Surgery – Respiratory method or device – Means for supplying respiratory gas under positive pressure
Reexamination Certificate
2000-03-24
2002-08-20
Dawson, Glenn K. (Department: 3761)
Surgery
Respiratory method or device
Means for supplying respiratory gas under positive pressure
C128S204180, C128S200240
Reexamination Certificate
active
06435182
ABSTRACT:
BACKGROUND OF THE INVENTION
For patients maintained on artificial mechanical ventilation, a primary concern of the clinician is the mechanical status of the lungs, which can often be inferred from estimates of total respiratory or pulmonary resistance (R) and elastance (E). Several techniques exist to assess R and E during mechanical ventilation at a given frequency, tidal volume, or mean airway pressure. Bates, J. H. T., and A. M. Lauzon, “A nonstatistical approach to estimating confidence intervals about model parameters: application to respiratory mechanics”,
IEES Trans. Biomed. Eng.
39: 94-100, 1992; Kaczka, D. W., G. M. Barnas, B. Suki, and K. R. Lutchen, “Assessment of time-domain analyses for estimation of low-frequency respiratory mechanical properties and impedance spectra”,
Ann. Biomed. Eng.
23: 135-151, 1995; Lutchen, K. R., D. W. Kaczka, B. Suki, G. Bamas, G. Cevenini, and P. Barbini, “Low-frequency respiratory mechanics using ventilator-driven forced oscillations”;
J. Appl. Physiol.
75: 2549-2560, 1993; and Peslin, R., J. Felico de Silva, F. Chabot, and C. Duvivier, “Respiratory mechanics studied by multiple linear regression in unsedated ventilated patients”,
Eur. Respir. J.
5: 871-878, 1992.
Such estimates, however, do not permit inference on the level or distribution of obstruction in the airways, or the relative stiffness or viscance of the lung or respiratory tissues. Recent studies have suggested that the frequency dependence of R and E in breathing rates from 0.1 to 8 Hz embodies information needed to partition airway and tissue mechanical properties, or establish the dominant pattern of constriction in the airway tree. Petak, F., Z. Hantos, A. Adamicza, and B. Daroczy, “Partitioning of pulmonary impedance: modeling vs. alveolar capsule approach”,
J. Appl. Physiol.
75: 513-521, 1993; Lutchen, K. R., and H. Gillis, “Relationship between heterogeneous changes in airway morphometry and lung resistance and elastance”,
J. Appl. Physiol.
83: 1192-1201, 1997.
The frequency dependence of R and E can be derived from input impedance (Z), the complex ratio of pressure to flow during external forcing as a function of frequency. It is difficult to measure Z at low frequencies in ventilator-dependent patients, especially those with chronic obstructive pulmonary disease (COPD). Measurements relying on small-amplitude forced oscillations generated by loud-speakers require that the patient be disconnected from ventilatory support, which becomes impractical if low frequency information is desired, and becomes dangerous in patients with COPD. Navajas, D., R. Farre, J. Canet, M. Rotger, and J. Sanchis, “Respiratory input impedance in anesthetized paralyzed patients”,
J. Appl. Physiol.
69 1372-1379, 1990. Other investigators have used spectral analysis on standard volume-cycled ventilator waveforms to extract impedance information without disrupting mechanical ventilation (see Barnas, G. M., P. Harinath, M. D. Green, B. Suki, D. W. Kaczka, and K. R. Lutchen, “Influence of waveform and analysis technique on lung and chest wall properties”,
Respir. Physiol.
96: 331-344, 1994), but these attempts have been unsuccessful due to the waveforms' poor signal-to-noise ratio at harmonics above the frequency of ventilation as well as nonlinear harmonic distortion of the resulting pressure signal. Suki, B., and K. R. Lutchen, “Pseudorandom signals to estimate apparent transfer and coherence functions of nonlinear systems: applications to respiratory mechanics”,
IEEE Trans. Biomed. Eng.
39: 1142-1151, 1992 Recent studies have demonstrated that a broad-band Optimal Ventilator Waveform (OVW) can be used to measure Z in awake subjects with mild-to-moderate obstruction during tidal-like excursions with minimal harmonic distortion. Kaczka, D. W., E. P. Ingenito, B. Suki, and K. R. Lutchen, “Partitioning airway and lung tissue resistances in humans: effects of bronchoconstriction”,
J. Appl. Physiol.
82: 1531-1541, 1997; Lutchen, K. R., K. Yang, D. W. Kaczka, and B. Suki, “Optimal ventilation waveforms for estimating low-frequency respiratory impedance”,
J. Appl. Physiol.
75: 478-488, 1993. However the original OVW is oscillatory, which presents two problems. First, the active expiratory component of the OVW makes it impossible to use in patients whose obstruction may be so severe that their airways can be dynamically compressed during expiration. Mead, J., I. Lindgren, and E. A. Gaensler, “The mechanical properties of the lungs in emphysema”,
J. Clin. Invest.
34: 1005-1016, 1955. Second, it is generated via a closed system in which no fresh gas is delivered to the patient, and therefore long-term ventilation is impossible.
Besides the experimental limitations of measuring Z, an additional problem arises in its physical interpretation in patients with COPD. Regardless of the methods used to generate or acquire the data, Z has been computed using traditional spectral methods, which assume system linearity and time invariance. Daroczy, B., and Z. Hantos, “An improved forced oscillatory estimation of respiratory impedance”,
Int. J. Bio.
-
Med. Comput.
13: 221-235, 1982; Michaelson, E. D., E. D. Grassman, and W. Peters, “Pulmonary mechanics by spectral analysis of forced random noise”,
J. Clin. Invest.
56: 1210-1230, 1975. Depending on the patient's pathology, such assumptions can be grossly invalid. For example, the phenomena of dynamic airway compression and expiratory flow limitation are highly nonlinear processes, in which flow is no longer determined by the pressure drop across the airways. Fry, D. L., R. V. Ebert, W. W. Stead, and C. C. Brown, “The mechanics of pulmonary ventilation in normal subjects and in patients with emphysema”,
American Journal of Medicine
16: 80-97, 1954. In general, linear approximations in patients with severe obstruction must be restricted to data in which such processes are known to be absent, such as inspiration.
Current mechanical ventilators are operable in passive or active modes. With active modes, an effort by the patient triggers the delivery of a breath. With passive modes, only the ventilator is active and it delivers the breath at a pre-set breathing rate (frequency), volume per breath (tidal volume (VT)) and waveform. The most common active mode is referred to as volume support. With volume support, a pre-set flow wave shape is delivered via a ventilator pressure source during inspiration. Ventilator controlled solenoid valves then enable the patient to passively expire to the atmosphere. Current volume support waveforms include a) a step to a constant flow rate (i.e. step waveform) delivered to produce the desired VT within the inspiratory time (TI); b) a ramp in which a peak flow is delivered immediately in the inspiration followed by a linear decrease in flow for the remainder of TI; and c) a sinusoid in which peak flow is reached proximal to the middle of inspiration.
The primary goal of all waveforms is to maintain blood gas levels (O
2
and CO
2
) to sustain normoxia and nomocapnia. Because ventilation via pressure produced at the mouth is not natural, another requirement is that the ventilator not produce excessive pressure at the airway opening. This can lead to barotrauma (intralung airway damage), which in turn can cause respiratory distress.
During mechanical ventilation, the clinician is concerned (among other things) with the mechanical status of the patient. Specifically, it is desirable to estimate the degree of airway obstruction or constriction, the distribution of constriction in the airways, and the relative stiffness and viscosity of the lung tissues. This assessment can be made by examining the resistance R and elastance E at several frequencies surrounding normal breathing rates (i.e., from about 0.1-8 Hz). The behavior of the R and E spectra over this frequency range are very distinct for particular forms and degrees of lung disease. Such information is helpful in a) determining the severity of any lung disease that is present and its response to therapy and the mechanical ventilation itself; b)
Kaczka David W.
Lutchen Kenneth R.
Suki Bela
Dawson Glenn K.
Samuels , Gauthier & Stevens, LLP
Trustees of Boston University
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