Ventilating system for supplying a patient with respiratory gas and method for triggering the respiratory phases in said system

A ventilating system for supplying a patient with respiratory gas has an apparatus for measuring a respiratory gas flow curve 27 and a respiratory gas pressure curve 28 and has an evaluation circuit 8 for processing at least the respiratory gas flow curve and switching over the respiratory phases. As switchover values from one respiratory phase to the other, the evaluation circuit 8 utilizes predetermined trigger thresholds T.sub.IL, T.sub.EL of the respiratory gas flow curve 27. The ventilating system is improved in such a manner that a good adaptation thereof to the breathing effort of the patient 2 is provided even when leakage is present. This task is solved in that, in the evaluation circuit, the respiratory gas flow curve 27 and the respiratory gas pressure curve 28 are logically coupled during both the inspiratory phase and the expiratory phase. This logic operation takes place in accordance with a predetermined function. From the foregoing, an inspiratory trigger ancillary threshold Z.sub.I and an expiratory trigger ancillary threshold Z.sub.E are computed as an additional switchover criterion. These trigger ancillary thresholds Z.sub.I and Z.sub.E are added to the trigger thresholds T.sub.IL and T.sub.EL, respectively. A method for triggering the respiratory phase in the system is also disclosed.

FIELD OF THE INVENTION 
The invention relates to a ventilating system for supplying a patient with 
respiratory gas and includes a device for measuring a curve of respiratory 
gas flow and a curve of respiratory gas pressure. The system further 
includes an evaluation circuit which switches between respiratory phases 
and processes at least the curve of respiratory gas flow. The evaluation 
circuit utilizes predetermined trigger thresholds of the respiratory gas 
flow curve as values for switchover from one respiratory phase to the 
other. The invention also relates to a method for triggering the 
respiratory phases in said system. 
BACKGROUND OF THE INVENTION 
U.S. Pat. No. 5,303,700 discloses a method for detecting the respiratory 
phases of a patient and for controlling a ventilating apparatus with 
trigger thresholds obtained from the respiratory gas flow curve. In the 
ventilating apparatus disclosed in this patent, a significant increase of 
the steepness of the respiratory gas flow curve between respective zero 
crossovers of the respiratory gas flow is used as a trigger threshold. 
It is also known to utilize the drop of the respiratory gas flow to 25% of 
the maximum value as a trigger criterion for the switchover from 
inspiration to expiration. The respiratory gas pressure is measured 
synchronously to the respiratory gas flow and evaluated. 
Problems with respect to the switchover between the respiratory phases can 
occur when there is leakage, for example, leakages within the system 
because a mask is not seated sufficiently tight or because of an unblocked 
tube or even because of leakages caused by a fistula in the patient. The 
above-mentioned switchover is referred to specific function values of the 
respiratory gas flow curve. 
In a pressure-controlled ventilating operation, a compensating flow is 
metered during expiration and during inspiration to compensate for 
leakages of this kind. In this way, the respiratory pressure can be 
maintained at the preselected value. A leakage compensation of this kind 
is disclosed, for example, in the German publication entitled "Evita 2, 
Das universelle Beatmungsgerat fur die Intensivtherapie" published by 
Dragerwerk Aktiengesellschaft as publication no. 9048262, pages 14 and 15. 
However, by metering the compensating flow, the zero crossovers of the 
respiratory gas flow curve are shifted whereby the synchronization with 
the respiratory gas efforts of the patient is affected. 
The drop of the respiratory gas flow to 25% of the maximum value can be 
used as a switchover criterion. However, if this switchover criterion is 
used, for example, in an assisted ventilation and there is large leakage 
present, then the switchover is not triggered when the leakage lies above 
this 25%, limit. For this case, it is known to input a maximal inspiration 
time as an additional switchover criterion in accordance with which a 
switchover to the expiration takes place in each case. A criterion of this 
kind is disclosed, for example, in the German publication entitled 
"Gebrauchsanweisung Evita Intensivpflege-Ventilator", pages 82 and 83 
(June 1991), and published by Dragerwerk Aktiengesellschaft of Germany. 
The leakage flow is generally not constant and is, however, dependent upon 
respiratory gas pressure in each case. For this reason, no satisfactory 
criteria for the switchover of the respiratory phases can be obtained only 
from the trigger thresholds obtained from the respiratory gas flow curve 
or additional time inputs. 
U.S. Pat. No. 3,903,881 discloses a ventilating system wherein the leakage 
at the lower pressure level is compensated by a compensating flow. The 
next inspiration stroke is triggered when the inspiration flow exceeds an 
adjusted trigger threshold. The trigger threshold must then be so adjusted 
that it lies just above the compensating flow. The trigger threshold must 
be adjusted for each change of the respiratory gas pressures because the 
leakage flow is influenced by the ventilating pressure. For a trigger 
threshold which is set too low, the next inspiration stroke is triggered 
too soon. On the other hand, the system becomes insensitive when the 
trigger threshold is set too high and the trigger threshold is only 
exceeded with the first forceful inhalation. 
SUMMARY OF THE INVENTION 
It is an object of the invention to improve a ventilating system of the 
kind described above so that a good adaptation of the ventilating system 
to the breathing effort of the patient is provided even when leakages are 
present. 
The ventilating system of the invention is for supplying a patient with 
respiratory gas during inspiratory and expiratory phases of breathing. The 
ventilating system includes: a pressure measuring device for measuring a 
first curve representing respiratory gas pressure; a flow measuring device 
for measuring a second curve representing respiratory gas flow; valve 
means for providing predetermined pressures of the respiratory gas during 
the inspiratory and expiratory phases; an evaluation circuit for 
processing the curves and for controlling the valve means to switch over 
between the phases in dependence upon predetermined trigger thresholds 
T.sub.IL and T.sub.EL of the second curve; and, the evaluation circuit 
including means for executing a logic operation pursuant to a 
predetermined function between the first and second curves during the 
inspiratory and expiratory phases to form an inspiratory ancillary trigger 
threshold Z.sub.I and an expiratory ancillary trigger threshold Z.sub.E 
and to add the ancillary trigger thresholds (Z.sub.I and Z.sub.E) to the 
inspiratory and expiratory thresholds (T.sub.IL and T.sub.EL), 
respectively, to form composite trigger thresholds (T.sub.IL +Z.sub.I) and 
(T.sub.EL +Z.sub.E) for controlling the switchover between the phases. 
The method of the invention is for triggering breathing phases in a 
ventilating system for supplying a patient with respiratory gas during 
inspiratory and expiratory phases of breathing. The method includes the 
steps of: adjusting an inspiratory pressure P.sub.I during an inspiratory 
phase and adjusting an expiratory pressure P.sub.E during an expiratory 
phase; measuring a first curve representing respiratory gas pressure; 
measuring a second curve representing respiratory gas flow; providing 
predetermined trigger thresholds T.sub.IL and T.sub.EL of the second curve 
for use in forming switchover values for switching over between the 
phases; determining a volume V.sub.MI of respiratory gas averaged over the 
inspiratory phase and a volume V.sub.ME of respiratory gas averaged over 
the expiratory phase; determining the mean values of the inspiratory 
pressure P.sub.I and the expiratory pressure P.sub.E ; forming an 
inspiratory ancillary trigger threshold Z.sub.I from at least the product 
of the mean expiratory pressure P.sub.E and the difference (V.sub.MI 
-V.sub.ME ); forming an expiratory ancillary trigger threshold Z.sub.E 
from at least the product of the mean inspiratory pressure P.sub.I and the 
difference (V.sub.MI -V.sub.ME ); switching from the inspiratory phase to 
the expiratory phase in response to a drop of the second curve to a 
composite trigger threshold (T.sub.EL +Z.sub.E ); and, switching from the 
expiratory phase to the inspiratory phase in response to an increase of 
the second curve to a composite trigger threshold (T.sub.IL +Z.sub.I). 
The advantage of the invention is seen essentially in that the switchover 
values are dynamically adapted to the compensating flow metered for 
compensating a leakage caused either by the patient or by the system. This 
adaptation is provided in that an additional criterion in the form of an 
inspiration trigger ancillary threshold Z.sub.I and an expiration trigger 
ancillary threshold Z.sub.E is obtained by logically coupling the 
respiratory gas flow to the respiratory gas pressure. With this criterion, 
an adaptation of the respiratory phase switchover time points of a system 
having leakage to the switchover time points of a leakage-free system is 
possible. The magnitude and the time-dependent trace of the leakage flow 
do not have to be known. 
The coupling between the respiratory gas flow and the respiratory gas 
pressure advantageously have the following form: 
##EQU1## 
wherein: V.sub.MI is the value of the respiratory gas flow V averaged over 
the inspiration; V.sub.ME is the value of the respiratory gas flow V 
averaged over the expiration. The respiratory pressure P.sub.I is the 
averaged respiratory pressure during the inspiration and P.sub.E is the 
averaged respiratory pressure during the expiration. The new trigger 
thresholds are T.sub.EL plus Z.sub.E and T.sub.IL plus Z.sub.I.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION 
FIG. 1 shows a ventilating system 1 for supplying a patient 2 with 
respiratory gas via an inspiration line 3 and for discharging the exhaled 
gas via an expiration line 4 and a PEEP valve 5 leading to an expiration 
outlet 6. The PEEP valve 5 adjusts an expiration pressure P.sub.E on the 
patient during the expiration. The PEEP valve 5 is driven by a linear 
drive 7 which is connected via a signal line 13 to an evaluation circuit 8 
of the ventilating system 1. The inspiration line 3 and the expiration 
line 4 come together in a Y-piece 9 from which a common respiratory gas 
line 10 leads to the patient 2. The respiratory gas line 10 conducts 
respiratory gases for both inspiration and expiration in common. 
A respiratory gas pressure sensor 11 is connected into the inspiratory line 
3 to measure the respiratory gas pressure P and a respiratory gas flow 
sensor 12 is mounted in the respiratory gas line 10 to measure the 
respiratory gas flow V. The sensors (11, 12) are connected via signal 
lines 13a and 13b, respectively, to the evaluation circuit 8. In this 
connection, the respiratory gas flow V is understood to be the 
time-dependent discharge of the respiratory gas volume, that is, the 
volume per unit of time. 
The evaluation circuit 8 includes a microprocessor (not shown) which 
functions as a central control unit of the ventilating system 1. The 
microprocessor stores and evaluates the respiratory gas pressure signals 
supplied by sensor 11 and the respiratory gas flow signals supplied by 
sensor 12. The microprocessor also controls the respiratory phases and 
controls the respiratory gas flow control valve 15 for supplying the 
patient 2 with respiratory gas. A desired-value transducer 14 is connected 
to the evaluation circuit 8 for supplying the inspiratory pressure P.sub.I 
and the expiratory pressure P.sub.E. The desired-value transducer 14 can 
be adjusted to preselected values of P.sub.I and P.sub.E by the operator. 
The respiratory gas flow V to the patient 2 is metered via the respiratory 
gas flow control valve 15 which is connected to a pressure gas source 30. 
During inspiration, the respiratory gas flow control valve 15 receives 
input values from a ramp generator 16 in such a manner that, during 
inspiration, a pregiven inspiratory pressure P.sub.I is present and, 
during expiration, and together with the PEEP valve 5, a pregiven 
expiratory pressure P.sub.E is provided. 
The respiratory gas flow V and the respiratory gas pressure P are 
represented graphically in FIG. 2 as functions of time (t). Thus, 
reference numeral 17 identifies the respiratory gas flow curve and 
reference numeral 18 identifies the respiratory gas pressure curve. The 
curves 17 and 18 are presented in ideal form, that is, the respiratory gas 
flow V reaches the maximum inspiration flow V.sub.I as maximum value in a 
very short time and the respiratory gas pressure P increases to the 
inspiratory gas pressure P.sub.I at time point t.sub.1 likewise in a short 
time. 
From curve 17, it can be seen that the respiratory gas flow V becomes 
decreasingly and continuously less between the points (19, 20), that is, 
between time point t.sub.1 and t.sub.2, in dependence upon the fill level 
of the lungs of the patient 2. The respiratory gas flow V has dropped to 
25% of the maximum inspiratory flow V.sub.I at point 20. The time point 20 
is an expiratory trigger threshold T.sub.E and is the time point at which 
the switchover from the inspiratory phase to the expiratory phase takes 
place. The expiratory trigger threshold T.sub.E is stored in the 
evaluation circuit 8. 
At the start of the expiratory phase (that is, starting at time point 
t.sub.2, namely, point 20), the sign of the curve 17 reverses and the 
respiratory gas flow V reduces continuously from a maximum expiratory flow 
V.sub.E down to zero at time point t.sub.3 (at point 21). The zero 
crossover of the respiratory gas flow V is an inspiratory trigger 
threshold T.sub.I which is likewise stored in the evaluation circuit 8 and 
operates to initiate a new inspiratory stroke. In the embodiment described 
above, the time interval between t.sub.2 and t.sub.3 is approximately 
twice as long as the time interval between t.sub.1 and t.sub.2 so that the 
breathing time ratio between inspiration and expiration is approximately 
1:2. 
The curves (17, 18) shown in FIG. 2 refer to a so-called tight ventilating 
system 1, that is, without leakage so that the leakage flow V.sub.L is 
zero. In a real ventilating system 1, leakages as a rule do occur. These 
leakages can be caused, for example, by a mask 22 which is not seated 
tightly on the face of the patient 2 or by fistulas in the patient's 
lungs. If, for example, a leakage flow V.sub.L flows in the direction of 
arrows 26 because of the loose mask 22 into the ambient, then, if the same 
inspiratory and expiratory pressures P.sub.I and P.sub.E are to be 
reached, a higher maximum inspiratory flow V.sub.IL is needed during the 
inspiration and, during expiration, additional respiratory gas must be 
metered into the inspiratory line 3 in order to maintain the expiratory 
pressure P.sub.E constant. Contributing to the difficulty in this 
situation is that the leakage flow V.sub.L is generally dependent upon 
pressure, that is, V.sub.L is greater during inspiration than during the 
expiration. The letter "L" refers here to the ventilating system 1 with 
leakage. 
FIG. 3 shows, as an example, a respiratory gas flow curve 27 and a 
respiratory gas pressure curve 28 for a ventilating system 1 wherein 
leakage is present. For a better overview, the curves 17 and 18 of FIG. 2 
are shown in FIG. 3 in phantom outline. 
Because of the leakage flow V.sub.L, the respiratory gas flow curve 27 now 
increases to the maximum value V.sub.IL (point 23) and then drops 
continuously to point 24. The expiratory trigger threshold T.sub.EL is 
reached when the respiratory gas flow has dropped to 25% of the maximum 
value V.sub.IL, that is, at point 24. A switchover to the expiratory phase 
takes place at point 24. Since the leakage flow V.sub.L is likewise 
present during expiration, additional expiratory gas must be metered into 
the inspiratory line 3 via the respiratory gas flow control valve 15 in 
order to maintain the constant expiratory pressure P.sub.E. Because of 
this metering of respiratory gas, the respiratory gas flow curve 27 does 
not approach the abscissa asymptotically as does the curve 17; instead, 
the respiratory gas flow curve 27 intersects the abscissa at point 25 and 
thereafter assumes a positive sign. 
Since the zero crossover of the respiratory gas flow V is stored in the 
evaluation circuit 8 as the inspiratory trigger threshold T.sub.IL also 
for the ventilating system 1 subjected to leakage, the ventilating system 
1 would execute a new inspiratory stroke at time point t.sub.31 (point 
25). This new inspiratory stroke is not shown in FIG. 3 so that a better 
overview is provided. 
T.sub.I and T.sub.IL are numerically the same in the above-described case 
because the trigger criterion is the zero crossover of the respiratory gas 
flow. They are, however, different when the trigger criterion is, for 
example, referred to a specific percentage component of a respiratory gas 
flow. If the respiratory gas pressure curve with leakage 28 is compared to 
the respiratory gas pressure curve without leakage 18, then, for a 
ventilating system 1 with leakage and the trigger thresholds T.sub.IL and 
T.sub.EL in accordance with the state of the art, the inspiratory time is 
lengthened, that is, the patient inhales too long and the expiratory time 
is shortened and the patient receives the next inhalation stroke during 
the expiratory phase even without spontaneous breathing. For a better 
overview, the inspiratory stroke is not shown in the respiratory gas 
pressure curve 28 which would otherwise start at time point t.sub.31. 
The invention now provides that the trigger thresholds T.sub.IL and 
T.sub.EL are corrected by including trigger ancillary thresholds Z.sub.I 
and Z.sub.E in such a manner that the switchover of the respiratory phases 
takes place at the time points t.sub.1, t.sub.2 and t.sub.3, points (19, 
20, 21) as with a leakage-free ventilating system 1. 
The inspiratory trigger ancillary threshold Z.sub.I and the expiratory 
trigger ancillary threshold Z.sub.E can be derived from a functional 
relationship between the respiratory gas pressure P and the leakage flow 
V.sub.L. This functional relationship can be a linear dependency between 
the respiratory gas pressure P and the leakage flow V.sub.L as it is 
present in a laminar flow or the root of the leakage flow V.sub.L is 
proportional to the respiratory gas pressure P as, for example, for a 
turbulent flow. 
Since the leakage flow V.sub.L cannot be measured directly, a mean value of 
the respiratory gas flow V is computed in the evaluation circuit 8 as 
V.sub.MI and V.sub.ME during inspiration as well as during expiration and 
the mean respiratory gas pressures P.sub.I and P.sub.E are measured. 
For the exception of a laminar leakage flow V.sub.L, the trigger ancillary 
thresholds Z.sub.I and Z.sub.E result as follows: 
##EQU2## 
The switchover between the inspiratory phase and the expiratory phase and 
vice versa takes place, in a ventilating system 1 having leakage, at the 
trigger thresholds T.sub.EL plus Z.sub.E and T.sub.IL plus Z.sub.I. The 
mean inspiratory volume V.sub.MI is approximately equal to the expiratory 
volume V.sub.ME for a leakage-free ventilating system 1. For this reason, 
Z.sub.I and Z.sub.E are zero for this case. 
It is understood that the foregoing description is that of the preferred 
embodiments of the invention and that various changes and modifications 
may be made thereto without departing from the spirit and scope of the 
invention as defined in the appended claims.