Servo-Controlled Ventilator Alveolar Ventilation Control
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Solution Overview
Problem
Ventilators that target total ventilation often fail to accurately meet a patient's respiratory needs due to anatomical and physiological deadspace variations, leading to inadequate alveolar ventilation, especially at high respiratory rates.
Innovation Solution
A ventilator that uses flow triggering and phase cycling, with a servo-controlled system that adjusts pressure support based on the error between target and actual ventilation, and accounts for anatomical and physiological deadspace to maintain alveolar ventilation, providing a higher pressure during inspiration and a lower pressure during expiration.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If ventilators target total ventilation, then ventilation support is provided, but alveolar ventilation accuracy deteriorates due to deadspace variations
Solution Approach 1:
The patent segments total ventilation into two distinct components: deadspace ventilation and alveolar ventilation. By measuring and calculating each component separately using flow rate integration over different time intervals, the system achieves accurate targeting of alveolar ventilation independent of deadspace variations. This segmentation allows the ventilator to distinguish between ventilation that does not participate in gas exchange and ventilation that does, resolving the contradiction between measuring total ventilation and ensuring alveolar ventilation reliability.
Solution Approach 2:
The patent changes the control parameter from total ventilation to alveolar ventilation specifically. By adjusting the target parameter to focus on the alveolar component rather than the aggregate total ventilation, the system directly addresses the reliability issue. The control algorithm modifies pressure support based on alveolar ventilation error rather than total ventilation error, ensuring that the primary control objective maintains reliable alveolar gas exchange despite variations in deadspace.
2Reliability
If pressure support is increased to compensate for hypoventilation, then ventilation support improves, but patient comfort deteriorates due to aggressive responses
Solution Approach 1:
The patent implements dynamic, asymmetric gain adjustment where the system's response characteristics change based on the direction of ventilation error. For hypoventilation (negative error), the system applies a higher gain for more aggressive correction. For hyperventilation (positive error), the system applies a lower gain for more gradual correction. This dynamic adaptation of control aggressiveness based on error direction resolves the contradiction by providing reliable support when needed while maintaining comfort during correction phases.
Solution Approach 2:
The patent employs feedback control with asymmetric response characteristics. The servo-controlled system continuously monitors alveolar ventilation and adjusts pressure support based on the sign and magnitude of the ventilation error. The feedback mechanism incorporates different correction strengths for under-ventilation versus over-ventilation scenarios, allowing aggressive intervention when alveolar ventilation is insufficient but gentle adjustment when it exceeds targets, thereby maintaining both reliability and patient comfort.
Data Source
AI summary
A method and apparatus for providing ventilatory assistance to a spontaneously breathing patient an error signal (56) is computed that is the difference between a function of respiratory airflow (54) over a period of time and a target value (52). Using a servo loop, air is delivered to the patient at a pressure that is a function of the error signal, the phase of the current breathing cycle, and a loop gain that varies depending on the magnitude of the error signal. The loop gain increases with the magnitude of the error signal, and the gain is greater for error signals below a ventilation target than for error signals above the ventilation target value. The target value (52) is an alveolar ventilation that takes into account the patient's physiologic dead space.