11 April 2023
8 min Read
Effective Ventilation and Respiratory Care:The Importance of High-Quality Patient Circuits
Mechanical ventilation is a life-saving procedure for patients with respiratory failure. Patient circuits are a vital component of the mechanical ventilation system, linking the patient to the ventilator and delivering the appropriate volume of gas to the lungs. However, the role of patient circuits in ventilation is often undervalued, and their quality is overlooked. The compliance of patient circuits has a substantial influence on the ventilator's performance accuracy, which consequently has an impact on patient outcomes. In this blog, we will discuss the importance of quality patient circuits in ventilation and how compliance compensation can augment the precision of volume delivery.
Role of Patient Circuit in Ventilation
Patient circuits serve as an essential link between the ventilator and the patient's airway, ensuring the precise delivery of gas volume into the lungs. The compliance of a patient circuit, which measures the expandability of the circuit when pressurized, holds the key to accurately measuring the delivered volume and is thus of significant importance in the treatment.
The compliance of a system is defined as the change in volume that occurs per unit change in the pressure of the system. In the context of patient circuits, circuit compliance refers to the measure of the expandability of the patient circuit under pressure.
Relevance of Compliance
During the inspiration phase of ventilation, a fraction of the gas volume delivered by a ventilator does not reach the lungs of the patient. This occurs due to the expansion of tubing under inspiratory pressure, leading to a portion of the gas being trapped in the breathing circuit. Once the expiratory valve opens, this trapped gas is released. In essence, the ventilator's gas is utilized to expand both the patient's lungs and the circuit, resulting in loss of gas volume that was intended for the patient. The magnitude of this loss of volume depends on the compliance of the circuit and the inspiratory pressure.
To account for the volume lost within the tube due to its expansion, it is important to explore ways to prevent or offset this loss. This leads us to the idea of "Tube Compliance Compensation." This feature compensates for the lost volume within the tube by adding it to the target tidal volume. For instance, if the intended tidal volume is 400 ml, but 30 ml is lost in the circuit, the ventilator compensates by delivering a total of 430 ml, thus effectively accounting for the loss.
The amount of gas lost in the patient circuit is influenced by its compliance, which is assessed during the pre-use check through the patient circuit test. To determine the volume lost in the circuit as breaths are administered, the following formula is used:
The aforementioned calculation is performed for every breath, and the resulting volume loss is subsequently added into the targeted tidal volume.
Avoiding Alarm Fatigue
For precise volume delivery and compensation, especially for neonatal and pediatric patients, the use of high-quality circuits is crucial as circuit compliance has a direct impact on accuracy.
As per ISO 5367: 2014, circuit compliance should not exceed 1 ml per 1 cm H₂O pressure for every 1 meter of tubing.For instance, a typical 3.6-meter ventilator circuit should only have a volume change of 30-40 ml when ventilated with a driving pressure of 10 cm H₂O.High-quality circuits with overall compliance of less than 3 ml/cmH₂O have a gas compression factor of approximately 1/3 of the compliance, regardless of the material used (e.g., plastic or silicone). This empirical data enables accurate determination of circuit compliance and precise volume delivery to patients once compliance compensation is performed.
In contrast, inferior quality circuits exhibit significant variations in the proportions of gas participating in circuit expansion. As a result, determining the compliance of these circuits becomes challenging, leading to inaccuracies in volume delivery even after compliance compensation.
We conducted tests to assess volume delivery accuracy at our R&D centre and observed deviations in tests involving a wide variety of circuits. The following data depicts the accuracy of volume delivered by a ventilator and monitored using a lung simulator.
Case 1 - Good quality patient circuit, compliance < 3ml/cm H₂O
Case 2 - Inferior quality patient circuit, compliance > 3ml/cm H₂O
Based on the above data, it is evident that the accuracy of tidal volume delivery is highly dependent on the quality of the circuit. In addition to achieving precise tidal volume delivery, there are several other reasons why it is essential to use a patient circuit of high quality. These reasons include:
Leaks in circuit: Avoiding patient-ventilator asynchrony, which can lead to lung injuries
Resistance to airflow: Facilitating ease in spontaneous breathing
Flexibility: Preventing kinking and brittle fractures in the tubing
Inertness to humidity and drugs: Ensuring no chemical leaching from the circuit into the gas pathway or mixing with gases
Minimal dead space: Promoting efficient ventilation
Biocompatibility: Ensuring the circuit's material is compatible with the patient's biological system