Pressure control for constant minute volume

A method and apparatus for ventilating a patient wherein the inspiratory positive airway pressure (IPAP) is adjusted as a function of minute volume. The IPAP is gradually adjusted over extended periods of time in small increments to conform the patient's minute volume to a prescribed value. This gentle but effective approach ensures adequate ventilation while minimizing discomfort to the patient thereby rendering the system ideal for treating sleep disordered breathing in the homecare environment.

BACKGROUND OF THE INVENTION 
The present invention generally relates to ventilator systems that are used 
in assisting the respiration of a patient to treat disturbed breathing, 
snoring, mixed obstructive sleep apnea, and certain cardiovascular sleep 
conditions. More particularly, the present invention pertains to the 
control of the air pressure to which the patient is subjected to by the 
ventilator during each respiratory cycle to provide a system well-suited 
for homecare applications. 
Obstructive sleep apnea is a sleep disorder characterized by the relaxation 
of the airway, including the upper airway muscle tissue during sleep. When 
this occurs, the relaxed muscles can partially or completely block the 
patient's airway, a condition more prevalent in overweight patients. 
Partial blockage can result in snoring while complete blockage can result 
in sleep apnea. When complete blockage occurs, the patient's ventilation 
efforts do not result in the intake of air and the patient becomes oxygen 
deprived. In reaction, the patient begins to awaken and upon reaching a 
nearly awakened state, the upper airway muscles resume normal tension 
which clears the airway and allows inhalation to occur. The patient then 
falls back to the deeper sleep whereupon the upper airway muscles again 
relax and the apneic cycle repeats. Central apnea is a condition wherein 
no inspiratory effort occurs or is delayed. Both central apnea as well as 
obstructive sleep apnea may be as present simultaneously, a condition 
referred to as mixed apnea. Other breathing irregularities are known which 
involve apneic intervals, Cheyne-Stokes breathing, being an example 
thereof. 
In some patients, sleep apnea events can occur hundreds of times during a 
sleep session. As a consequence of the repetitive arousal to the nearly 
awakened state, the patient never achieves fully relaxed deep sleep and is 
deprived of REM (rapid eye movement) sleep. Additionally, the patient's 
blood oxygen falls to subnormal levels. People afflicted with sleep apnea 
are continually tired even after an apparently normal night's sleep, while 
the continual or repeated oxygen depravation may have an adverse affect on 
the patient's cardiovascular system. 
In order to treat obstructive sleep apnea, so-called continuous positive 
airway pressure (CPAP) systems have been devised in which prescribed 
levels of positive airway pressure are continuously imposed on the 
patient's airway. The presence of such positive pressures within the 
airway provides a pressure splint to offset the negative inspiratory 
pressure thereby maintaining tissue in position and the patient's airway 
open. The positive pressure is typically generated by a blower, the output 
of which is ducted to the patient and connected to the airway by a nasal 
pillow which seals with the patient's nares. Control valves in the system 
control the pressure to which the patient's airway is subjected. 
In prescribing the CPAP therapy, it is usually necessary for a patient to 
spend one or two nights in a sleep treatment laboratory where it is first 
determined whether the patient has a respiratory disorder such as sleep 
apnea. If so, the patient is then fitted with a CPAP device whereupon 
pressure and volume parameters are determined for providing the necessary 
air splint and satisfying the patient's respiratory requirements. 
A number of shortcomings, are associated with the previously known CPAP 
systems. Two fundamentally different approaches have heretofore been taken 
with respect to the manner in which the breathing is controlled each 
suffering from a number of disadvantages. Initially, ventilator systems 
were designed to deliver a predetermined volume during the inspiration 
phase of each breathing cycle. While this approach positively ensures 
adequate respiration even for patients completely incapable of breathing 
on their own, the rigorous routine is perceived as quite uncomfortable by 
patients requiring less breathing assistance. The prescribed volume of air 
is after all forced into the patient's airways without regard to the 
pressures that may be generated and independent of what rate the patient 
would consider comfortable. Such systems are therefore not well matched to 
the needs of the homecare market, especially in the treatment of sleep 
disordered breathing, and are today reserved exclusively for very critical 
care applications. 
Substantially more comfortable breathing assistance is provided by 
ventilator systems wherein the respiratory cycle is pressure driven. Such 
systems may be configured to supply air at a predetermined inspiratory 
positive airway pressure (IPAP) upon sensing the onset of inspiration and 
until the patient initiates exhalation. Upon exhalation, system pressure 
is immediately reduced to a predetermined expiratory positive airway 
pressure (EPAP) to facilitate the expulsion of air from the patient's 
airway. This type of system augments a patient's spontaneous tidal volumes 
and was the accepted mode for assisting the patient to overcome the work 
of breathing associated with an artificial airway, the mechanics of the 
ventilator and for the weaning of the patient from the full support of 
mechanical ventilation. In its simplest form, such system does not take 
into account the actual volume of air respirated by the patient. 
Consequently, despite satisfying the prescribed pressure parameters, the 
patient may nonetheless suffer from hypoventilation, reduced PAO.sub.2, 
reduction in daytime alertness and increased CO.sub.2 levels. 
More recently, several hybrid forms of pressure support have been 
introduced which vary in method and adjustment but strive to overcome the 
problem associated with varying tidal volumes using pressure limited modes 
of ventilation. One way of accomplishing this is to vary the amount of 
pressure to achieve satisfactory gas exchange in light of changing 
compliance/resistance components by estimating or actually monitoring such 
parameters. Since such approach may still require fairly close monitoring 
of the patient in order to avoid hypoventilation and the consequences 
thereof, many of the corresponding devices are still better suited for use 
in hospitals rather than in the homecare environment. U.S. Pat. No. 
5,134,995, which discloses a variety of systems that undertake to adjust 
air pressure to accommodate various conditions, is hereby incorporated by 
reference. 
A system is needed that ensures adequate ventilation of a patient while 
eliminating the need for monitoring and supervision so as to provide a 
device suited for the homecare market. Previously known systems have been 
unable to adequately satisfy this need. 
SUMMARY OF THE INVENTION 
The present invention overcomes the disadvantages associated with the prior 
art CPAP devices by providing for the automatic adjustment of the pressure 
support level in order to ensure that adequate ventilation is achieved. 
Only minimal monitoring of the patient is necessary thereby rendering the 
system ideally suited for homecare applications. 
The system of the present invention provides ventilation by supplying air 
at elevated pressure during the inspiration phase of the respiratory 
cycle. The pressure level (IPAP) is set as a function of the patient's 
actual minute volume in relation to a preselected minute volume target 
wherein minute volume is defined as the total volume of air respirated 
over the course of a minute. As a result, adequa ventilation is readily 
achieved despite fluctuations in the patient's respiration rate, 
compliance and resistance and without the disadvantage s associated with 
prior art devices. 
A target minute volume is initially prescribed based on various 
physiological parameters of the patient. An initial baseline IPAP is then 
calculated as a function of a such prescribed minute volume, the 
anticipated breath rate and nominal compliance and resistance values. Upon 
being subjected to ventilation, the actual minute volume respirated by the 
patient is calculated, which in turn is compared with the prescribed 
minute volume pursuant to which the IPAP is gradually adjusted in order to 
reconcile the actual value with the prescribed value. The adjustment of 
the IPAP is very gradual both in terms of the frequency of adjustments as 
well as the magnitude of such adjustments in order not to arouse the 
patient while sleeping. Adequate ventilation is ensured because the 
system's function is ultimately based on the volume of air that is 
actually respirated. 
These and other features and advantages of the present invention will 
become apparent from the following detailed description of a preferred 
embodiment which, taken in conjunction with the accompanying drawings, 
illustrates by way of example the principles of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The ventilation system of the present invention provides a method and 
apparatus for automatically adjusting the IPAP supplied to a patient 
during the inspiration phase of each respiratory cycle. The automatic 
operation ensures that the patient is adequately ventilated while the 
manner in which such function is achieved is sufficiently subtle so as not 
to awaken a sleeping patient which is, of course, essential in the 
treatment of sleep disordered breathing. 
FIG. 1 provides a schematic representation of the system 12 of the present 
invention in its most fundamental form. A blower 14 pressurizes the system 
with fresh air. Conduit 16 ducts such pressurized air to a standard nasal 
mask 18 which is fitted about the nose or a nasal pillow that is fitted to 
the nose and extends directly into the patient's nares. Port 19 
continuously vents a small amount of air from the nasal mask or pillow in 
order to prevent moisture buildup and subsequent condensation therein. The 
port also prevents buildup of exhaled gases including CO.sub.2. The actual 
pressure within the system is controlled by relief valve 20 which vents 
superfluous air volume to the atmosphere. The position of the relief valve 
is in turn controlled by controller 24 pursuant to a number of different 
signals. Flow meter 26 provides information as to the volume of air 
inhaled by the patient, while pressure sensor 28 provides information as 
to the pressurization of the system at any given moment. The function of 
the controller is additionally subject to various parameters that are 
input such as through keyboard 30. The controller is also operative to 
control the position of valve 21 which is closed when relief valve 20 is 
opened and to tailor the output of blower 14 in relation to pressure 
demands. 
The general function of the ventilator entails oscillating the system 
pressure between an IPAP value and a much lower EPAP value during the 
inhalation and expiration phase, respectively, of each respiratory cycle. 
The device senses the onset of each phase and immediately adjusts the 
airway pressure accordingly. Consequently, the IPAP serves to maintain a 
positive pressure in the patient's airway in order to avoid the negative 
pressure that would result pursuant to the patient's inspiratory efforts 
thereby splinting the otherwise obstructive tissue into position. The 
reduction of pressure to the EPAP minimizes the work the patient must 
expend in order to exhale. 
Additionally, the system monitors the tidal volume of each cycle and more 
particularly the sum of the tidal volumes over a given period of time to 
calculate the minute volume. The calculated value is compared with a 
target to determine whether an insufficient or an excessive volume of air 
is being respirated. In the event the actual minute volume exceeds the 
target value, the IPAP is gradually reduced such as by a small increment 
every few minutes. If, on the other hand, the actual minute volume is less 
than the target value, the IPAP is gradually increased, again by a small 
increment every few minutes. 
The flowchart shown in FIGS. 2a-c illustrates in detail the method by which 
the system automatically adjusts the IPAP. By considering various 
physiological parameters, the sleep professional first determines a minute 
volume target and breath rate target for a particular patient along with 
the EPAP, the initial IPAP and the maximum deviation from IPAP. These 
values are input into the controller 24 via keyboard 30 at step 40. The 
nasal mask or pillow 18 is then fitted to the patient's nose and treatment 
is commenced at step 42. At step 44, the blower motor 14 is energized and 
the relief valve 20 is actuated to maintain the initial IPAP setting 
during inhalation and the EPAP setting during exhalation. The onset of 
each respiratory phase is sensed by methods well known in the art. 
As ventilation continues, the tidal volumes respired by the patient are 
averaged over the previous 5 minutes and updated with every breath at step 
46. This calculation must also take into consideration an initial 
adjustment factor indicative of the volume escaping through port 19 and 
any miscellaneous leakage occurring throughout the system. Such 
information is provided by the output of flow meter 26 and an internal 
clock (not shown). The average breath rate is then calculated at step 48 
by dividing the number of breath cycles sensed by flow sensor 26 by the 
elapsed time. At step 50, it is determined whether 5 minutes have expired 
since the last IPAP change. If not, the system continues to function at 
the initial IPAP setting, if more than 5 minutes have come to pass, the 
program moves on to the subroutine shown in FIG. 2b. 
At step 52, the target minute volume is adjusted as a function of the 
breath rate. This is necessary as the tidal volume and the efficiency of 
air exchange in the lungs is linked to the respiration rate. A typical 
adjustment factor used in this calculation is 
FACTOR=[0.0375.times.breaths/minute] +0.55. The two coefficients may vary 
as a function of patient parameters and are individually entered via 
keyboard 30. In the succeeding step, step 54, the patient's actual minute 
volume is calculated by taking the average tidal volume obtained in step 
46 and multiplying it by the breath rate obtained in step 48. The actual 
minute volume is then compared to the adjusted minute volume at step 56 
and in the event such calculated error is less than a preselected error 
limit, no adjustment of the IPAP is deemed necessary and the system 
continues to cycle at the initially set levels. If on the other hand, the 
preselected error limit is exceeded, an adjustment of the IPAP is 
necessary. At step 60, it is determined whether the actual minute volume 
exceeds the adjusted minute volume. If not, the program skips to step 66. 
If so, a determination is made at step 62, as to whether the IPAP is 
already less than a preselected minimum value. If yes, no adjustment is 
made, if not then IPAP is reduced by a single increment, such as 1 cm 
H.sub.2 O. At step 66, the determination is made as to whether the IPAP 
exceeds a preselected maximum value. If so, no adjustment is made, if not, 
the IPAP is increased by a single increment such as 1 cm H.sub.2 O. 
As a direct consequence of this approach to controlling a ventilation, the 
patient is assured of receiving sufficient oxygen. The system is flexible 
enough to allow for a change of breath rate and fluctuation of tidal 
volume from breath to breath. When a change in IPAP is deemed necessary, 
the change is made gradually, i.e. varied in small increments and spread 
out over time to provide for a fairly transparent operation. This is, of 
course, essential in treating sleep apnea as any arousal from sleep, or 
other sleep disorder breathing would defeat the purpose of the treatment. 
While a particular form of the invention has been illustrated and 
described, it will also be apparent to those skilled in the art that 
various modifications can be made without departing from the spirit and 
scope of the invention. Various types of ventilation systems may be used, 
additional factors may be taken into consideration in adjusting the 
various target values and limits and features may be incorporated to 
accommodate certain contingencies. Accordingly, it is not intended that 
the invention be limited except by the appended claims.