Surgery – Respiratory method or device – Means for supplying respiratory gas under positive pressure
Reexamination Certificate
2002-09-03
2004-11-23
Bennett, Henry (Department: 3743)
Surgery
Respiratory method or device
Means for supplying respiratory gas under positive pressure
C128S204180, C128S202220
Reexamination Certificate
active
06820618
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the respiratory care of a patient and, more particularly, to a ventilator that monitors the pressure of the breathing gas supplied to and exhaled from the patient, and controls the pressure and/or flow rate of the breathing gas supplied by the ventilator to the patient to nullify the work of breathing imposed on the patient by the breathing apparatus.
2. Prior Art
Mechanical ventilatory support is widely accepted as an effective form of therapy and means for treating patients with respiratory failure. Ventilation is the process of delivering oxygen to and washing carbon dioxide from the alveoli in the lungs. When receiving ventilatory support, the patient becomes part of a complex interactive system which is expected to provide adequate ventilation and promote gas exchange to aid in the stabilization and recovery of the patient. Clinical treatment of a ventilated patient often calls for monitoring a patient's breathing to detect an interruption or an irregularity in the breathing pattern, for triggering a ventilator to initiate assisted breathing, and for interrupting the assisted breathing periodically to wean the patient off of the assisted breathing regime, thereby restoring the patient's ability to breath independently.
A patient whose breathing is being supported by a ventilator typically receives breathing gas through a ventilator conduit. The ventilator conduit generally consists of two flexible conduits, an inhalation conduit and an exhalation conduit, connected to a wye fitting. The free ends of the conduits are attached to the ventilator so that the inhalation conduit receives breathing gas from the ventilator's pneumatic system and the exhalation conduit returns gas exhaled by the patient to the ventilator. The wye fitting is typically connected to the patient's breathing attachment, which is oftentimes an endotracheal tube, which conducts breathing gas into the lungs of the patient, and exhaled gas from the lungs of the patient to the exhalation conduit.
In those instances where a patient requires mechanical ventilation due to respiratory failure, a wide variety of mechanical ventilators are available. Most modern ventilators allow the clinician to select and use several modes of inhalation either individually or in combination. These modes can be defined in three broad categories: spontaneous, assisted or controlled. During spontaneous ventilation without other modes of ventilation, the patient breathes at his own pace, but other interventions may affect other parameters of ventilation including the tidal volume and the baseline pressure, above ambient, within the system. In assisted ventilation, the patient initiates the inhalation by lowering the baseline pressure, and then the ventilator “assists” the patient by completing the breath by the application of positive pressure. During controlled ventilation, the patient is unable to breathe spontaneously or initiate a breath, and is therefore dependent on the ventilator for every breath.
During spontaneous or assisted ventilation, the patient is required to “work” (to varying degrees) by using the respiratory muscles in order to breath. The work of breathing can be measured and quantified in Joules/L of ventilation. In the past, techniques have been devised to supply ventilatory therapy to patients for the purpose of improving patient efforts to breath by decreasing the work of breathing to sustain the breath. Still other techniques have been developed that aid in the reduction of the patient's inspiratory work required to trigger a ventilator system “ON” to assist the patient's breathing. It is desirable to reduce the effort expended by the patient in each of these phases, since a high work of breathing load can cause further damage to a weakened patient or be beyond the capacity or capability of small or disabled patients.
The work of breathing of a patient breathing spontaneously while intubated and attached to the ventilator during ventilatory support by may be divided into two components: first, the imposed work of breathing of the breathing apparatus; and second, the physiologic work of breathing of the patient. The imposed work of breathing is the resistive work of breathing imposed by the breathing apparatus (the physical construct of the entire ventilation support external to the patient' lungs, i.e., the endotracheal tube, the ventilator conduit, the medical ventilator, etc.) upon the spontaneously breathing patient receiving ventilator support. The physiologic work of breathing of the patient consists of two components: first, the resistive work of breathing of the airways of the patient, and two, the elastic work of breathing of the lungs and the chest wall. It is desirable to reduce or, even more desirable, to nullify the imposed work of breathing as the patient may be detrimentally affected by an excessively high expenditure of energy early in the inspiration process caused by the respiratory muscle force required to overcome the imposed work of breathing of the breathing apparatus. Patient's may fatigue under the imposed work of breathing workload which predisposes the patient to respiratory muscle fatigue, respiratory distress, respiratory or ventilator dependancy, and/or failure. Nullification of the imposed work of breathing also allows for the contemporaneous determination of the physiologic work of breathing.
The conventional methods and apparatus for reducing or minimizing the imposed work of breathing are inadequate. Typically, these conventional efforts rely upon a means of “triggering” the ventilator to supply inspiratory ventilation support upon the sensing of an inspiration effort. The conventional means for triggering the ventilator may be classified as either pressure or flow-by triggering. In conventional pressure triggering, the withdrawal of the small volume of gas that occurs as a breath is initiated by the patient results in a corresponding drop in pressure which is monitored via a pressure sensor that is typically disposed within the ventilator conduit at or near the wye piece or within either the inhalation conduit or the exhalation conduit. At the onset of spontaneous inhalation by the patient, the pressure change is detected in the breathing circuit which functions to trigger the ventilator “ON” to then actively inflate the lungs of the patient during ventilation support. Several disadvantages are associated with the use of conventional pressure triggering to reduce the imposed work of breathing. First, the chosen pressure measurement sites produce a pressure signal that measures the pressure of the breathing gas proximate the sensors which is remote from the actual intratracheal pressure drop occurring within the patient's trachea throughout the spontaneous inhalation effort. The measured sensors are then used as a basis for regulating or controlling the amount of pressure or flow rate (to generate the requisite pressure) of breathing gas applied to the lungs. Because the chosen sites are so remote from the lungs of the patient, the resulting pressure measurements are an inherently inaccurate measurement of the pressure on the airways and lungs of the patient which causes a marked increase in the effort or work to inhale by the patient as the regulated amount of breathing gas applied to the patient is calculated in error due to the “approximated” value of the pressure drop sensed.
Second, and once again because of the remote pressure sensing measurement sites, in conventional pressure triggering there is a significant amount of lag time and associated negative pressure that always occurs between the onset of the patient's inspiratory effort and the time that the gas pressure or flow reaches the patient's airway. This lag time is generally referred to as a ventilator's response time, and commonly occupies a small but significant portion of a patient's total inspiration time. The pressure waves that are indicative of the pressure
Banner Michael Joseph
Blanch Paul Bradford
Akerman & Senterfitt
Ragonese Andrea M.
University of Florida Research Foundation Incorporated
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