Surgery – Respiratory method or device – Means for supplying respiratory gas under positive pressure
Reexamination Certificate
1999-05-21
2003-03-18
Lewis, Aaron J. (Department: 3761)
Surgery
Respiratory method or device
Means for supplying respiratory gas under positive pressure
C128S204180
Reexamination Certificate
active
06532959
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to methods and apparatus for the provision of positive pressure ventilatory assistance for patients with cardiac failure or Cheyne-Stokes breathing from any cause, including central sleep apnea, cardiac failure or stroke.
EXPLANATION OF TERMS
In this specification, respiratory airflow is intended to refer to the instantaneous flow of gas into or out of the lungs. The term “average” is intended to mean any measure of central tendency or the result of any low pass filtering operation. Ventilatory support is intended to mean any procedure which has a similar effect as the respiratory muscles, particularly the supply of breathable gas under varying positive pressure to the airway via a nosemask, face mask, endotracheal tube, tracheotomy tube, or the like, but also including other procedures such as negative pressure ventilation, cuirasse, iron lung, external chest compression, or rocking bed ventilation. According to common usage, ventilation can mean either a procedure, as in the expression “positive pressure ventilation”, or a measure of average respiratory airflow over a period of time. Instantaneous ventilation is intended to mean the volume inspired over a short period of time less than several seconds. Equally it can be calculated as the volume expired over such a period, or it can be the average of the two. For example, measures of instantaneous ventilation would include half the average of the absolute value of the respiratory airflow, calculated over a time interval short compared with several seconds, or half the absolute value of the respiratory airflow, low pass filtered with a time constant short compared with several seconds. For technical reasons to be explained below, in the best embodiment, instantaneous ventilation is taken as half the absolute value of the instantaneous respiratory airflow, ie averaged over an arbitrarily short period of time. However, it is not intended that the invention is limited to calculating instantaneous ventilation in this way.
The term “varying A inversely with B” is intended in the broad sense of increasing A if B is decreasing, and decreasing A if B is increasing.
The term “servo-controller” here refers to a feedback controller accepting an input, or controlled, variable (for example actual measured ventilation) and a reference quantity (for example a desired or target ventilation), and producing an output (for example the settings of a ventilator) which is used to subsequently bring the value of the input (controlled) variable towards the value of the reference variable.
The term “oppose” can include reduce, limit dampen, or prevent.
The terms “recent average ventilation” and “longterm average ventilation” are to be understood to be equivalents.
BACKGROUND OF THE INVENTION
Patients with cardiac failure have reduced cardiac ejection fraction, are typically very breathless, and often wake at night with extreme breathlessness called paroxysmal nocturnal dyspnea, due to accumulation of fluid in the lungs.
Patients with cardiac failure also often have Cheyne-Stokes breathing, particularly during sleep. Cheyne-Stokes breathing is an abnormal limit cycle instability of the patient's respiratory controller in which there are rhythmic alternating periods of waxing and waning ventilation, causing repetitive deoxygenation and reoxygenation of the arterial blood. The cause of the waxing and waning of ventilation is not entirely clear, but there is an increase in chemoreceptor gain [Wilcox I et al. Ventilatory control in patients with sleep apnoea and left ventricular dysfunction; comparison of obstructive and central sleep apnoea. 1998; 11:7-13], possibly related to stimuli arising in the heart or lungs, a change in the chemoreceptor set point leading to overventilation and alkalosis in the awake state with apneas during sleep, and an increase in circulation time leading to delays between ventilation and chemoreception [Naughton M et al. Role of hyperventilation in the pathogenesis of central sleep apneas in patients with congestive heart failure.
Am Rev Respir Dis
1993; 148:330-338]. Cheyne-Stokes breathing is associated with high mortality [Andreas et al. Cheyne-Stokes respiration and prognosis in congestive heart failure.
Am J Cardiol
1996; 78:1260-1264]. It is possible that it is harmful because of the repetitive hypoxia, which will lead to hypoxic pulmonary vasoconstriction and high right heart afterload, and to increased sympathetic activity, systemic vasoconstriction and high left heart afterload. It may also be harmful because of repetitive alkalosis during the waxing period of the cycle. Finally, in some patients it is associated with repetitive arousal from sleep, which causes severe sleep disruption, increased sympathetic activity, and increased afterload.
Continuous positive airway pressure (CPAP) has been used for decades for the emergency treatment of pulmonary oedema, and is more recently being used longterm during sleep for the treatment of cardiac failure. Nasal CPAP leads to an improvement in cardiac output and ejection fraction, and an improvement in quality of life [Naughton MT et al, Treatment of congestive heart failure and Cheyne-Stokes respiration during sleep by continuous positive airway pressure.
Am J Respir Crit Care Med
1995; 151:92-97], and a reduction in sympathetic nervous system activity [Naughton MT et al, Effects of nasal CPAP on sympathetic activity in patients with heart failure and central sleep apnea.
Am J Respir Crit Care Med
1995; 152:473-479]. The precise mechanism of action is unclear. Making the alveolar pressure and right atrial pressure positive with respect to the inferior vena caval pressure, and making the left ventricular pressure more positive with respect to abdominal aortic pressure, will tend to dry the lungs, improve gas exchange, relieve paroxysmal nocturnal dyspnea, reduce reflex pulmonary vasoconstriction, reduce sympathetic activity and reduce cardiac afterload via multiple complex mechanisms. Standard nasal CPAP masks may also help stabilize Cheyne-Stokes breathing, because the effective ventilation cannot exceed the fresh gas flow, which is in turn set by the exhaust flow. Finally, many patients with cardiac failure also have coexisting obstructive sleep apnea, which worsens cardiac failure but is treated by nasal CPAP.
Unfortunately, despite excellent effectiveness, nasal CPAP is often poorly tolerated by patients with cardiac failure, particularly early on in treatment, and it has not become widely used. The reasons for the poor tolerance are unknown. In addition, nasal CPAP reduces, but unfortunately does not immediately suppress the Cheyne-Stokes breathing [Naughton MT et al. Effect of continuous positive airway pressure on central sleep apnea and nocturnal PCO2 in heart failure.
Am J Respir Crit Care Med
1994; 150:1598-1604].
Various other approaches using known methods of ventilatory assistance suggest themselves in order to provide the same benefit as CPAP while also reducing either respiratory work or Cheyne-Stokes breathing or both. Unfortunately no known device is completely satisfactory, either because of discomfort, overventilation, or both. For example,
FIG. 1
shows persistent Cheyne-Stokes breathing in a patient with cardiac failure being treated with bilevel ventilatory support with timed backup. The subject is in stage
3
non-REM sleep. The polygraph tracings are arterial haemoglobin oxygen saturation (top tracing), chest wall movement (middle tracing), and mask pressure (bottom tracing). The Cheyne-Stokes breathing persists. Note, in the middle trace, the cyclical waxing and waning of the amplitude of chest wall movement indicating periods of overbreathing and underbreathing, and resultant regular decreases in arterial haemoglobin oxygen saturation despite the ventilatory support.
Many classes of ventilator, far from increasing comfort, actually decrease comfort. Volume cycled ventilators (regardless of the trigger variable) and time triggered ve
Gottlieb Rackman & Reisman P.C.
Lewis Aaron J.
Mitchell Teena
ResMed Ltd.
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