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Letters

Letters addressing topics of current interest or material in RESPIRATORY CARE will be considered for publication. The Editors may accept or decline a letter or edit without changing the author's views. The content of letters as published may simply reflect the author's opinion or interpretation of information--not standard practice or the Journal's recommendation. Authors of criticized material will have the opportunity to reply in print. No anonymous letters can be published. Letters should be submitted electronically via Manuscript Central. Log onto RESPIRATORY CARE's web site at http://www.RCJournal.com. Instructions on how to submit a manuscript are on the site and also printed in every issue.

Performance of Transport Ventilators

The paper by Chipman et al in the June 2007 issue of RESPIRATORY CARE represented a monumental undertaking to evaluate 15 transport ventilators.1 We reviewed the paper on several occasions to reconcile their findings with our work on transport ventilators.2-9 We write to point out several inaccuracies in the study and ask Chipman et al to more clearly explain their findings and recommendations. In their Table 1, Chipman et al list the positive end-expiratory pressure (PEEP) of the LTV1000 as 0 ­30 cm H2O. The actual range is 0 ­20 cm H2O. The table also lists the PEEP of the Crossvent 3 as 0 cm H2O, but the Crossvent 3 is capable of PEEP up to 35 cm H2O. Were Chipman et al referring to an earlier model of the Crossvent 3 that couldn't supply PEEP? Also, the Vortran is listed as being capable of providing continuous mandatory ventilation (CMV) and intermittent mandatory ventilation (IMV), but in fact the Vortran can provide only pressure-cycled CMV. Similarly, the Percussionaire TXP provides only IMV, not CMV. With the Percussionaire TXP, if the patient breathes spontaneously, air is entrained from the inspiratory port of the Phasitron. We are unclear as to why Chipman et al listed both assist-control and CMV for some ventilators, as those 2 modes are the same. Did they mean to distinguish between ventilators that can be patient-triggered and those that can deliver only mandatory breaths? If so, then errors still remain. The Pneupac ParaPac ventilators have a mode termed SMMV (synchronized minimum mandatory volume), not MMV (mandatory minute volume) or IMV, which are quite different, in that if the patient breathes spontaneously at a respiratory rate equal to or greater than the set rate, the ventilator does not provide any mandatory breaths. Also the Crossvent 3 has a selection for pressure support but does not have the ability to flow cycle. This mode results in patient-triggered, pressure-limited, timecycled support. The Crossvent 3 is listed as having both volume and pressure control, but in fact it is capable of only pressure-

limited ventilation via a mechanical pressure-relief valve. In that instance the ventilator continues to deliver the set flow and volume according to the volume control settings, but vents gas to the atmosphere if the pressure threshold is reached. These errors may have resulted from Chipman et al depending on the manufacturer's literature for their data. However, since their intention was to compare available devices, we believe these errors should be pointed out so readers can determine the actual performance of the devices, not simply mimic the manufacturer's specifications. Chipman et al stated that in their bench protocol the Impact/Uni-Vent Eagle 754 operated on an E-size cylinder for 35 min, which is substantially less than we have observed and is not consistent with that ventilator's function.2 Was this test repeated more than once? This finding suggests either a leak in the system, a malfunctioning ventilator, or unfamiliarity by the operator. Was the tidal volume (VT) continuously measured to assure that the correct minute volume was being delivered? As Chipman et al are well aware from their experience, repetition of laboratory experiments is invaluable in the detection of one-time errors. In Table 2, Chipman et al concluded that neither the Impact/Uni-Vent Eagle 754 nor the Pulmonetic LTV1000 are capable of ventilating injured lungs. As in our previous comment, that conclusion is inconsistent with the bench data and the worldwide experience with those 2 devices. For over a decade the Impact/Uni-Vent Eagle 754 has been successfully used to transport critically ill soldiers (including many with acute respiratory distress syndrome) between military care centers. Similarly, the LTV1000 is capable of volume and pressure control at up to 80 breaths/min, and is used by many centers for transport of critically ill patients. We request that Chipman et al explain the nature of the failures with the Impact/UniVent Eagle 754 and the Pulmonetic LTV1000, and reconcile their findings with the common experience. Our review of the animal-model experiments in the Chipman et al study led us to understand that a single animal was used to test 5 separate ventilators, in random sequence. Is it possible that the small number

of animals resulted in a ventilator being disadvantaged in the trial because of the sequence? As an example, Figure 2 indicates that each ventilator was used in 2 animal experiments. What was the progression of the lung injury? If the first 4 ventilators failed to provide adequate oxygenation or ventilation, would that worsen the lung injury and thus make the fifth ventilator fail to provide adequate gas exchange? Does their surfactant-wash-out model improve with time, or does gas exchange steadily worsen? Also, the performance of every ventilator is affected by the device characteristics, the condition of the animal, and the understanding of the operator. It seems incongruent that a given ventilator is capable of maintaining gas exchange in one animal, but not the other, unless the ventilator model is substantially different or the operator fails to make the appropriate changes. Finally, though Chipman et al concluded that the Impact/Uni-Vent Eagle 754 is unable to ventilate injured lungs, it is one of the ventilators they recommend in their conclusions. Though we agree, and experience clearly demonstrates that this recommendation makes sense, how did they come to this conclusion? At the end of their exhaustive study, how did Chipman et al determine which ventilators to recommend? Were characteristics ranked or weighted? Was each ventilator given a score for performance in each of the evaluations? Though we agree that both the Impact/Uni-Vent Eagle 754 and Newport HT50 meet the requirements for a "front-line ventilator for rescue situations," we are not sure why the LTV1000 and VersaMed iVent do not.10 Did Chipman et al eliminate the LTV1000 because of its high gas consumption? Was the VersaMed eliminated because of short battery life and excessive weight? We request that Chipman et al describe the system they used to come to their conclusions. If gas consumption, size, and battery life were the only factors considered, the animal experiments seem unnecessary. We appreciate the substantial effort by Chipman et al in this project and their commitment to provide the respiratory care community with much-needed data. However, we believe they should correct some errors and explain the findings that seem to run

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counter to our experience and common sense. Richard D Branson MSc RRT FAARC Division of Trauma and Critical Care Department of Surgery University of Cincinnati Medical Center Cincinnati, Ohio SMSgt Dario Rodriquez RRT Center for Sustaintment of Trauma Readiness Skills United States Air Force Cincinnati, Ohio

Mr Branson has received honoraria and/or research support from Cardinal, Drager, and Co¨ vidien. The authors report no other conflicts of interest related to the content of this letter. REFERENCES 1. Chipman DW, Caramez MP, Miyoshi E, Kratohvil JP, Kacmarek RM. Performance comparison of 15 transport ventilators. Respir Care 2007;52(6):740-751. 2. Lyons E, Johannigman J, Davis K, Tsuei B, Branson R. Portable ventilators: a comparison of duration of operation from an E-cylinder (abstract). Respir Care 2006; 51(11):1292. 3. Austin PA, Campbell RS, Johannigman JA, Branson RD. Work of breathing characteristics of seven portable ventilators. Resuscitation 2001;49(2):163-172. 4. Austin PN, Johannigman JA, Simmons LW, Camden MP, Campbell RS, Branson RD. Surface temperature of two portable ventilators during simulated use under clinical conditions. Mil Med 2001;166(10):843847. 5. Barnes SL, Branson R, Gallo LA, Beck G, Johannigman JA. En-route care in the air: snapshot of mechanical ventilation at 37,000 feet. J Trauma 2008;64(2 Suppl): S129-S135. 6. Salas N, Wisor B, Agazio J, Branson RD, Austin PA. Comparison of ventilation and cardiac compressions using the Impact 730 automatic transport ventilator compared to a conventional bag valve with a facemask in a model of adult cardiopulmonary arrest. Resuscitation 2007;74(1):94-101. 7. Stevenson VW, Haas CF, Wahl WL. Intrahospital transport of the adult mechanically ventilated patient. Respir Care Clin N Am 2002;8(1):1-35. 8. Austin PN, Campbell RS, Johannigman JA, Branson RD. Impsed work of breathing during ventilator failure. Respir Care 2002; 47(6):667-674. 9. Campbell RS, Johannigman JA, Branson RD, Austin PN, Matacia G, Banks GR. Battery duration of portable ventilators: effects

TO THE

EDITOR

With the Puritan Bennett 840 the clinician must first select a ventilation mode, and we will again use the example of assist/control, followed by the selection of a mandatory breath type, such as volume control or pressure control.4 In the neonatal arena, the Drager ¨ Babylog has a mode in which the ventilator delivers a set number of nonsynchronized, time-cycled, pressurelimited breaths, and the patient is allowed to breath spontaneously in between the breaths from a clinician-adjustable, continuous gas flow. Although we have known this ventilation mode for many years as intermittent mandatory ventilation (IMV), on the ventilator the manufacturer describes it as CMV.5 We did not evaluate any of these ventilators during spontaneous ventilation, which we clearly identified as a major limitation of our study. Consequently, we relied on the manufacturers' documentation regarding triggering and spontaneous breathing capabilities. Several years ago, Chatburn and Primiano, in an attempt to promote standardization of nomenclature, published an extensive description of ventilation modes.6 However, no consensus on the use of that nomenclature has been established. Neither the engineers who design ventilators, the marketing and sales people, nor clinicians use Chatburn and Primiano's nomenclature. In the absence of such consensus, it has been our practice to describe ventilation modes as controlled, assist/control, synchronized intermittent mandatory ventilation (SIMV), or spontaneous, and the breath-delivery types as volume-controlled, pressure-controlled and/or pressure-supported. In our paper we intentionally disregarded the manufacturers' descriptions of modes and applied the above classification system. Thus, since the Percussionaire delivers a set number of mechanical breaths to a patient making no spontaneous respiratory efforts, it was classified as

of control variable, positive end-expiratory pressure, and inspired oxygen concentration. Respir Care 2002;47(10):1173-1183. 10. American Association for Respiratory Care. Guidelines for acquisition of ventilators to meet demand for pandemic flu and mass casualty incidents. http://www.aarc.org/ headlines/ventilator_acquisitions/ vent_guidelines.pdf. Accessed July 28, 2008.

The authors respond: We thank Branson and Rodriquez for their letter.1 We agree there were mistakes in one of our tables that we missed during editing, and that there is considerable disagreement about terminology. However, we also believe that most of the issues they raise were covered in the paper. Branson and Rodriquez are correct that the positive end-expiratory pressure (PEEP) ranges for the Pulmonetic LTV1000 and Crossvent 3 are 0 ­20 cm H2O and 0 ­35 cm H2O, respectively, and we thank them for pointing this out. We do, however, take exception to what they describe as inaccuracies with respect to various ventilation modes. It is unfortunate that no standardized nomenclature exists for classifying ventilation modes. Ventilator manufacturers use various names to describe their devices' modes. Consider 2 commonly used ventilators, the Puritan Bennett 840 and the Drager ¨ Evita 4. The Evita 4 offers a continuous-mandatory-ventilation mode that allows timed or patient triggering of constant-volume breath delivery, as well as the PCV Assist mode that allows timed or patient triggering of pressure-controlled breaths.2 Both of those modes may be defined as a form of assist/control ventilation (ie, all breaths are delivered by the ventilator at a set minimum rate [mechanical breaths], but the patient may trigger the ventilator and thus cause a rate greater than the set rate).3 Contrast this to controlled mechanical ventilation, in which all breaths are delivered by the ventilator at a set rate and patient triggering is not allowed.

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a controlled mode. And because spontaneous efforts are met with unsupported air entrainment, it was also classified as having an IMV mode. We used similar logic in classifying the Vortran. It functions in a controlled mode with a patient who does not have a spontaneous respiratory rate. However, upon further inspection, despite the manufacturer's claim that "spontaneous breathing patients may entrain room air," the one-way valve is pressurized during normal operation and does not allow spontaneous breathing. It functions only as a fail-safe valve, allowing spontaneous breathing of room air if gas flow to the system is lost (eg, empty cylinder). The Parapac devices provide a mode identified as synchronized mandatory minute ventilation (SMMV), which is intended to allow spontaneous breathing but also provides mandatory breaths at the set tidal volume (VT) in the event the patient does not meet the minute volume requirement. To more carefully evaluate this, we set the tidal volume at 1,000 mL and the respiratory rate at 10 breaths/min. If the patient has a small spontaneous VT ( 200 mL), the ventilator delivers the set VT at the set rate (IMV). If the spontaneous VT is 200 mL but less than the set VT, the ventilator delivers the set VT at a rate lower than the set rate. And if the spontaneous breaths equal the set VT, no mandatory breaths are delivered. This is consistent with the manufacturer's description: "The tidal volume required to completely inhibit the ventilator is fixed at 450 mL, but the frequency is determined by that set on the ventilator."7 Regarding the Crossvent 3, a VT and pressure limit are set. If the pressure limit is above the pressure needed to deliver the set VT, the set volume is delivered every breath (volume ventilation)! However, if the pressure is set lower than the pressure needed to deliver the set VT, the excess volume is vented to the atmosphere and the set pressure is held for the remainder of

TO THE

EDITOR

300 ­1,100 psi, and the delivered VT of alternate breaths decreased to approximately 50% of the set value. Continued operation eventually resulted in further variance in delivered VT in all breaths. Performance was unaffected by the addition of PEEP. Regarding the animal studies, all the ventilators that failed to ventilate the injured lung model failed because of the development of auto-PEEP, which prevented further increase in the rate. We should have explained this in more detail. The lung model we used is the same that had been used by us in numerous other animal studies and is stable for longer than 4 hours, which exceeded the time needed to evaluate the group of 5 ventilators.8-10 Could the sequence of ventilators evaluated have affected their ability to ventilate and the development of auto-PEEP? Absolutely. This is in fact one of the reasons we did not use this failure as a "strike" against a ventilator in the final analysis In the introduction of our paper we listed the 6 criteria we used to select the most suitable ventilator for use in the out-of-hospital setting; we focused on forward military positions. As we stated above, because of the size of some of the injured animals, we disregarded the failure to ventilate injured lungs in our final evaluation. The specific criteria that separated the Newport HT50 and Impact/Uni-Vent Eagle 754 from the VersaMed iVent and Pulmonetic LTV1000 was the duration of the internal battery. We did not consider the attachment of additional battery capabilities, and we stated that both the VersaMed iVent and Pulmonetic LTV1000 would also be considered at the same level as the Newport HT50 and Impact/Uni-Vent Eagle 754 if their internal battery life was longer. Daniel W Chipman RRT Robert M Kacmarek PhD RRT Respiratory Care Services Massachusetts General Hospital Boston, Massachusetts

the inspiratory time (pressure ventilation)! In no case did we indicate that any ventilator provides both controlled mechanical ventilation and assist/control ventilation, as Branson and Rodriquez indicated. Gas consumption was defined as the amount of time the ventilator functioned on one full E-size oxygen cylinder (capacity 660 L of oxygen) at a VT of 1,000 mL and a respiratory rate of 10 breaths/min, on 100% oxygen. We performed this test once with each ventilator. In theory the maximum time of operation would be 66 min, assuming there were no leaks in the system, no bias flow or other diversion of gas by the ventilator, 100% of the gas was devoted to minute volume, the cylinder contents were correct, and the ventilator functioned as expected throughout the test. As is well known, the actual content of oxygen cylinders differs considerably. However, the reason for the less-than-expected duration of operation of the Impact/Uni-Vent Eagle 754 was its inability to maintain set parameters during this aspect of the evaluation. A peculiar finding with the Impact/Uni-Vent Eagle 754 was that before the cylinder became depleted, the ventilator alarmed "O2 LOW/ FAIL" and substantially decreased the VT of alternate breaths. We identified this as the point at which the ventilator was unable to maintain set parameters. To more carefully address Branson and Rodriquez's concern, we recently repeated this portion of the test with the Impact/Uni-Vent Eagle 754, with 3 different ventilators, at the previously described settings, and with and without PEEP set at 5 cm H2O. All cylinders were verified to be full, with pressure range 2,000 ­2,300 psi. Predicted duration of operation was 60 ­ 69 min. All 3 ventilators exhibited similar changes in VT delivery, but at various cylinder pressures. The range of duration of normal operation was 22­ 61 min. The cylinder-pressure range when the malfunction began was

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Dr Kacmarek has been a consultant for Space Laboratories, and has received research grants and/or honoraria from Puritan Bennett, Maquet, Cardinal Health, Newport Medical, Hamilton Medical, Respironics, and Drager. Mr Chipman ¨ has been a consultant for and has received lecture honoraria from Maquet. The authors report no other conflicts of interest related to the content of this paper. REFERENCES 1. Chipman D, Caramez MP, Miyoshi E, Kratohvil J, Kacmarek RM. Performance comparison of fifteen transport ventilators. Respir Care 2007;52(6):740-751. 2. Operating instructions. Drager Evita 4 in¨ tensive care ventilator, 7th edition. Telford, Pennsylvania: Drager; 2004;70-78. ¨ 3. Martinez-Ruis R, Bigatello RM, Hess D. Mechanical ventilation. In: Critical Care Handbook of the Massachusetts General 6.

TO THE

EDITOR

Kac-marek RM. Open-lung protective ventilation with pressure control ventilation, high-frequency oscillation, and intratracheal pulmonary ventilation results in similar gas exchange, hemodynamics, and lung mechanics. Anesthesiology 2003; 99(5):1102-1111. 9. Caramez MP, Schettino G, Suchodolski K, Nishida T, Harris RS, Malhotra A, Kacmarek RM. The impact of endotracheal suctioning on gas exchange and hemodynamics during lung-protective ventilation in acute respiratory distress syndrome. Respir Care 2006;51(5):497-502. 10. Caramez MP, Miyoshi E, Harris RS, Kacmarek RM, Malhotra A. Gas exchange impairment induced by open suctioning in acute respiratory distress syndrome: impact of permissive hypercapnia. Crit Care Med 2008;36(2):560-564.

4.

5.

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Hospital, 3rd edition. Hurford WE, editor. Philadelphia: Lippincott Williams and Wilkins; 2000:87-88. 840 ventilator system operator's and technical reference manual, part 4-075609-00 revision C. Carlsbad, California: Nellcor Puritan Bennett; 1999:4.3. Babylog 8000 infant care ventilator operator's manual, software release 3. Chantilly, Virginia: Drager; 1993. ¨ Chatburn RL, Primiano FP Jr. A new system for understanding modes of mechanical ventilation. Respir Care 2001;46(6): 604-621. Parapac Transport 200D Ventilator user's manual. PN 504-1117A_NUS, Issue 2, 07/ 2003, Draft B, Pneupac Ltd. Bedfordshire, England, page 31. Sedeek KA, Takeuchi M, Suchodolski K, Vargas SO, Shimaoka M, Schnitzer JJ,

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