By Edwin Coombs, MA, RRT, NPS, ACCS, FAARC
A recent study published in Respiratory Care examined performance characteristics of high-frequency infant ventilators and may raise concerns over how measurement accuracy can be affected, with respect to effective tidal volume delivery and/or impact on delivered pressure/amplitude.
In response, this brief article will review gas laws and how their respective variables can have an impact on the delivery of mechanical breaths.
A Review of Some Basics
The ideal gas law defines a relationship between pressure, volume, temperature, and the number of gas molecules. Pressure and volume are inversely related; whereas, temperature is directly proportional to volume or pressure.1
Boyle’s law states that at a constant temperature, pressure is inversely proportional to volume. This law predicts the relationship of a volume of gas to a pressure change.1-2 Temperature (ie, warm ventilator tubing) also contributes to changes in tubing compliance.
Gay-Lussac’s law of pressure and temperature describes the direct relationship between pressure and temperature. If the absolute temperature of a fixed volume of gas is increased, the pressure will increase proportionally.1-2
Charles’s law predicts the effect of temperature on a fixed amount of dry gas. At a constant pressure, gas expands proportionally to changes in absolute temperature.1-2
Mechanical Ventilation
The patient circuit of a mechanical ventilator conducts the breathing gas from the ventilator to the patient and from the patient to the expiratory valve of the ventilator. Tubing expansion can be seen when high pressure is generated within the patient circuit.
The circuit can often be seen to expand during inspiration and then return to its original size during exhalation. Part of the inspiratory pressure or tidal volume delivered from the ventilator contributes to this tubing expansion, and, as a result, it is attenuated before reaching the distal airways. This phenomenon is extremely important when ventilating infants and children, especially when utilizing high-frequency ventilation. For this reason, a low-compliance circuit must be utilized.2
The use of high-frequency oscillatory ventilation (HFOV) in infants is to protect extremely premature and fragile lungs from ventilator-induced lung injury through a protective ventilation strategy.
Typically, initial HFOV settings in infants are:
- a MAP of 2-3 cmH2O above conventional ventilation;
- power to achieve an amplitude that results in visible “chest wiggle;” and
- a frequency of 10 Hz.3
While bench studies are an integral component that contributes to the advancement of science and our understanding of respiratory care products, researchers and clinicians must consider all factors that can potentially impact how devices, accessories, and related consumables deliver mechanical ventilation. Understanding research study design and methodology is critical when evaluating the results of equipment tests.
RT
Edwin Coombs, MA, RRT, NPS, ACCS, FAARC, is the senior director of Marketing, Portfolio Solutions Training, Clinical Affairs, & Intensive Care, North America, Dräger Inc.
References
- Hess, D. MacIntyre, N. Galvin, W. Mishow, S. “Respiratory Care: Principles and Practice” Third Edition; Jones & Bartlett Learning, 2016, pp 1210-1211.
- Cairo, JM. Pilbeam, S. “Mosby’s Respiratory Care Equipment” Seventh Edition; Mosby, 1999, pp 13-14.
- High Frequency Oscillatory Ventilation (HFOV) : a guide to the use of HFOV in the neonate (scot.nhs.uk). https://www.clinicalguidelines.scot.nhs.uk/nhsggc-guidelines/nhsggc-guidelines/neonatology/high-frequency-oscillatory-ventilation-hfov-a-guide-to-the-use-of-hfov-in-the-neonate/
