.1 (range 7.5– 7.8). Comparable values have been reported for respiratory mucosa in
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rabbits (pH = 6.7) and rats (pH = 7.4–7.6) [18]. In humans, the pH of tracheal mucus ranges from 6.1 to 7.9, depending on the measurement technique employed [18]. This small variability in pH may be due in part to technical differences between studies. After the addition of acidic solution (pH = 2.7) above the ETT cuff, the mean pH remained fairly constant with only a small drop at the end of the 2 h observation period. This change in the mucosal pH may represent micro aspiration due to inadequate sealing of high volume, low pressure cuffs which has been demonstrated in various studies [17,19,20]. ETTs with high volume cuffs have cuff diameters ranging from 1.5 to 2 times the tracheal diameter, causing them to develop folds that eventually lead to microaspiration [19]. Seegobin placed dye above large volume ETT cuffs and using bronchoscopy, found evidence of dye tracking past the cuff in 100% of cases [19]. A bench model assessing aspiration past various high volume low pressure ETTs found that leakage (>20 mL water in 5 min) occurred even at the highest intracuff pressure (50 cm of H2O) in the majority of ETTs. Our model is consistent with this previous evidence for aspiration across ETT cuffs in a continuous and quantitative fashion. To establish a positive control, we deflated the ETT cuff at the end of our experiment which led to a mean voltage drop of 419.3 mV 32.6 mV (range 368.9–455.3 mV) to a mean voltage of 497.3 mV 24.8 mV (range 435.7–567.1 mV). This confirmed that the probe was capable of detecting and registering an increase in acidity as the acidic solution entered the trachea. The software recorded the drop in voltage accurately. It was difficult to assess the drop in pH because the pH begins to rise immediately after the acidic solution passes the electrodes. Evaluating for changes in voltage appears to be a more sensitive method of detecting microaspiration because small changes in pH convert to large changes in voltage making it easier to detect. A limitation of this study is that the experiment lasted for only 2 h. We arbitrarily selected 2 h for the study, which was tolerated well by the animals. This suggests that the experiment could be run for longer, which could have shown a greater degree of microaspiration prior to deflation of the ETT cuff. Nevertheless, having a dynamic experimental model of aspiration that is possible to run for long periods of time will likely prove useful in future studies aimed at establishing the safety of interventions that could prevent aspiration. 5. Conclusion This porcine model allows for the continuous quantitative assessment of aspiration over time. Such a model could allow the evaluation of techniques for reducing aspiration in the future.
Funding No external funding was received in association with this study, and no financial interests are held by any of the authors. References [1] P.E. Marik, Aspiration pneumonitis pneumonia: a clinical review, N. Engl. J. Med. 344 (2001) 665–672. [2] J.D. Hunter, Ventilator associated pneumonia, BMJ 344 (May) (2012) e3325. [3] J. Rello, D.A. Ollendorf, G. Oster, M. Vera-Llonch, L. Bellm, R. Redman, et al., Epidemiology and outcomes of ventilator-associated pneumonia in a large US database, Chest 122 (2002) 2115–2121. [4] W.G. Melsen, M.M. Rovers, M. Koeman, M.J. Bonten, Estimating the attributable mortality of ventilator-associated pneumonia from randomized prevention studies, Crit. Care Med. 39 (2011) 1–7. [5] M.B. Drakulovic, A. Torres, T.T. Bauer, J.M. Nicolas, S. Nogue, S.M. Ferrer, Supine body position as a risk factor for nosocomial pneumonia in mechanically ventilated patients: a randomised trial, Lancet 354 (1999) 1851–1858.
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