Surgery – Diagnostic testing – Respiratory
Reexamination Certificate
2001-07-26
2003-09-02
Nasser, Robert L. (Department: 3736)
Surgery
Diagnostic testing
Respiratory
C128S204230, C600S538000
Reexamination Certificate
active
06612995
ABSTRACT:
The present invention relates to a method and an apparatus to determine the alveolar opening and/or closing of a lung.
Such a method and such an apparatus are especially useful to optimally set the control variables of an artificial ventilator as both the alveolar opening and the alveolar closing are important parameters of an atelectatic (=partially collapsed) lung.
In German intensive care units (ICUs), approximately 8.000-10.000 are artificially ventilated each day. The ventilator control variables, such as airway pressure (P
aw
) and respiratory rate (RR), are usually chosen based on known standard procedures, but often left constant afterwards and not adapted to the changing needs of a specific patient.
Today, the success of artificial ventilation is evaluated by using arterial blood gas analysis during which the partial pressures of oxygen (paO
2
) and carbon dioxide (paCO
2
) are determined. However, quite often these values are measured only 1-4 times a day. Since a human performs about 20.000 breath strokes per day, it becomes obvious that such a low “sampling rate” may not be sufficient to evaluate the status in critical and unstable patients.
Patients with an acute respiratoy distress syndrome (ARDS) usually belong to this group of critical patients. Despite all sucesses in intensive care medicine, ARDS still is a pathological state with a mortality of 50%. The basic patho-physiological mechanism is the lack of “surfactant”, a substance which reduces suface tension resulting in a collapse of major lung fractions and a dramatically reduced gas exchange area.
To prevent undesirable sequelae and consecutive multiorgan failure, one important goal of protective ventilator therapy should be a gentle and early “reopening” of the lung. Choosing the airway pressures properly has an important impact on this.
Through the identification of the alveolar opening and especially of the alveolar closing pressures, a distressed lung may be kept open by proper choice of the airway pressure. However, the manual determination of opening and closing pressures is arduous and time consuming. To use the present invention in clinical practice, an automatic, computerized strategy is strongly recommended.
Prior to citing known methods to identify a lung collapse, a basic introduction to artificial ventilation shall be given:
The major function of the lung is the gas exchange, i.e. providing sufficient O
2
to the circulation and eliminating CO
2
from the body. If a human is not capable to perform this gas exchange himself anymore, he must be ventilated artificially.
FIG. 1
shows the human bronchial tree and an enlargement of some human alveoles. As in spontaneous ventilation, during artificial ventilation fresh air must be transported via the conducting parts of the brochial tree into the respiratory zone of the lung. The gas exchange actually happens in the so called “alveoli”, grape-shaped structures with an average diameter of about 70 &mgr;m which are located in the termial part of the bronchial tree.
During spontaneous ventilation, contraction of the diaphragm produces a subathmospheric pressure within the lung which causes air to be sucked into the lung. By contrst, in most modern forms of artificial ventilation a positive airway pressure is applied to the patient which presses air into the lung (“excess pressure ventilation”).
There are two major forms of ventilatory support: assisted (=augmented) and mandatory (=controlled) artificial ventilation.
In augmented artificial ventilation, the activity of the patient is monitored, either by detecting inspiratory flows sufficient to trigger an artificial breath stroke or by allowing the patient to breathe on top of a basic mandatory ventilatory support. These ventilation modes are especially used during weaning from the ventilator. By contrast, controlled mechanical ventilation (i.e. artificial ventilation without spontaneous breathing activity) is usually applied to more severly ill patients in which complete control of the breathing is desirable or necessary.
There are two major forms of controlled mechanical ventilation, namely pressure- and volume-controlled ventilation.
During pressure-controlled ventilation, the airway pressure is kept at desired levels during inspiration as well as during expiration. The corresponding pressure levels may be named, “peak inspiratory pressure” (PIP) and “positive end-expiratory pressure” (PEEP). Note that the alveolar pressure P
alv
actually varies in between these two pressure levels.
FIG. 2
illustrates the time course of airway and alveolar pressure during pressure controlled ventilation.
On the ventilator, several control variables must be adjusted according to the patient needs including the respiratory rate (RR) and the inspiration to expiration ratio (I/E). The following eqn. describe the relationships
RR
=
1
T
insp
+
T
exsp
·
60
min
and
(
1
)
I
/
E
=
T
insp
T
exsp
(
2
)
with T
insp
the inspiration and T
exp
the expiration time. The inspired and exhaled volume during quiet breathing is named tidal volume (V
T
). Assuming a stationary operation and no leakage in the breathing system, V
T
is given by
V
T
=
∫
0
T
insp
V
.
atem
ⅆ
t
=
∫
T
insp
T
exsp
V
.
atem
ⅆ
t
(
3
)
During volume-controlled ventilation, a konstant air flow is applied during inspiration while expiration occurs passively against a given PEEP.
FIG. 3
illustrates the time course of airway pressure, alveolar pressure and air flow during volume-controlled ventilation.
Note that volume-controlled ventilation guarantees delivery of a certain tidal volume while pressure-controlled ventilation does not. For this reason, some clinicians still prefer this form of mandatory ventilation. However, depending on the actual lung condition there are major disadvantages. In patients with a stiff lung, for example, P
alv
may reach undesirable limits and cause barotrauma. Furthermore, due to lung inhomogenities, local lung air flows may arise (so called “Pendelluft”).
From Leonhardt, S., Böhm, S. and Lachmann, B. “Optimierung der Beatmung beim akuten Lungenversagen durch Identifikation physiologischer Kenngroessen”, Automatisierungstechnik (at), Vol. 46, No. 11, pp 532-539, 1998 as well as from U.S. Pat. Nos. 5,660,170, 5,738,090 and 5,752,509, it is known that the airway pressures required to open or close a specific lung can generally be identified from measurements of the arterial oxygen partial pressure (paO
2
). After the identification procedure, the authors suggest to ventilate above the closing pressure.
It is known that both the identification and the later selection of ventilator parameters for long-term ventilation can be accomplished automatically by using a computer. A major disadvantage of this known method is that the measurement of this physiological parameter requires expensive and very sensible catheter systems and introduce possible damage to the patient (infections, bleeding, etc.).
Object of the present invention is to automatically provide a setting of ventilator parameters in critical patients.
This object is solved by a method according to the claims 1, 5 and 6 as well as an apparatus according to the claims 9, 11 and 12. By using the new feedback signals as claimed in this invention, in claim 13 an apparatus is presented aiming at automatic protective artificial ventilation of human lungs.
The invention is based on the cognition that the hemoglobin oxygen saturation (SO
2
), the endtidal CO
2
concentration (etCO
2
) and the CO
2
output (the elimination of CO
2
volume from the body per unit time) can easily be obtained noninvasively and may be used, either solely or combined as parameters to identify the alveolar opening and closing pressure levels of the lung. An invasive arterial line is not necessary anymore. All three parameters may be measured outside the body and may well be used as feedback signals for automatic artificial ventilation.
Similar to using the arterial oxygen partial pressure (PaO
2
) as a parameter to
Bohm Stephan
Leonhardt Steffen
Nasser Robert L.
Wolf Greenfield & Sacks P.C.
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