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11 July 2023
7 min Read
Understanding ICU Ventilator Oxygen Consumption : Unmasking the Facts
ICU ventilators, essential for critically ill patients, support maintaining necessary oxygen levels. However, many are unaware of how these machines work and the importance of their oxygen consumption, a knowledge gap further highlighted by the COVID-19 pandemic. Understanding oxygen consumption in ICU ventilators is crucial for resource management, cost control, and patient safety.
ICU ventilators combine air and oxygen to reach the required concentration before delivering it to the patient. The 'oxygen consumption' is the amount of oxygen the ventilator uses. These machines are adjusted to deliver a specific fraction of inspired oxygen (FiO2), from 21% (room air concentration) to 100% for severely hypoxic patients.
Though one might assume that oxygen consumption equals the amount given to the patient, it's not always so. Ventilators also use oxygen for self-testing and powering pneumatics, contributing to overall consumption. Specific ventilator designs, especially those with poorly designed internal gas pathways, may cause additional oxygen wastage.
Why is Oxygen Consumption Important?
Cost Implications: Oxygen is a significant cost in healthcare, particularly in resource-constrained settings. By understanding and optimizing the oxygen consumption of ICU ventilators, healthcare facilities can make more cost-effective decision.
Resource Management: Hospitals and ICUs must monitor and manage their resources efficiently. If a healthcare facility doesn't accurately track its ventilators' oxygen consumption, it could run into serious problems, such as running out of oxygen. This is specifically critical at hospitals that do not have oxygen plants and depend on oxygen cylinders.
Emergency Planning: In situations such as a pandemic or a mass casualty incident, where there is a sudden increase in demand for ventilators, understanding oxygen consumption becomes crucial for effective disaster management.
Influence of Ventilator Design on Oxygen Consumption
Absence of a controlling valve: Some ventilator designs release oxygen ahead of the air inlet without a regulating or controlling valve. This results in a continuous oxygen flow into the mixing chamber, often exceeding the patient's needs. The excess oxygen, not required for patient care, leaks out from the air inlet ports, causing oxygen wastage. Consequently, despite delivering the same FiO2 levels, these ventilator designs have higher overall oxygen consumption. This can be countered using a variable control valve for FiO2 delivery, which regulates oxygen flow based on the patient's needs. This valve adjusts the oxygen and air mixture in real time, reducing waste and enhancing efficiency.
Feedback sensor for FiO2 control algorithms: The feedback sensor type in FiO2 control algorithms significantly affects oxygen wastage in ventilators. Galvanic oxygen sensors are commonly used but have limitations like slower response times and occasional inaccurate readings. These sensor characteristics can result in over-delivery of the set FiO2, as the system errs towards excess to ensure patient safety. This 'better safe than sorry' approach can lead to higher oxygen consumption and wastage. However, these challenges can be addressed by implementing flow rate-based metering. This method precisely measures and controls oxygen delivery feedforward, guaranteeing the accurate supply of the set FiO2 , resulting in efficient oxygen delivery, reduced wastage, and improved overall ventilator efficiency.
Oxygen wastage during the expiratory phase: Another reason for increased oxygen consumption and wastage in ventilators is maintaining FiO2 levels during the expiratory phase of a respiratory cycle when the patient is exhaling. Standard ventilator operation involves delivering the set FiO2 throughout the entire respiratory cycle. This means that even when the patient is exhaling, the ventilator continues releasing the same oxygen concentration. While this design ensures continuous oxygen availability, it leads to significant wastage as the patient doesn't utilize the oxygen delivered during the expiratory phase and is essentially expelled.
Reducing Oxygen Consumption in ICU Ventilators: An Experimental Study in Design and Control Implementation
Noccarc introduced the V310+ ventilator to address design and control challenges seen in previous ventilators, conducting a study to evaluate improvements in oxygen consumption. This study compared two designs: traditional control with feedback from an oxygen sensor without an active control valve at the oxygen supply (Figure 1) and active oxygen flow control with flow metering (Figure 2).
In Figure 1, the design had issues such as delayed FiO2 response and inefficient oxygen use. These issues were addressed in the design in Figure 2 using a feed-forward control strategy, determining the required flow rate in advance for optimal response. The oxygen flow rate is then achieved via valve regulation based on flow sensor feedback.
The innovative control strategy of the second pneumatic system significantly reduces oxygen wastage by only opening the oxygen valve proportionally and as needed. A comparative test was conducted using two identical H-type oxygen cylinders (also known as Jumbo cylinders), with both designs operating at the same respiratory rate and FiO2 percentage.
Results showed the first design only lasted 2 hours while the second lasted an impressive 8 hours. This fourfold increase in operational time meant a 400% reduction in oxygen wastage (Figure 3). Furthermore, the second design accurately maintained the desired FiO2 percentage, ensuring optimal oxygen delivery to patients. The reduced oxygen wastage and enhanced FiO accuracy demonstrated potential improvements through innovative design and control strategies.
CUnderstanding ICU ventilator oxygen consumption is not just for medical professionals. Anyone involved in healthcare, including patients, families, administrators, and policymakers, can benefit from understanding this critical aspect of patient care. And an efficient system with minimal oxygen consumption can be achieved through innovative designs and control strategies. With the knowledge comes the power to advocate for better resource management, improved patient care, and efficient emergency response.
In conclusion, ICU ventilators are lifelines for those unable to breathe independently. As we continue to grapple with respiratory illnesses, pandemics, and the everyday reality of critical care, let's strive to deepen our understanding of ICU ventilator oxygen consumption and its impact on our healthcare systems. This awareness can lead to improved patient outcomes, more efficient resource use, and better preparedness for future challenges.
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