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Noninvasive COPD screening of expiratory flow limitation within home therapy
Symposium sponsored by Philips Respironics ERCA–JIVD 3rd Joint International Meeting, Lyon, France, March 2018 Tidal expiratory flow (EFL) limitation can occur in COPD patients when they exercise, or in more severe cases, when they aren’t exercising – in either case creating what is sometimes described as a “severe mechanical constraint.”1 Recently Dr. Peter Calverley and Professor Raffaele Dellacà led a symposium to discuss tidal expiratory flow limitation and its effects, how to measure it noninvasively and effectively, and how using ventilators to minimize tidal EFL can produce physiological improvements. Along the way, they emphasis the importance of understanding respiratory physiology and respiratory mechanics.
Introduction and physiology behind the expiratory flow limitation
Dr. Peter Calverley discusses why understanding respiratory physiology is vital to understanding COPD and how to best treat patients. Respiratory physiology simply enables a gas exchange—O2 from the air to the circulatory system, and CO2 subsequently expelled from the circulatory system. When patients can’t expel the corresponding amount of CO2, this leads to a buildup of CO2 in the body. Calverley talks about the phenomenon of tidal expiratory flow limitation (EFL) in patients. This occurs when an increased expiratory rate fails to increase the expiratory volume flow. This can be seen when a patient exhales along the same flow-volume curve during quiet, tidal breathing as they do during a forced expiratory effort. In other words, the patient is working harder to breath, yet the flow of air doesn’t increase proportionally. EFL results in the “waterfall concept,” where a narrowing in a critical point of the airway system means that no more gas can leave the system at the same rate. Health people have an “expiratory flow reserve.” Unhealthy people with emphysema, small-airway disease, COPD, suffer from this tidal expiratory flow limitation—where the breath stacks up within the lungs, known as hyperinflation. How is tidal EFL currently detected and measured today? The intra-esophageal balloon technique is invasive, and it can be tricky to place the balloon. Applying abdominal pressure during expiration can work, though timing the pressure is an issue. Professor Raffaele Dellaca explains the physics behind a new technique that can be used for detecing EFL.
Respiratory mechanics and the forced oscillation technique
Dellaca defines respiratory mechanics as the relationship between atmospheric pressure (forces) and flow (displacements) of the respiratory system. Tidal EFL can be detected by measuring how respiratory mechanics change between inspiration and expiration during tidal breathing. Measuring the atmospheric pressure is more difficult because of the effect of respiratory muscles; these muscles are used to breathe in and out, and their effect needs to be negated. The forced oscillation technique (FOT), in which pressure is applied from outside of the lungs, allows the study of structural and mechanical properties of the respiratory system deduced from its mechanical response to small time-varying forces. Loudspeakers are used to create the oscillation frequencies that are used in this technique. In the resulting spectra, the high-frequency oscillation can be clearly identified from the quiet breathing of the patient via a digital filter. FOT measures respiratory system impedance and reactance. Impedance is the value that the forced oscillation produces. Reactance is the measurement of the compliance of the respiratory system, that is, its stiffness. When FOT is applied during EFL, the forced oscillation can’t get past the choke points in the respiratory system, located primarily in the central airways. Oscillation can’t reach the alveoli, so only the mechanical pressure of the central airway is measured. The reactance becomes more negative because the system is much stiffer. Dellacà’s group has found that using one continuous frequency, rather than several during FOT, and performing the analysis very quickly, means that resistance and reactance can be measured at the same time, within a single breath.
How common is tidal EFL?
Calverley discusses the occurrence of tidal EFL in COPD populations and finds that people fall into three categories: below critical threshold, above critical threshold, and with variable critical threshold. For some people, a lot of breaths were flow limited; in others, only a few breaths were flow limited. In other words, there is a difference between a tidally flow limited breath and a tidally flow limited patient. (Note that an elderly population was used in these studies because they are most likely to have lost elastic recoil in the lungs; in other words, their airway system has become stiffer.) Further, patients’ conditions and capacities didn’t remain static—they varied from day to day, so the clinician must take this into account when considering treatment options.
What are the clinical implications for COPD patients with and without EFL? Flow-limited patients had a lower inspiratory capacity than those who were not flow limited. They also had a worse exercise component, and higher degrees of breathlessness. In a 6-minute walking study, flow-limited patients declined over time; that is, their ability to walk decreased over time. Regarding possible treatment, it’s well known that pulmonary rehabilitation doesn’t have an effect on lung mechanics. It’s possible to change it with bronchodilators. Mechanical support, that is, ventilator support, might be an option.
But what about the patient that needs ventilatory support?
Raffaele Dellacà describes a method that incorporates both the forced oscillation technique and a self-regulating mechanical ventilator into a continuous positive airway pressure (CPAP) therapy system for COPD patients. In one study to test this idea, the forced oscillation (FOT) was delivered through the nose, a pathway with much greater resistance, while the patient was receiving CPAP therapy. Flow and pressure were measured as usual. The patient’s airway flow was measured while seated and in the supine position. While seated, CPAP as low as 4 cm of water was effective in abolishing EFL. When the same patient was placed in the supine position, the situation changed dramatically. Because of the role of the chest wall and the abdominal contents that push up the diaphragm, a pressure of 4 cm was not enough; the patient needed a much greater pressure to abolish EFL. Dellacà and his group started working with Philips Respironics on a prototype of an automatic system loop that would adapt mechanical ventilation to the COPD patient’s need.
Among the several studies that he discussed, Dellaca describes an overnight study with patients with moderate to very severe COPD who were classified as EFL in the supine position. The aim was to compare prescribed EPAP (expiratory positive airway pressure) with automatically tailored EPAP in these patients, and to characterize the effect of a continuously adjusting EPAP applied for a long-term period, on physiological variables. He notes that the pressure needed to abolish EFL is an independent feature of COPD.
This study, a randomized crossover trial, spanned two nights per patient. During night 1, the ventilator was set with fixed parameters as prescribed by the patient’s physician (IPAP and EPAP). For night 2, the ventilator was set to the FOT auto-EPAP mode; that is, IPAP and EPAP self-adjusted according to EFL. The results showed that during night 1, with fixed parameters, the CO2levels in the patient remained constant. During night 2, with the auto-EPAP parameters, there was tremendous variation in pressure, and hence a significant improvement of CO2 levels in the same patient. As observed previously, changing posture had huge impact on the pressure needed; the ventilator adjusts constantly to accommodate this. With the auto-EPAP method, ineffective respiratory efforts were reduced by ~ 25% (sleep efficacy was not affected)
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Dr. Calverley Concludes:
Symposium Faculty Peter Calverley, Professor of Respiratory Medicine and Honorary Consultant Respiratory Physician at Aintree Hospitals, Liverpool, UK Robert Romano, Biomedical Engineer at Philips Medical Systems, Philips Home Healthcare Solutions, Pittsburgh, PA, US
Raffaele L Dellacà, Associate Professor, Politecnico di Milano University, Milan, Italy
David White, Chief Scientific Officer, Philips Sleep & Respiratory Care, Denver, CO, US
1Expiratory_Flow_Limitation_Definition_Mechanisms_Methods_and_Significance. Accessed 15 May 2018.