Portable oxygen concentrators (POCs) that deliver continuous flow (CF) oxygen do exist, offering patients mobility while providing a steady stream of supplemental oxygen. A POC draws in ambient air, filters out nitrogen, and delivers concentrated oxygen to the user. While CF portable concentrators are available, they represent a smaller, heavier segment of the market compared to the widely used pulse dose devices, which deliver oxygen only on demand.
Understanding Flow Types
The primary difference between portable oxygen concentrators lies in their method of oxygen delivery, known as flow type. Continuous flow (CF) delivers a steady stream of oxygen at a constant rate, measured in liters per minute (LPM), regardless of the user’s breathing pattern. This flow is consistent, meaning oxygen is flowing even during the exhalation phase or pauses between breaths. CF devices must operate constantly to maintain this uninterrupted output, which requires a significant and steady power draw.
Pulse dose (PD) delivery, also called on-demand or intermittent flow, is a more sophisticated approach that conserves oxygen and energy. These devices use a sensitive sensor to detect the precise moment a user begins to inhale, and they immediately deliver a concentrated burst or “pulse” of oxygen. Since oxygen is only delivered during inhalation, there is no wasted oxygen during exhalation, making this method highly efficient. The pulse size or frequency is adjusted automatically to ensure the patient receives the necessary amount of oxygen, even if their breathing rate changes during activity.
Physical Constraints of Continuous Flow Portability
Achieving continuous flow in a portable unit introduces several physical trade-offs related to power and size. Continuous delivery requires the compressor and sieve beds to work without rest, creating a much higher energy demand compared to pulse dose devices. This constant power draw results in significantly shorter battery life for CF POCs, often providing only one to three hours of use on a standard battery, especially at higher flow settings. Users frequently must carry multiple spare batteries or rely on a nearby power outlet for extended use.
The components necessary to sustain a consistent stream of oxygen also contribute to increased size and weight. Continuous flow requires larger, more powerful compressors and greater quantities of the sieve material. Consequently, CF portable concentrators are inherently heavier and bulkier than their pulse dose counterparts, posing practical challenges for daily use and travel.
There is also a physical limit to the maximum continuous flow rate a portable unit can maintain while ensuring high oxygen purity. Most portable concentrators use a pressure swing adsorption process to achieve an oxygen concentration of approximately 90% or higher. Pushing a portable unit to provide a very high continuous flow rate can compromise the final oxygen concentration, reducing the therapeutic benefit. Therefore, portable continuous flow devices offer a limited maximum LPM compared to larger, stationary home units.
Clinical Rationale for Continuous Flow Prescription
A physician may mandate the use of continuous flow over a pulse dose device for specific medical reasons. Patients who require a higher flow rate, often three liters per minute or more, may find that continuous flow is necessary because pulse dose devices cannot adequately replicate the required oxygen volume. The consistent, steady delivery of CF ensures that the patient’s blood oxygen saturation levels remain stable, particularly during periods of rest or inactivity.
Continuous flow is often the standard prescription for patients who require oxygen during sleep. Shallow breathing or mouth-breathing during sleep may not reliably trigger the breath sensor on a pulse dose unit, potentially leading to oxygen desaturation. The uninterrupted stream from a CF device ensures a constant supply of oxygen regardless of the depth or pattern of the patient’s breathing. This consistent supply is also essential for patients with severe respiratory conditions, such as advanced Chronic Obstructive Pulmonary Disease (COPD), where a stable oxygen level is important for supporting vital organ function.