Continuous vs. Pulse Dose Point & Counterpoint
Bob Messenger is the Clinical Respiratory Education Specialist for Invacare Corp. His 30-year respiratory career includes acute care, as well as operational and ownership experiences in both homecare and sleep diagnostics, and he has served on the respiratory care program faculty at Cuyahoga Community College in Cleveland. Prior to joining Invacare, he was a clinical specialist and supervisor of clinical education at university-affiliated MetroHealth Medical Center in Cleveland. His research has been published in trade and professional journals and he lectures on a variety of respiratory-and sleep-related topics.
Patrick Dunne is president of HealthCare Productions Inc., a respiratory care training and development enterprise in Fullerton, Calif. Patrick is a veteran respiratory therapist and has been practicing in the home care setting since 1985. He is a past-president of the American Association for Respiratory Care (AARC) and a former chair of the AARC’s Home Care Specialty Section. He presently serves the AARC as a governor and executive committee member on the International Council for Respiratory Care (ICRC). Dunne has a clinical consulting relationship with SeQual Technologies Inc.
Robert Chatburn, RRT-NPS, FAARC, is a clinical research manager in the Cleveland Clinic’s respiratory therapy section, associate professor of medicine at Case Western Reserve University’s Lerner College of Medicine in Cleveland, and vice president of research and clinical services for Strategic Dynamics Inc.
Tom Williams is managing director of Strategic Dynamics, Inc., Scottsdale, Ariz. Williams assists clients in strategy formulation, market research, sales training and clinical and benchmark studies. He can be reached via e-mail: at TWilliams@StrategicDynamicsFirm.com.
Topic 1: Clinical Differences Between Continuous and Pulse Dose Devices
How do these devices function differently? What are the advantages and drawbacks of each? Are some patients more suitable for each type of device?
Messenger: The controversy of continuous flow vs. conserving technology for long-term home oxygen delivery is a debate of flawed systems. By their nature, each is a low-flow system, and each falls victim to the variations in oxygen concentration that result from changes in respiratory rate, tidal volume, inspiratory flow rate and other factors.1-5 Despite all of these variables, clinicians use low-flow systems because they are practical — they offer the advantages of patient comfort and convenience. It is within these practical aspects that the advantages and limitations of each system need to be evaluated. Oxygen-conserving technology is essential to reducing the size and weight of portable oxygen systems without sacrificing duration. In a study of the effect that the weight of a portable oxygen system has on exercise capacity, Leggett and Flenley6 demonstrated that when the system weighs more than 10 pounds, all of the endurance benefit of the oxygen is consumed in the process of carrying the system. Indeed, the Fifth Oxygen Consensus Conference7 recognized the benefits of meeting the practical needs of ambulatory COPD patients. That conference established three criteria for portable oxygen systems: the system must weigh less than or be equal to 10 pounds, it must be able to be carried by most patients, and it needs to provide an equivalent of 2 liters per minute for a minimum of fourhours. All of the self-sustaining systems that meet these criteria utilize oxygen conserving technology.
Dunne: A continuous flow device administers oxygen in liters per minute (L/min), whereas a pulse dose device intermittently administers a volume (or bolus) of oxygen in milliliters per breath (mL/breath). Continuous flow delivery — the traditional method for oxygen therapy — is considered by many to be the “gold standard.” However, the introduction of pulse dose delivery in the 1980s to conserve oxygen contents during ambulation, proved to be as effective in maintaining adequate oxygenation when properly applied to a patient by a respiratory therapist (RT) using pulse oximetry. Hence, both approaches to oxygen therapy can be considered efficacious. However, the introduction of portable oxygen concentrators (POCs) capable of only delivering oxygen in the pulse dose mode raises an important clinical question: Can pulse dose oxygen delivery be used on a 24/7 basis to maintain adequate oxygenation? Unfortunately, the answer is not so clear-cut since there are two important factors that impact oxygen delivery with a POC: (1) the maximum bolus size a particular POC is capable of producing; and (2) the time it takes to actually deliver the bolus. Both of these performance characteristics vary widely between POCs. Other factors include: the patient’s systemic oxygen requirement (especially during exercise and sleep), as well as the ability of a particular POC to produce sufficient concentrated oxygen with increased respiratory rates.
Moderators: A continuous flow device provides oxygen in liters per minute (L/min) usually in the therapeutic range of 1-3 L/min. A pulse dose device intermittently administers a volume or bolus of oxygen in milliliters per breath. Settings on pulse dose devices are arbitrary and are not an absolute equivalent to continuous flow. As of this writing, there are two manufacturers that provide continuous flow delivery (CFD) and pulse dose oxygen delivery (PDOD) within the same device, with a third manufacturer slated to enter the market within the next few months. There are six different manufacturers that offer a PDOD POC. Another difference in the two technologies is their size and weight. To provide continuous flow, pulse dose oxygen delivery (PDOD) requires a larger pump/motor assembly, additional electronics and a more molecular sieve. All of this must then be placed within a larger case. This is what increases the device’s size and weight (approximately 18-20 lbs). PDOD POCs weigh less than 10 lbs. The market niche for PDOD/CFD POCs versus PDOD POCs is hotly debated for many reasons. A prominent reason is that some clinicians believe that CFD is required for all oxygen-dependent patients during sleep. Others believe that if a patient does not desaturate during sleep, as proven by an overnight oximetry test, then a PDOD POC is fine to use. The issue is not whether a person desaturates at night but why they desaturate. If desaturation is due to sleep apnea, then neither continuous flow nor pulse dose systems will be effective. On the other hand, if patients desaturate because they have shallow breathing and are not triggering a pulse dose system, then continuous flow may offer an advantage. In practice, nobody is really motivated to fund the sleep studies required to answer these kinds of questions. Therefore, at a minimum, we believe a patient should be evaluated for desaturation at night using an appropriate level of continuous flow (perhaps a higher flow than required for normal awake rest). If the patient desaturates, then the issue of sleep apnea should be addressed. If not, then at least we know one effective therapy mode. If there is sufficient motivation to use a PDOD POC then a second study can be done using a specific device. Regarding the issue of whether continuous flow should be considered a “gold standard,” the fact is that, as Mr. Messenger pointed out, the Fifth Oxygen Consensus Conference established a definition for portable oxygen systems that includes a criterion that the system must provide an equivalent of 2 l/m for a minimum of four-hours. By default, then, a constant flow of 2 L/min becomes the standard. However, the fallacy of this definition is that the term “equivalent of 2 L/min” has no definite meaning. If you require a flow meter manufacturer to produce an instrument that outputs 2 L/min plus or minus an acceptable tolerance, then you have created a genuine standard that can be checked by anyone. But referring to oxygen delivery to a patient using a nasal cannula as being equivalent to anything is like requiring a broken clock to read the correct time, which it indeed can do only for an instant twice a day. This ambiguity in the Consensus definition allows the wide variety of performance from commercial POCs and perhaps, more importantly, reinforces the practice of physicians writing prescriptions in terms of flow settings as if they were actual drug dosages. Aside from the issue of ambiguous dosing with a nasal cannula, there are at least two other factors that invalidate any assumption of “equivalence” between continuous flow and pulsed dose devices. First, the FiO2 delivered by a continuous flow device must decrease as the patient’s minute ventilation increases. At least in theory, pulsed dose systems can avoid or mitigate this problem by maintaining a constant pulse volume per breath. Second, the standard “rule of thumb” for estimating the FiO2/flow relationship relies heavily on the assumption of an “anatomic reservoir” that is filled by a continuous flow device between breaths. Our lab has data to show that patients with COPD may have end expiratory flow in excess of the cannula flow setting, thus eliminating the reservoir effect and lowering the FiO2. Again, pulse devices may avoid this effect because they do not require a reservoir effect in the first place. The bottom line is that portable devices have widely varying performance characteristics and probably always will. The only rational way to deal with this variability is by titrating each patient to a specific device for the expected usage conditions including rest, activity and sleep.
Topic 2: Industry Standards
Within the industry, there remains a lot of confusion regarding how to compare the settings on a pulse dose device and the lpm settings on continuous flow. Does the industry need standards? Who should regulate these industry standards? What terminology is needed?
Messenger: Industry standards for oxygen conserving devices (OCDs) may reduce some confusion, though they would do little to solve the perceived “equivalence” problem. Much of the confusion results from variations in bolus volumes between manufacturers for any given setting. Manufacturers claim these bolus volumes to be “equivalent” to continuous flow at that same setting. Despite the differences in bolus volumes, all of these manufacturers are both correct and incorrect in their claim of equivalency to continuous flow. Continuous flow is flawed because it does not provide a stable oxygen concentration in the face of variations in respiratory rate, tidal volume, dead space and the many other variables associated with chronic obstructive disease. In a comparison of eight OCDs to continuous flow, Bliss et al8 demonstrated that the OCDs offered greater FiO2 stability in the face of variations in respiratory rate. So, depending on the respiratory rate and other factors at any given time, conserverdelivered oxygen may result in better, equivalent, or worse oxygenation than continuous flow. Although OCDs offer many clinical and practical advantages, they should never be assumed to offer equivalence based on a number on the knob. A reasonable industry standard would be for all OCD manufacturers to clearly state that proper setting of their device requires titration by pulse oximetry.
Dunne:The primary source of confusion within the industry is the claim that a particular numerical setting on a pulse dose POC is “equivalent” to the same number on a flow meter used for continuous flow. For example, a pulse setting of “2” is equal to 2 L/min. This is not the case. The numerical setting on a pulse dose device is reflective of the bolus size in mL, not a flow in L/min. However, a patient receiving oxygen at 2 L/min, when switched to a particular pulse dose POC also set on number “2,” might indeed achieve the same degree of oxygen saturation, but as could only be determined by pulse oximetry. In this scenario, the POC would be said to be “clinically or functionally “equivalent” to the 2 L/min prescription. However, on another patient, it may take a numerical setting of “3” (or even higher) on the POC to achieve functionally equivalency. The confusion is all about the pervasive mis-representation that the numerical settings on a POC are “equivalent” to L/min. The easiest way to address this issue would be for all POCs to have each numerical setting on the control panel specifically indicate the respective bolus size in mL.
Moderators: In our opinion industry standards, for example, requiring labeling of pulse volume instead of arbitrary dimensionless number settings could be beneficial, but they are only a short-term solution. The major problem in the industry is that “continuous flow” does not imply “constant oxygen delivery” and thus should not be considered a “gold standard” (or for that matter a clinical standard of any type) with which to compare PDOD device performance. This is a common mistake that many experienced clinicians often make. The problem for the HME provider is that while oxygen is prescribed by the physician in liters per minute the under 10 lbs, POCs only provide PDOD, and those POCs in the 18-to 20-lb. range provide both PDOD and continuous flow delivery (CFD). But as we will discuss in the next question, having manufacturers agree on any standards is really the “tail wagging the dog” and not likely either to happen or to be of any real benefit.
Topic 3: Educating Providers and Physicians
What must providers do to properly educate themselves on the differences between these devices? Who should teach them? What do they need to know? How should the industry go about educating physicians and patients?
Messenger: In hospitals, oxygen therapy is exclusively focused on meeting the clinical needs of the patient. In the home, oxygen therapy must address those clinical needs, in addition to meeting the practical needs of patient ambulation and social interaction. Size, style and efficiency of the oxygen system, all of which are influenced by OCDs, are of no concern to either the patient or the clinician during acute care hospitalizations, yet these are the factors that help to minimize the intrusiveness of home oxygen and influence long-term acceptance and use of ambulatory oxygen systems. There is a great tendency within the acute care community to dismiss the technology of long-term oxygen therapy as overly -simple and undeserving of educational time and attention. Yet it is from this acute care community that the prescriptions arise. Given these challenges, possibly the best first step may be an alignment of the industry with the current practice of evidence/outcomes-based medicine. Educating physicians to prescribe home oxygen therapy based on titration by pulse oximetry ensures that, regardless of the oxygen system used, the needs of the patient are met, rather than merely meeting the settings of the prescription. Educating physicians on the benefits of saturation-targeted prescribing is an essential component of any provider’s outcomes-based oxygen therapy program.
Dunne: There is no question that the rampant confusion surrounding the performance capabilities of pulse-only POCs requires a concerted educational undertaking to ensure prescribing physicians and home care providers truly understand the role of these devices in maintaining adequate arterial oxygenation. For their part, home care providers must have knowledgeable respiratory therapists (RTs) on staff possessing a detailed understanding of all methods of oxygen delivery and dosing. Said therapists should also be capable of objectively assessing the performance capabilities (and limitations) of the various POCs. Ideally, the respective POC manufacturers should provide such training on performance capabilities. Once the RTs have all of the necessary information on the available POCs, they in turn can assist their company in selecting the best brand of POC to meet their non-delivery LTOT needs. The homecare RT is, likewise, the best resource in helping prescribing physicians gain a better understand of the potential advantages these devices can offer their patients, as well as any limitations that might compromise adequate oxygenation. Most of this information will be new, since physicians will not have seen pulse dose delivery devices used in the hospital setting. Hospitals uniformly still use continuous flow dosing for oxygen therapy.
Moderators: There is no easy answer to this question. The fundamental problem is that oxygen cannot be prescribed like other drugs for the simple reason that the delivered dose cannot be determined a priori as with other drugs. For example, the Ohio Board of Pharmacy stipulates that for a prescription to be valid, it must “indicate the quantity to dispense,” that is, the dosage the patient is to consume. The problem with oxygen as a drug is that in no case is a prescription such as “continuous flow of oxygen at 2 L/min” the quantity of oxygen consumed by the patient. Therefore, strictly speaking, a prescription like that is just not valid. Yet we have a Catch-22 because substituting the phrase “titrate to patient requirement” is also no indication of the “quantity to dispense” and indeed, it leaves that determination up to a non-physician. This of course flies in the face of the prevailing paradigm of western medicine and is doomed to failure. Physicians certainly “experiment” with different dosages to determine the optimum therapeutic value for a specific patient (e.g., so-called “n of 1 trials”). But there is no practical way a physician can do this with oxygen because: (a) there is no practical way to actually measure the dosage (FiO2) and (b) even if there was a way to measure FiO2 the physician cannot expend the resources required to observe the effects of a given dosage during activities of daily living. The ideal solution would be for pharmacy boards to revise their rules, in effect, allowing non-physicians (e.g. registered respiratory therapists) to prescribe oxygen and then let these alternate-care providers evaluate the effects of a proposed dosage with a recording oxygen saturation monitor in the home. This does not seem likely unless convincing research shows that harm is being done to patients as a result of the current prescription ambiguity. That leaves manufacturers with a requirement to label their products in a quantitative manner, and physicians with a requirement to prescribe with the only quantitative information they have at hand, flow rates. And we are back to the same problem. Thus, the sad fact seems to be that there is no solution in sight without some major motivating force that is yet to be discovered. Manufacturers have neither the resources nor the motive to change physician behavior, and physicians do not perceive a problem. Viewed in this context, the whole debate may be futile.
Topic 4: Current Research
Discuss the latest findings on continuous flow and pulse-dose devices that are relevant to the industry.
Messenger: Early conserver technology offered demand-oriented flow. These devices provided normal flow that was limited to inspiration.9 The frequent result was poor patient oxygenation10 and a negative impression of the technology within the physician community. Contemporary oxygen conserving technology provides a pulsed volume of oxygen timed to maximize alveolar delivery.11 These devices extend the duration of compressed gas cylinders, and they provide the essential technology for the development of small, light and clinically effective portable concentrators. Probably, the area of greatest contention for OCD use is during sleep. This concern arises from fear that mouth breathers will be unable to trigger the device resulting in severe hypoxemia. This concern has been reinforced by the experiences of home care clinicians that have conducted overnight oximetry and discovered that their patient desaturated. Baseline testing these same patients on continuous flow oxygen may have showed similar desaturation. In a study of 82 stable, oxygen-dependent COPD patients receiving continuous flow oxygen at night, 48 percent of patients had significant desaturation.12 Subsequent studies of portable oxygen concentrators13-15 that established appropriate settings during sleep demonstrate adequate oxygenation and favorable clinical outcomes. The American Association for Respiratory Care specifically supports the nocturnal use of OCDS in the revised long-term oxygen therapy Clinical Practice Guideline.16
Dunne: Regrettably, at this writing, there are relatively few objective studies published in peer review publications comparing the efficacy of pulse dose delivery from a POC on a 24/7 basis to continuous flow delivery. However, several bench studies have been conducted on related applications, with the results presented as poster sessions at various medical meetings. In essence, these bench-study simulations have looked at the effect of increases in respiratory rate on the ability of certain pulse only POCs to maintain oxygen concentrations or greater than or equal to 90 percent. As expected, the results indicated that the smaller the POC, the more likely it is that FiO2 will be compromised when the respiratory rate increases, as is often the case with exertion secondary to ambulation. No doubt, this dearth of published data on the performance capabilities and limitations of POCs will be addressed as more in-vitro and in-vivo research is conducted and eventually published. Clearly, the industry is slowly but steadily transitioning to the non-delivery model of LTOT, and the full impact of this novel technology must be objectively assessed to ensure patient efficacy and safety are not compromised by a lack of knowledge or misunderstanding of the performance capabilities of the various devices available.
Moderators: It takes time for published research to catch-up with product innovation. Numerous studies have been published in peer review journals on the efficacy of oxygenconserving devices. There is a dearth, however, of literature published in peer review journals comparing one POC device to another and to other oxygen non-delivery devices. There are studies in process and they will be forthcoming. Each of these published papers will stimulate thought and require additional studies to be performed to add clarity and further direction. In the meantime, clinicians will be required to rely on bench and/or clinical studies presented via Web-cast, white paper, lecture or poster at industry-sponsored events or through educational sessions at medical meetings. Perhaps the greatest area where further research is needed is the use of PDOD vs. CFD during sleep. Many clinicians believe that POCs cannot be used with oxygen-dependent patients at night because they rely on an OCD to deliver the oxygen with each breath. This belief might be more conjecture than clinical fact. There are a number of published clinical studies that demonstrate the effective use of oxygen conserving devices in a variety of patient-care situations, including during sleep. In fact, there are more total patients in clinical studies validating the use of OCDs during sleep than the number of patients studied in the two landmark studies establishing the use of long term oxygen in hypoxemic patients (Nocturnal Oxygen Therapy Trial-NOTT and British Medical Research Council -BMRC studies). While we wait for additional studies to be performed and published, HME providers who are concerned that their patients may desaturate during night time use of a POC should test their blood oxygen saturation levels by oximetry before placing them on the specific device, or they can purchase either a POC with pulse dose/continuous flow or a stationary concentrator plus a POC.
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This article originally appeared in the Respiratory Management September 2009 issue of HME Business.