DIFFERENTIAL VENTILATION OF THE LUNGS. A NEW EXPERIMENTAL MODEL
S. NAZARI, A. SAVINI*, L. RICCIARDI**, G. BELLINZONA***, U. PRATI, P. DIONIGI, F. MONCALVO, A. ZONTA
Dept of Surgery, University of Pavia
*Dept of ELECTRONICS, University of Pavia,
**Institute of Physiology, University of Pavia
***Div Anaesthesia, IRCCS San Matteo, Pavia
Many recent reports (Powner et al 1977, Carlon et al 1978, Gallageret al 1980, Kuetan et al 1982) emphasise the role of differential ventilation in patients with severe respiratory failure caused by asymmetric lung disease. The aim is to prevent the dangerous ventilation-perfusion inequality induced in these cases by standard mechanical ventilation (Carlon et al 1978, West et al 1964). Under these conditions in fact, the compliance of the more affected lung may be lower and the airway resistances may be increased. If the ventilation is delivered through a standard tracheal tube, the less affected lung would receive a greater part of the tidal volume. This disproportionate increase of the tidal volume in one lung may cause a greater mismatching of alveolar ventilation and perfusion and a consequent elevation of the pulmonary venous admixture. This effect may be aggravated by positive end expiratory pressure (PEEP) commonly generated in the two hemisystems.(Kanaker and Shannon 1975). The separate ventilation is usually accomplished by means of two respirators, each one being connected to a channel of a double-lumen tracheobronchial tube (Carlon et al 1978).However the use of these tubes is troublesome (5~9) particularly in patients in Intensive Care Unit (ICU). They are occasionally difficult to put in place and uncoordinated patient's movements may cause displacement of the tip of the tube with serious consequences if not promptly identified and corrected. Another disadvantage lies in thefact that they cannot be put in place passing through the nostril, a method of intubation more suitable for long term ventilation of ICUpatients. Moreover, in long term treatments, the risk of bronchial wall injuries due to the tube may be relevant.The need for two, synchronised ventilators further increases the practical complexity of this ventilatory schedule (3~4).The purpose of this paper is to describe a device recently set upin our department that allows the out differential ventilation and offersmany advantages over double-lumen tube, double respirator technique.
MATERIALS AND METHODS
The device consists of a thin, triple lumen, double tip, cuffed, tube which is inserted through a standard nasotracheal tube; each cuffed tip is positioned into one main bronchus origin (Fig. 1).The three main lumens of the tube are connected to manometers so that a continuous monitoring of the pressures in the bronchi distally to the balloons and in the trachea can be carried out.Ventilation is delivered through the naso-tracheal tube by means of the respirator in the usual way.An electronic device was developed which, by properly inflating and deflating the balloons during the respiratory cycle, allows both the synchronous distribution of the Tidal Volume into the two lungs in the desired ratio, and the generation of differentiated positive end expiratory pressure (PEEP) values in the two hemisystems.The balloons' inflation and deflation is obtained by means of two pairs of electromagnetic valves. Each pair of valves controls a balloon,one being connected to an oxygen source for balloon deflation, the other to a vacuum source for balloon deflation.The signals to action the valves to obtain balloons' inflation and deflation are given by an electronic console that receives signals "in real time" from the manometers sited in the trachea and in the two bronchi.
Fig. 1. The device is positioned by passing it through a standardnasotracheal tube, without interference with the patient ventilation. The device advancement is stopped when the tip bifurcation is engaged in the tracheal carina. At this point,the proper position of the balloons in the bronchi is detectedby means of chest auscultation after having them alternatively inflated. A visual control of the position can be obtained with a fiberoptic bronchoscope passed through the tracheal tube.Separate lung pathophysiological data collection (i.e.hemithorax static compliance and dynamic inspiratoryvolume/pressure curve) may be easily carried out by alternatively excluding the lungs by means of balloons inflation.
Partition of the tidal volume between the two lungs in the desired ratio.
The desired partition of the Tidal Volume between the two lungs is obtained by blocking the main bronchus of the lung whose ventilation has to be decreased (in the example the less affected lung with higher compliance) during the last phase of the inspiration, by means of balloon inflation.In this way, during the first phase of inspiration, when both bronchiare patent, the Tidal Volume is shared among the two lungs accordingly to their respective compliance and inspiratory resistance (Fig. 2,I).In the final phase of the inspiration, when the balloon is inflated and the bronchus blocked, the rest of Tidal Volume delivered by the respirator is wholly conveyed into the contralateral lung (the less compliant lung (B) in the example) (Fig. 2,II).If the dynamic inspiratory volume/pressure curve of each hemisystem is determined separately, the pressure in the airway would allow to know, at any moment during the respiratory cycle, the amount of TidalVolume present in each lung whatever its compliance and resistance maybe.As a matter of fact, the dynamic inspiratory volume/pressure curve depends both on the compliance and inspiratory resistance (Derenne 1981);so, it is possible to predispose the amount of Tidal Volume to be delivered into one lung by choosing the value of pressure in the ascending slope of the inspiratory curve, to which to trigger the balloon inflation.Accordingly, the signal to inflate the balloon and consequently to block the bronchus is derived, "in real time", from the ascending slope of the inspiratory curve recorded in that bronchus.When one bronchus is blocked, likewise, the pressure recorded inthe contralateral hemisystem indicates the entered amount of Tidal Volume.Consequently, by varying the total amount of Tidal Volume delivered by the respirator, it is' possible to obtain the inflation of that lung at the desired degree.During the first phase of the expiration, the pressure in the trachea(C) exceeds that present into the excluded lung (Fig. 2, III). So, the balloon is deflated only when the pressure in the trachea falls to reach the value present into the excluded lung. This is necessary to avoid undesired pendular air movements from one lung to the other during this first phase of the expiration (Fig. 2, IV).Accordingly, the signal to deflate the balloon and open the bronchusis derived "in real time" from the descending slope of the expiratory curve recorded in the trachea.
Fig. 2. Partition of the Tidal Volume between the two lungs in the desired ratio. The lungs schematic excursions were drawn using interrupted lines to indicate the initial status in each section of the figure, likewise, continuous lines indicate the final status.
Graphs A, B and C show the pressure curves recorded in the right and left bronchi and in the trachea respectively.In this example, the right lung (A) is presumed to be more compliant than left lung (B). Air resistance is presumed to be the same in the two lungs. In this condition it could be advisable to limit the overdistension of the more compliant right lung by diverting a part of Tidal Volume into the leftone.
I. During the first phase of the inspiration the Tidal Volumeis shared among the two lungs accordingly to their respective compliance and resistance. So a greater part of the TidalVolume enter the right lung (A) in this phase.
II. When the balloon is inflated and the bronchus blocked in the final phase of inspiration, the rest of the Tidal Volume delivered by the respirator is wholly conveyed into the contralateral, less compliant lung.If the dynamic inspiratory volume/pressure curve was previously measured separately in each hemisystem, the pressure in the airway would indicate, at any moment during the respiratory cycle, the amount of gas present in each lung, thus allowing the proper timing of bronchial blockade. The signal to action the balloon's inflation device is taken "in real time" from the ascending slope of the inspiratory curve when the pressure reaches the desired value (graph A, arrow).
III. At the beginning of the expiration, the pressure in the carina is higher than that present in the excluded lung (comparegraph C and graph A). So the bronchus is maintained blocked in order to prevent further lung distension due to gas passage from the trachea during this first expiratory phase.IY. when the pressure in the trachea falls to reach the value recorded in right lung (A), the balloon is deflated and the expiration can normally occur from both lung (graph C, arrow).
Differential PEEP generation.
In order to selectively generate a PEEP in one bronchus, the balloon in that bronchus is inflated during the end expiratory phase and the bronchus is blocked (Fig. 3, I). The signal to action the balloon's inflation device is derived, "in real time", from the descending slope of the curve recorded in that bronchus during expiration, when the pressure reaches the presettled PEEP value. At the beginning of the next inspiration, when the pressure in the trachea reaches the PEEPvalue, the balloon is deflated and the inspiration can regularly occur(Fig. 3, II e III).A lower PEEP in the contralateral lung can be generated by acting on the external circuit in the usual way.
Planning a possible future use of the device in the experimental animals and eventually in man, alarms and safety servomechanisms were predisposed.An independent, battery driven, electronic circuit is activated when the pressure in any of the three manometers sited in the airways is higher than a presettled values. If this happens an acoustic alarm is actioned and the balloons' inflating valves are blocked; moreover the valves connected to the vacuum source are opened so that both balloons deflate, allowing standard bi-pulmonary ventilation to be automatically resumed. The same effect is triggered in case of electric black-out.An adjunctive timed circuit allows to periodically and automatically exclude the balloons inflation-deflation device for a variable time(for example 30-60 sec every 10 minutes). During this period, while standard bi-pulmonary ventilation is automatically resumed, a vacuum source connected to the A, B and C manometers' channels is activated so that mucus, blood and other secretions can be automatically removed.This experimental model of separate ventilation was tested in a lung simulator system (Drager LS 800) with a Siemens 900C servo respirator.
Fig. 3. Differentiated PEEP generation. Same example and symbols ofFig. 2.The aim is a selective PEEP generation into left lung(B)
I. During the expiration, when the pressure in B reaches the desired PEEP (Graph B, arrow) the balloon is inflated and the bronchus blocked (II). At the beginning of the next inspiration, the bronchus is maintained blocked until the pressure in the trachea (Graph C, arrows) reaches the PEEPvalue in B.III. At this point the balloon is deflated and both bronchi are patent for the inspiration. A lower PEEP in the otherlung can be generated by acting on the external circuit inthe usual way.
The application of the device to a lung simulator allowed to study indetail this experimental model of differential ventilation under various conditions of compliance and airway resistance.This experimentation showed that the device could induce the desired distribution of the Tidal Volume between the two hemisystems with accuracy and reproducible results.Differentiated PEEP could also be easily generated.The figures 2,3 show a theoretic differential ventilatory regimen that can be carried out with the device. In this example the right lung(A) is presumed to be less affected than left lung and has a higher compliance. The airway resistances are presumed to be the same in thetwo hemisystems.In this condition the aim of differential ventilation was to prevent overinflation of the more compliant right lung (A) by diverting a partof the Tidal Volume into the left lung during the last phase of the inspiration (Fig. 2).Moreover, a PEEP was selectively generated into the left, less compliant lung (Fig. 3).The experimental curves obtained with the device in the lung simulator in a similar example of different lung compliances are reported at fig.4 and 5. The fig. 4 shows the experimental curves recorded in the more compliant right lung (upper track) and in the left lung (lower track)before and after actioning the device to block the over ventilation ofthe right lung.The compliance was settled at 0.03 l/mbar for the right lung andat 0.02 1/mbar for the left lung. the resistance was similar in both lungs. Tidal volume, 500 ml, was delivered at a rate of 18 per min by a SIEMENS SERYOC ventilator. The device was settled to block the right lung inflation when the inspiratory pressure reached 19cm H20(first mark) and 16 cm H20 (second mark) respectively.It can be clearly seen that the ventilation in the right lung is efficiently and timely blocked; an increasing inspiratory pressure is correspondingly recorded in the left lung due to the fact that the ventilator conveyed, in that phase, the whole remaining tidal volume to that lung.In basal conditions the TV partition was measured to be 300cc inthe right lung and 200cc in the left; then it passed to 250cc in both lung when the device was first actioned; and to 200cc in the right and 300cc in the left at the end.Fig. 5 shows the experimental curves recorded in the less compliant left lung (upper track) and in the right lung (lower track) before and after actioning the device to generate a selective PEEP in the left lung.Compliances were settled as in the previous example and so were resistances, Tidal volume and respiratory rate as well. The device was settled to block the left lung deflation when the expiratory pressurereached 5 (first mark) and 10 (second mark) cm H20 respectively.It can be clearly seen that the corresponding PEEP was efficiently maintained in the left lung.
Fig. 4. Partition of the tidal volume between the two lungs in the desired ratio. Experimental curves obtained with a lung simulator (Dragar 900) in the more compliant right lung (uppertrack) and in the left lung (lower track) before and after actioning the device to block the overinflation of the right lung. The compliance was settled at 0.03 l/mbar for the right lung and at 0.02 l/mbar for the left lung. The resistances were similar in both lungs. Tidal volume, 500 ml, was delivered at a rate of 18 per min with a Siemens servo C ventilator.The device was settled to block the right lung inflation when the inspiratory pressure reached 19 cmH20 (first mark) and16 cmH20 (second mark) respectively. It can be clearly seen that the ventilation in the right lung is efficiently and timely stopped. An increasing inspiratory pressure is correspondingly recorded in the left lung due to the fact that the ventilator conveyed, in that phase, the whole remaining tidal volume into that lung. In basal conditions, the Tidal Volume partition was measured to be 300cc in the right lung and 200cc in then left.; theit passed to 250 cc in both lungs when the device was first activated and to 200 cc in the right and 300 in the left at the end.
Fig. 5. Selective PEEP generation. Experimental curves recorded by a lung simulator (Dragar) in the less compliant left lung(upper track) and in the right lung (lower track) before and after actioning the device to generate a selective PEEP in the left lung. Compliances were settled as in the previous example and so were resistances, tidal volume and respiratory rate as well.The device was settled to block the left lung deflation when the espiratory pressure reached 5 (first mark) and 10 (second mark) cm of H20 respectively. It can be clearly seen that the corresponding selective PEEPs were efficiently and timely maintained in the left lung.
COMMENT AND CONCLUSION
It is not the purpose of this paper to deal with the pathophysiological patterns of asymmetric lung disease, nor their possible modification by differential ventilation (7~13) or other methods (14~17).The main aim of the work was to evaluate from the physical point of view, the possible differential ventilation regimens that can actually be carried out with this device; that seems in fact to be easier to use than usual double-lumen tube, double respirator method and,consequently, can be expected to enhance the diffusion of this effective(3,8) but complex(4) therapeutic tool.A thorough experimentation in an animal model is needed to conclusively evaluate the device and to assess its possible clinical role.Nevertheless, the present experience showed that the device can efficiently and reproducibly induce the expected physical effects on the two hemisystems.Compared with the usual method for separate ventilation this device seems to offer many advantages. First, there is no need for double-lumen tubes, badly tolerated for long term treatments nor double, synchronised respirators. The thin, double-tip tube can be easily positioned by passingit through a standard naso-tracheal tube without interfering with the patient ventilation; this allows to avoid complex and disturbing manoeuvres such as extubation in order to put in place the double-lumentube. Moreover the device allows an easy and proper collection of separate lung physiological data (i.e. lung compliance) in order to assess thereal usefulness of differential ventilation upon an objective basis.Most important, this method can be expected to avoid the possible ischemic damage induced by the cuff of the double-lumen tube on the bronchial wall.As a matter of fact, the airtight contact between the balloons and the bronchial wall takes place only for a limited part of the respiratory cycle (end inspiration and end expiration). This intermittent contact,minimising the ischemic lesions' risk, can be expected to allow safer long term treatments. The periodic automatic secretions removal with no ventilation interruption is an adjunctive advantage of the device.The safety electronic servomechanisms included in the device allow to exclude, to a reasonable extent, the risk of serious consequences incase of malfunction. In fact these servomechanisms when put into action by any overpressure in the airway, deflate permanently both balloons, so that standard bi-pulmonary ventilation is automatically resumed.This electronic servomechanism is put into action also in case of cough or spontaneous breath when they cause a pressure increase overthe presettled value. This happens also if a malposition of the tube'stips occurs, causing partial airway obstruction (for example retractionof the tips in trachea).The ventilatory schedule shown in the figures is only an example of those possible with this device.In fact the two electronic units that co-ordinate the balloon's inflation and deflation for tidal volume partition and selective PEEPgeneration are independent and can be connected separately to each balloon. This theoretically allows to obtain any desired tidal volume partition between the two lungs and any selective PEEP generation.Thus, if in the example of Fig. 2 the aim of differential ventilation would be to further reduce the ventilation of the left lung, the tidal volume partition electronic unit should be connected to the balloonin B instead of A. In this way, the ventilation in that lung can be blocked at any moment when the pressure indicates the desired left lung distension has been reached (Fig. 6).
Fig. 6. Same example and symbols as Fig. 2. In this case the aim of the differential ventilation was decided to be to further reduce inflation of the left, less compliant lung. The TidalVolume partition electronic unit is then connected to theballoon in B. When the airway pressure indicates the desired left lung distension is reached, the balloon is inflated and the bronchus blocked. The balloon is deflated during expiration when the pressure in the trachea (C) reaches the value inB.
So in any conditions of different compliance and airway resistance of the two hemisystems, the tidal volume in each lung may be regulated from O to the whole amount delivered bythe ventilator. Likewise, no limitations are imposed also on selective PEEP generation. In fact both tidal volume partition and selective PEEP generation electronic units can be connected to the same balloon with the aim to obtain, both a lower ventilation and a higher PEEP in the same lung. In this case, during the inspiration the tidal volume partition unit would inflate the balloon to limit ventilation when the desired peak pressure is obtained. During the expiration, the PEEP generation electronic unit would again block the bronchus when the desired PEEP has been reached.A further practical advantage of the device lies in the fact that both the tidal volume partition and PEEP generation electronic units can be actioned independently and intermittently every 2,3,4 etc.respiratory cycles.This allows, for example, to obtain a certain tidal volume partition and/or a certain PEEP only every 2,3,4 etc normal respiratory cycles.In this regard this device can represent a versatile experimental ventilation model to study the physiological effects of different ventilatory schedules in the experimental animal in an easy way.An interesting field of possible application of this device can be hypothesised in single lung transplant. In this recently developed field(Patterson et al 1988, Patterson and Cooper 1988) in fact, the difference of compliance between the native and transplanted lung may become clinically relevant during acute rejection crisis which usually develops in the first postoperative week.This device could be used in this phase if the decreased compliance of the native lung due to rejection mismatches the ventilation-perfusion ratio to a dangerous level.In conclusion, the present experience confirms that the device allows the differential ventilation of the two lungs in a simpler and safer way when compared with double-lumen tube, double synchronised ventilator technique.Extensive animal experimentation is needed to conclusively evaluatethe possible clinical role of the device.
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Am Rev Respir Dis 110:49-55.last modified Feb 5, 2012