Surgery – Diagnostic testing – Respiratory
Reexamination Certificate
1999-11-10
2002-01-29
Lacyk, John P. (Department: 3736)
Surgery
Diagnostic testing
Respiratory
C600S532000
Reexamination Certificate
active
06342040
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention pertains to a patient monitor for monitoring and/or quantitatively measuring a physiological characteristic of the patient, and, in particular, to an apparatus and method for monitoring and/or quantitatively measuring a physiological characteristic based, at least in part, on a pressure differential between a pressure within a user interface and an ambient atmospheric pressure outside the user interface.
2. Description of the Related Art
There are many situations in which it is necessary or desirable to measure a physiological characteristic of a patient, such as characteristics associated with respiration. Examples of characteristics associated with respiration include the patient's flow, inspiratory period, expiratory period, tidal volume, inspiratory volume, expiratory volume, minute ventilation, respiratory rate, ventilatory period, and inspiration to expiration (I to E) ratio. It is also important in many situations to identify still other characteristics associated with respiration, such as identifying the start, end and duration of a patient's inspiratory phase and expiratory phase, as well as detecting patient snoring. For example, when conducting a sleep study to diagnose sleep disorders or when conducting other pulmonary monitoring functions, it is common to measure the respiratory rate and/or the air flow to and from the patient. Distinguishing between inspiration and expiration is useful, for example, in triggering a pressure support device that provides breathing gas to a patient.
There are several known techniques for monitoring patient breathing for these purposes. A first conventional technique involves placing a thermistor or thermocouple in or near the patient's airway so that the patient's breath passes over the temperature sensing device. Breathing gas entering the patient has a temperature that is generally lower than the exhaled gas. The thermistor senses this temperature difference and outputs a signal that can be used to distinguish between inspiration and expiration.
A primary disadvantage of the thermistor or thermocouple air flow sensing technique is that these devices cannot quantitatively measure the flow and/or volume of breathing gas delivered to and/or exhaled from the patient, because the signal from the sensor is a measure of air temperature, not air flow or pressure. Typically, a thermistor air flow sensor is only used to differentiate between inspiration and expiration. Sensors that detect humidity have similar uses and similar disadvantages.
A second conventional technique for measuring the airflow to and from a patient is illustrated in FIG.
1
and involves placing a pneumotach sensor
30
in a breathing circuit
31
between a supply of breathing gas, such as a ventilator or pressure support device, and the patient's airway. In a conventional pneumotach, the entire flow of breathing gas Q
IN
is provided to a patient
32
from a pressure source
34
. Conversely, all of the gas expelled from patient
32
, passes through pneumotach
30
so that during operation, there is a two-way flow of gas through pneumotach
30
.
In its simplest form shown in
FIG. 1
, pneumotach
30
includes a flow element
36
having an orifice
38
of a known size defined therein. Flow element
36
provides a known resistance R to flow through the pneumotach so that a pressure differential &Dgr;P exists across of flow element
36
. More specifically, flow element
36
causes a first pressure P
1
on a first side of the flow element to be different than a second pressure P
2
on a second side of the flow element opposite the first side. Whether P
1
is greater than P
2
or vice versa depends on the direction of flow through the pneumotach.
In a first type of conventional pneumotach, a major portion Q
1
of the total flow Q
IN
of gas delivered to pneumotach
30
passes through orifice
38
. The pressure differential &Dgr;P created by flow element
36
causes a lesser portion Q
2
of the gas delivered to the pneumotach to be diverted through a bypass channel
40
, which is connected to breathing circuit
31
across flow element
36
. An airflow sensor
42
in bypass channel
40
measures the flow of gas therethrough. Because the area of orifice
38
and the area of bypass channel
40
are known and fixed relative to one another, the amount of gas Q
2
flowing through bypass channel
40
is a known fraction of the total gas flow Q
IN
delivered to pneumotach
30
. Airflow sensor
42
quantitatively measures the amount of gas Q
2
passing through bypass channel
40
. Once this quantity is known, the total flow Q
IN
of gas passing through pneumotach
30
can be determined.
In a second type of conventional pneumotach, a pressure sensor, rather than an airflow sensor, is provided in bypass channel
40
. Gas does not pass through the pressure sensor. Instead, each side of a diaphragm in the pressure sensor communicates with respective pressures P
1
and P
2
on either side of flow element
36
. The pressure sensor measures pressure differential &Dgr;P across flow element
36
. For example, for flow in the direction illustrated in
FIG. 1
, pressure differential &Dgr;P across flow element
36
is P
1
-P
2
. Once pressure differential &Dgr;P is known, the flow rate Q
IN
of gas passing through pneumotach
30
can be determined using the equation, &Dgr;P=RQ
2
, where R is the known resistance of flow element
36
.
Another conventional pneumotach
44
is shown in FIG.
2
. Pneumotach
44
improves upon pneumotach
30
in
FIG. 1
by providing a first linear flow element
46
in place of flow element
36
. First linear flow element
46
functions in the same manner as flow element
36
by creating a pressure differential in breathing circuit
31
. However, flow element
46
has a plurality of honey-comb like channels that extend in the direction of gas flow to linearize the flow of gas through the pneumotach. The previous flow element
36
in
FIG. 1
can create downstream turbulence that hinders the flow of gas through the bypass channel or causes fluctuations in the downstream pressure, thereby degrading the airflow or pressure differential signal output by sensor
42
. Flow element
46
solves this problem by providing a plurality of honeycomb-like channels having longitudinal axis parallel to the axis of the breathing circuit. The honeycomb channels ensure that the flow across the downstream port of the bypass channel is linear, i.e., non-turbulent.
To ensure that the flow of gas across the port in bypass channel
40
upstream of flow element
46
is also linear, i.e., non-turbulent, other linear flow elements
48
and
50
are provided in the breathing circuit Flow elements
48
and
50
have the same honeycomb configuration as flow element
46
. Because gas can flow in both directions through pneumotach
44
, flow elements
48
and
50
are respectively located on each side of flow element
46
so that each entry port for bypass channel
40
is downstream of one of these additional flow elements regardless of the direction of flow through the pneumotach.
Although a pneumotach improves upon a theremistor in that it quantatively measures the flow and/or volume of gas passing therethrough, it also has significant disadvantages. For example, a pneumotach is relatively complicated and therefore difficult and costly to manufacture. It is also difficult to clean and is relatively large. Because of its size, which is dictated by the need to measure the pressure differential or flow across the flow element in the breathing circuit, it creates a relatively large amount of dead space in the patient breathing circuit, which is not conducive to minimizing rebreathing of CO
2
. Because of its complexity, a pneumotach may leak, and its operating capabilities can suffer as a result of heat and moisture buildup.
A third type of conventional airflow meter, illustrated in
FIG. 3
, is a nasal cannula airflow meter
52
. Nasal cannula airflow meter
52
is similar to a nasal oxygen ca
Kane Michael T.
Scarberry Eugene N.
Starr Eric W.
Haas Michael W.
Lacyk John P.
Natnithithadha Navin
Respironics Inc.
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