Curves and Loops in Mechanical Ventilation · 5 Contents Ventilation curve patterns 6 Pressure-time...

Frank RittnerMartin Döring

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Curves and Loopsin Mechanical Ventilation

Curves and Loopsin Mechanical Ventilation

Frank RittnerMartin Döring

5

Contents

Ventilation curve patterns 6

� Pressure-time diagram 6� Flow-time diagram 10� Volume-time diagram 12� Interpretation of curve patterns 14

Loops – a good thing all round 21

� PV loops 21� The static PV loop 21� The dynamic PV loop in ventilation 23� Interpretation of PV loops in ventilation 26� PV loops before and after the tube 34

� Loops – other possibilities 38� Flow-volume loop 38

Trends reviewed 40

� Documentation of a weaning process 41� Lung parameters based on peak and

plateau pressure 43

Capnography – keeping an eye on the details 44

� The physiological capnogram 46� Interpretations of capnograms 47

6

Ventilation curve patterns

All the ventilators of the Evita family offer graphicrepresentation of the gradual changes in ventilationpressure and breathing gas flow. Evita 4, Evita 2 duraand the PC software EvitaView additionally show thegradual changes in the breathing volume. Two or insome monitors three curves can be shown on thescreen at the same time, and particularly the fact that pressure, flow and volume can be displayedsimultaneously makes it easier to detect changescaused by the system or the lungs. The gradual change in pressure, flow and volume depend to anequal extent on the properties and settings of theventilator, as well as on the respiratory properties ofthe lung.

One respiratory cycle comprises an inspiratory andan expiratory phase. Under normal conditions thesetwo periods contain a flow phase and a no flow pausephase. No volume passes into the lung during the noflow phase during inspiration.

Pressure-time diagramVolume-controlled, constant flow

The pressure-time diagram shows the gradualchanges in the airway pressure. Pressure is given inmbar (or in cmH2O,) and time in seconds.

At a preset volume (volume-controlled ventilation)and constant flow the airway pressure depends on thealveolar pressure and the total of all airwayresistances, and can be affected by resistance andcompliance values specific to the ventilator and thelung. As the ventilator values are constant, thepressure-time diagram allows conclusions to be drawnabout the status of the lung and changes to it.

The gradual changes inpressure, flow and volumedepend to an equal extenton the properties andsettings of the ventilator, aswell as on the respiratoryproperties of the lung.


Resistance = airway resistanceCompliance = compliance of the entire system

(lungs, hoses etc.)

At the beginning of inspiration the pressure betweenpoints A and B increases dramatically on account ofthe resistances in the system. The level of the pressureat break point B is equivalent to the product ofresistance R and flow (*).

∆p = R ∗*

This relationship, as well as the following examples, isonly valid if there is no intrinsic PEEP. The higher theselected Flow * or overall resistance R, the greaterthe pressure rise up to point B. Reduced inspiratoryflow and low resistance values lead to a low pressureat point B.

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Pressure-time diagram forvolume controlled constantflow ventilation.

Ventilation curve patterns8

After point B the pressure increases in a straight line,until the peak pressure at point C is reached. Thegradient of the pressure curve is dependent on theinspiratory flow * and the overall compliance C.

∆p/∆t = * / C

At point C the ventilator applied the set tidal volumeand no further flow is delivered (* = 0).

As a result, pressure p quickly falls to plateaupressure. This drop in pressure is equivalent to therise in pressure caused by the resistance at thebeginning of inspiration. The base line between pointsA and D runs parallel to the line B - C.

Further on there may be a slight decrease inpressure (points D to E). Lung recruitment and leaksin the system are possible reasons for this. The level ofthe plateau pressure is determined by the complianceand the tidal volume. The difference between plateaupressure (E) and end-expiratory pressure F (PEEP) isobtained by dividing the delivered volume VT (tidalvolume) by compliance C.

∆P = Pplat - PEEP

By reversing this equation the effective compliancecan easily be calculated.

C = VT /∆p

The level of the plateaupressure is determined bythe compliance and the tidalvolume.


During the plateau time no volume is supplied to thelung, and inspiratory flow is zero. As alreadymentioned, there is a displacement of volume onaccount of different time constants, and this results inpressure compensation between differentcompartments of the lung.

Expiration begins at point E. Expiration is a passiveprocess, whereby the elastic recoil forces of the thoraxforce the air against atmospheric pressure out of thelung. The change in pressure is obtained bymultiplying exhalation resistance R of the ventilator byexpiratory flow *exp.

∆p = R ∗*exp.

Once expiration is completely finished, pressure onceagain reaches the end-expiratory level F (PEEP).

Pressure-oriented

In pressure-oriented ventilation (e.g. PCV/BIPAP) thepressure curve is quite different.

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Pressure-time diagramm for pressure controlledventilation.


Pressure increases rapidly from the lower pressurelevel (ambient pressure or PEEP) until it reaches theupper pressure value PInsp. and then remains constantfor the inspiration time Tinsp. set on the ventilator.

The drop in pressure during the expiratory phasefollows the same curve as in volume-orientedventilation, as expiration is under normal conditions apassive process, as mentioned above. Until the nextbreath pressure remains at the lower pressure levelPEEP.

As pressure is preset and regulated in the case ofpressure-oriented ventilation modes such as BIPAP,pressure-time diagrams show either no changes, orchanges which are hard to detect, as a consequence ofchanges in resistance and compliance of the entiresystem.

As a general rule it can be said that the pressurecurves displayed reflect the development of pressuremeasured in the ventilator. Real pressures in the lungcan only be calculated and assessed if all influentialfactors are taken into account.

Flow-time diagram

The flow-time diagram shows the gradual changes inthe inspiratory and expiratory flows *insp and *exsprespectively. Flow is given in L/min and time inseconds. The transferred volume is calculated as theintegration of the flow * over time, and is thusequivalent to the area underneath the flow curve.During inspiration the course of the flow curve isdependent on or at least strongly influenced by theventilation mode set on the ventilator. Only the course of the flow in the expiratory phase permitsconclusions to be drawn as to overall resistance andcompliance of the lung and the system.

The course of the flow in theexpiratory phase permitsconclusions to be drawn asto overall resistance andcompliance of the lung andthe system.


In normal clinical practice constant flow anddecelerating flow have become established as thestandard forms for ventilator control.

To date there has been no evidence to suggest thatparticular therapeutic success could be achievedusing other flow forms.

In the case of constant flow the volume flow rateduring inspiration remains constant throughout theentire flow phase. When inspiration starts the flowvalue very quickly rises to the value set on theventilator and then remains constant until the tidalvolume VT, likewise set on the ventilator, has beendelivered (this is the square area under the curve.) Atthe beginning of the pause time (plateau time) theflow rapidly returns to zero. At the end of the pausetime expiratory flow begins, the course of whichdepends only on resistances in the ventilation systemand on parameters of the lung and airways. Constantflow is a typical feature of a classic volume-orientedmode of ventilation.

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Flow-time diagram


In decelerating flow the flow falls constantly afterhaving reached an initially high value. Under normalconditions the flow returns to zero during the courseof inspiration. Decelerating flow is a typical feature ofa pressure-oriented ventilation mode.

The difference in pressure between the pressure inthe lung (alveoli) and the pressure in the breathingsystem, maintained by the ventilator at a constantlevel, provides the driving force for the flow.

As the filling volume in the lung increases thepressure in the lung also rises. In other words, the pressure difference and thus the flow dropcontinuously during inspiration. At the end ofinspiration the pressure in the lung is equal to thepressure in the breathing system, so there is nofurther flow.

If at the end of inspiration and at the end ofexpiration flow = 0, compliance can also be calculatedin a pressure-oriented ventilation mode using the VTmeasured by the ventilator.

C = VT / ∆P

where ∆P = Pinsp. - PEEP

Volume-time diagram

The volume-time diagram shows the gradual changesin the volume transferred during inspiration andexpiration. Volume is usually given in ml and time inseconds.

During the inspiratory flow phase the volumeincreases continuously. During the flow pause(plateau time) it remains constant as there is nofurther volume entering the lung. This maximumvolume value is an index of the transferred tidal

At the end of inspiration thepressure in the lung is equalto the pressure in thebreathing system, so there isno further flow.


volume and does not represent the entire volume inthe lung. The functional residual capacity (FRC) isnot taken into account. During expiration thetransferred volume decreases as a result of passiveexhalation.

The relationships between pressure, flow andvolume are particularly obvious when theseparameters are all displayed at the same time. Pressure, flow and volume

diagram of volume-orientedand pressure-orientedventilation

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Changes in compliance

When compliance changes the plateau and peakpressures change by the same amount of the pressuredifference ∆p.

increasing compliance → plateau and peak pressuresfall

decreasing compliance → plateau and peak pressuresrise

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Interpretation of curvepatterns


Changes in inspiratory airway resistance

When the inspiratory airway resistance changes thepeak pressure changes and the plateau pressureremains the same.

increasing resistance → peak pressure risesdecreasing resistance → peak pressure falls

The expiratory lung resistance cannot be seen onthe pressure curve as the alveolar pressure wouldneed to be known. Conclusions can be drawn howeverfrom the expiratory flow curve (see «Flow curve atincreased expiratory resistances»).

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The expiratory lungresistance cannot be seenon the pressure curve asthe alveolar pressure wouldneed to be known.


Spontaneous breathing

During a ventilator breath the patient will try tobreathe spontaneously, and will «fight» against themachine. Reducing the amount of time for inspirationor, even better, changing to a mode of ventilationwhere the patient is allowed to breathe spontaneouslyeven during a mandatory breath, is an option worththinking about. BIPAP or AutoFlow® are examples ofsuitable modes.

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Adaptation of the flow curve

In volume-controlled modes of ventilation, AutoFlow®

results in automatic flow adaptation with the aim ofapplying the set tidal volume at the lowest possibleairway pressure. The constant flow typical of volume-oriented ventilation modes (square) becomes at thesame time a decelerating flow form, while tidalvolume remains constant even if the compliance inthe patient’s lung changes.

Pressure limitation at a constant tidal volume canalso be achieved in Dräger ventilators by using thePmax setting. If the compliance of the patient changesthis set value may need to be checked and reset.

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The flow curve in the case of insufficient inspiration time

If the flow does not return to zero during inspirationthis means that the inspiration time is insufficient toapply the volume which could be achieved for the setpressure.

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The flow curve in the case of insufficient expiration time

If the flow does not return to zero during expiration,the expiration time is not sufficient for full expiration.This indicates the presence of an intrinsic PEEP.

This results in an increase in lung pressure in thecase of volume-controlled ventilation.

In Evita ventilators it is possible to measureintrinsic PEEP and trapped volume directly. Anintrinsic PEEP can have considerable effects on theexchange of gases and pulmonary blood circulation.

In some applications, however, there may beattempts to establish an intrinsic PEEP on purpose(Inverse Ratio Ventilation IRV), due to the fact thatthis will probably then only occur in certain desiredsections of the lung, while a PEEP set on the ventilatorwill affect the entire lung.

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In Evita ventilators it ispossible to measure intrinsicPEEP and trapped volumedirectly.


Flow curve in the case of increased expiratory resistances

A more gentle expiratory flow curve indicatesincreased expiratory resistances which may be causedby expiratory filters which have become damp orblocked as a result of nebulization. This may lead to aconsiderable increase in expiration time and adeviation from the set PEEP value.

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21

Loops – a good thing all round

The static PV loop (classic)

The static PV loop (pressure-volume curve) is obtainedas a result of the «super-syringe» method, and is usedpredominantly in scientific papers [1]. Most of what isknown about the PV loop is thus based on this method.The most important feature of this PV loop is that theindividual points of measurement (pressure andvolume) are recorded when breathing gas flow = 0.Using a super-syringe, the volume in the lung isincreased step by step. A few seconds after eachincrease in volume the resulting pressure is measured[2]. By connecting the individual points the PV loop iscreated.

PV Loops

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PV-Loop measured by«super-syringe» method.

Loops – a good thing all round22

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The relationship of volume to pressure reflectscompliance (C = ∆V / ∆P). Thus the PV loop shows howcompliance develops as volume increases. The lowerand upper inflection points can be taken from the PVloop. When the super-syringe method is used thevolume measured value does not return to zero duringexpiration, but the reasons for this are as yet notentirely clear. However, errors in measurement andoxygen consumption during measurement play asignificant role [2].

In the lower section (A) the pressure per volumeincrease rises particularly rapidly and only continuesin a straight line (B) once a lung-opening pressure(lower inflection point) has been exceeded. If the lungreaches the limits of its compliance, the rise inpressure per volume increase becomes bigger again(upper inflection point) (C).

It is generally accepted that ventilation should takeplace as far as possible within the linear compliancearea (B), as dangerous shear forces occur as a resultof the collaborating and reopening of individual areasof the lung. The lower inflection point can beovercome by setting a PEEP. The ventilation volume(in IPPV/CMV, SIMV) or inspiratory pressures (inBIPAP, PCV) must then be selected such that the upperinflection point will not be exceeded.

Dynamic PV loops in ventilation

PV loops which are generated during ventilation do not fulfil the condition that at the time when the individual measured values are recorded thebreathing gas flow should equal zero. The breathinggas flow generates an additional pressure gradientdue to the inherent resistances like tube, airways etc.(see also page 7).

PV-Loop with upper andlower inflection point.


For this reason the PV loop does not give an accuratepicture of the course of compliance. The greater theinspiratory breathing gas flow the greater theadditional pressure gradient and thus the degree ofinaccuracy.

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Since ventilators open the exhalation valve either toambient pressure or set PEEP at the beginning ofexpiration, the pressure displayed by the PV loop alsofalls almost immediately to this value.

In the static PV loop, on the other hand, thereduction is again a gradual process.

As regards the PV loop obtained for controlledventilation it can generally be said that the slower thelung is filled the better the rising branch reflects thecourse of compliance.

A number of studies and publications have shown thatPV loops recorded during the course of ventilationcorrelate well with loops from standard procedures, solong as the inspiratory flow is constant [3]. Thestudies are based on the assumption that the drop inpressure resulting from inspiratory resistances willalso remain constant at constant flow, and that thesteepness of the inspiratory loop will thus reflect onlythe elastance of thorax and lung. Although as a resultthe PV loop recorded by the ventilator is offset (therising branch shifts position), it otherwise retains itsoriginal shape, on the basis of which conclusions canbe drawn about compliance.

This also shows that in ventilation modes withdecelerating flow (BIPAP, PCV etc.) it is not possible todraw conclusions from the PV loop concerning thedevelopment of compliance of the lung.

In ventilation modes withdecelerating flow (BIPAP,PCV etc.) it is not possible todraw conclusions from thePV loop concerning thedevelopment of complianceof the lung.


Volume-controlled ventilation with constant flow

During inspiration the lung is filled with a preselectedconstant flow of gas, during which process thepressure in the breathing system gradually increases.The pressure in the lung increases to the same extentand, at the end of inspiration, reaches the same valueas the pressure in the breathing system (plateaupressure).

During expiration the ventilator opens theexhalation valve wide enough to maintain the level ofPEEP set. Due to the pressure difference, which isnow inverted (pressure in the lung greater than PEEPpressure), the breathing gas now flows out of the lungand the lung volume slowly falls. This is why PV loopsduring controlled ventilation run anti-clockwise.

Interpretation of PV loops inventilation.

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Pressure-controlled ventilation (decelerating flow)

Even during pressure-controlled ventilation thePV loops run anti-clockwise. However, in this case thelung is not filled with an even gas flow. At thebeginning of inspiration the ventilator generates agreater pressure in the ventilation system than in thelung, which is then kept constant by the ventilatorthroughout the entire inspiration. As a result of thisdifference in pressure air flows into the lung and thevolume of the lung slowly increases. As the volumeincreases the pressure in the lung also rises and thedifference between the lung pressure and the pressurein the breathing system becomes smaller.

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Since due to the laws of physics the difference inpressure determines the resulting breathing flow, the breathing flow becomes ever smaller duringinspiration, thus creating a decelerating flow.

The pressure in the breathing system is kept at aconstant level by the ventilator throughout inspiration,giving the PV loop during pressure-controlledventilation a more or less box-like shape.

Assumptions cannot be made on the basis of this loopabout the course of lung compliance. When thebreathing gas flow is equal to zero at the end ofinspiration, however, the steepness of a line drawnbetween the start of inspiration (A) and the point atthe end of inspiration (B) does represent a measure ofdynamic compliance. This presupposes, however, thatflow equals zero both at the end of inspiration and atthe end of expiration.

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CPAP spontaneous breathing

In spontaneous breathing the PV loops run clockwise.The patient’s inspiratory effort creates a negativepressure in the lung, which then has an effect in thebreathing system where the pressure is measured bythe ventilator.

The ventilator always tries to supply the patient withenough breathing gas to ensure that the set CPAPpressure is maintained at a constant level, although aslight negative deviation is inevitable. The area to theleft of an imaginary vertical axis (A) at the set CPAPpressure is thus a measure of the patient’s efforts tocombat the inspiratory resistances of the ventilator.

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PV loop in CPAP with ASB/P.supp.

A characteristic feature of respiratory supportsynchronized with the inspiratory effort of the patient(ASB/P.supp., SIMV etc.) is a small twist just above thezero point. The patient first generates a negativepressure in the lung. Once the trigger threshold hasbeen passed, however, the ventilator generates apositive pressure in the breathing system. The area tothe left of the vertical axis which is enclosed by thesmall twist (A) is a measure of how much work thepatient needs to do to trigger the ventilator. The areato the right of the axis (B) represents the work done bythe ventilator to support the patient, in so far as thepatient only triggers and does not actually breathe aswell.

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PV loops in the case of compliance changes

As compliance decreases, in other words as the lungbecomes less elastic, and the ventilator settingsremain the same, the PV loop in volume-controlledventilation takes an increasingly flat course.

The change in steepness of the inspiratory branchof the PV loop is proportional to the change in lungcompliance.

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The change in steepness of the inspiratory branch ofthe PV loop is proportionalto the change in lungcompliance.


PV loops in the case of resistance changes

If resistance changes during constant flow ventilationthe steepness of the right branch of the loop remainsunchanged, but changes position.

PV loop section reflecting lung over-extension

If during constant flow ventilation the loop starts to become flatter in the upper part of the inspiratorybranch, then this could be an indication of over-extension of certain areas of the lung. See also «The static PV loop».

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PV loop in ASB/P.supp.

If during ASB/P.supp. the patient is only able tomanage the trigger impulse and then does notcontinue to breathe, then only a volume equal to thesupport pressure in accordance to current lungcompliance will be reached. However, if the patientcontinues an inspiratory effort throughout the entiresupport phase then he or she will be able to inhalegreater volumes, whilst the support pressureremaining unchanged.

A change in the height of the PV loop is thus ameasure of the strength of the patient’s inspiratoryeffort.

A patient is only prompted to make an inspiratoryeffort if the tidal volume which can be achieved by theset pressure support (without the patient’s ownbreathing) is smaller than the patient’s individualneeds. On the other hand, the support pressure shouldat least compensate for the artificial airwayresistances (tube) (see also «PV loops before and afterthe tube»).

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PV loops before and after the tube

The PV loop measured by the ventilator gives only halfthe picture. As described in the section «dynamic PVloop», further pressure drops occur after the pointwhere the ventilator takes the pressure measurement(Y-piece) e.g. across the tube length and the physio-logical airways.

PV loop in CPAP before and after the tube

The PV loop displayed by the ventilator shows a narrowloop for purely spontaneous patient breathing at anincreased pressure level (CPAP). The narrower thearea to the left of the vertical axis, the less additionalwork of breathing needs to be done to combat theventilator’s inspiratory resistances. The area to theright of the axis, on the other hand, is only deter-mined by the expiratory resistances of the ventilator.The entire area which the loop encompasses is thus at the same time a measure of the quality of theventilator, although it should be remembered that forthe purposes of a direct comparison of ventilators thesame system of measurement needs to be used for allventilators, since the specifications of the varioussystems may differ considerably from one another. Afurther consideration is the fact that some ventilatorsapply a small additional support pressure (some3 mbar) even when no support pressure has been set,thus making a direct comparison no longer possible.

The narrower the area to theleft of the vertical axis, theless additional work ofbreathing needs to be doneto combat the ventilator’sinspiratory resistances.


The belief that a narrow PV loop is synonymous withless work of breathing for the patient is not correct inevery case.

A comparison with a loop recorded directly afterthe tube shows that this loop covers a considerablygreater area. Due to the comparatively small diameterof the tube the patient must do considerably morework of breathing.

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The smaller the tube diameter the more the patienthas to work to combat the tube’s resistance [4], a factwhich is shown by the different area covered by theloops recorded after the tube.A PV loop taken downstream from pathologicallyincreased airway resistances would cover an evengreater area.

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Assistance from pressure support (ASB)

Increased airway resistances, whether caused bydisease or intubation, thus result in increased work ofbreathing for the patient.

The reason for setting assisted spontaneous breath-ing (ASB/P.supp.) is generally to try to compensate forthese airway resistances. A comparison with loopsrecorded during CPAP shows that the area of the looptaken after the tube can also be reduced withASB/P.supp.

If the inspiratory branch of the loop lies exactly on thevertical line of the CPAP set value, the tube resistancewill only just be compensated for. If the inspiratorybranch lies to the right of the CPAP line then supportis provided above and beyond merely compensating forthe tube resistance, thus compensating for possiblepathological resistances in the lower airways. If the

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support pressure is inadequate and the patient isbeing forced to inhale, however, negative pressuremay still occur at the distal end of the tube.

Unfortunately, the PV loop at the distal end of thetube is not usually available. In the Evita 4 andEvita 2 dura with ATC® (Automatic Tube Compensation),it is now possible for the first time to display a PV loopafter the tube. The flow-dependent drop in pressureacross the tube is measured in real-time, and thepressure after the tube (tracheal pressure Ptrach) isthen calculated on this basis. The PtrachV loop thusallows the physician to assess to what extent theventilator is actually supporting the patient's breathingand how much of the work of breathing the patient isdoing himself.

Loops – other possibilities

In addition to pressure-volume (PV) loops, othercombinations of parameters are also possible. Some ofthese are already used in pulmonology but are notparticularly widespread in intensive care medicine.Some diagnostic procedures require the patient’scooperation.

Flow-volume loop

The flow-volume loop is occasionally used to obtaininformation about airway resistance, when aspirationshould be carried out and about the patient’s reactionto bronchial therapy.

Increased airway resistances as a result of sputumetc. can in many patients be recognized by a saw-toothed-shape loop. A smoother loop then verifies thatmeasures such as suctioning which have been takento improve airway resistance have been successful. [5]

ATC® is a registered trademark of Drägerwerk AG in Germany and the USA


In patients with obstructive diseases the expiratorybranch of the loop only changes shape when the setPEEP is greater than intrinsic PEEP. The fact that theshape of the loop does not change, however, does notnecessarily have anything to do with flow limitation. [1]

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40

Trends reviewed

Graphic trend displays enable ventilation processes tobe assessed at a later stage, with the development ofcontinuously measured values displayed in graphicform. Trend displays may be of interest in a variety ofdifferent applications, and each application willrequire a different period of observation. For instance,for assessing a process of weaning, several days oreven weeks will need to be displayed in one diagram,while an event which occurs suddenly calls for asmuch detail as possible to be shown in the diagram.

The areas of application for trend displays in ventilationare extremely varied due to the wide range of possiblecombinations of the individual parameters. Thefollowing are just a few examples, designed to makethe reader think of further possible applications.

Trends reviewed 41

Documentation of a weaning process

In the above trend it can be seen how the SIMV fre-quency was gradually reduced. Each time the frequencywas reduced there was a drop in minute volume (MV),although this drop was compensated for after a shorttime by the MV (MVspont.) spontaneously breathed bythe patient.

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Trends reviewed42

The second trend display shows the further course ofthe weaning process and its effects. Above a particularIMV frequency this was kept constant and only thepressure support was reduced. Initially this reductionwas also compensated for by the patient, though lateron a lasting reduction in MV can be seen, after whichthe ventilator support was once again increased.

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Trends reviewed 43

Lung parameters based on peak and plateau pressure

As already described in the chapter on curves, thechanges in peak pressure and plateau pressure allowstatements to be made about airway resistance andlung compliance. Thus the trend display of both theseparameters is at the same time a trend of R and C.

If during volume-controlled ventilation withoutpressure-limiting peak pressure (Ppeak) increaseswhile plateau pressure (Pplat) remains constant, thenthis indicates an increase in airway resistance. Thismay be caused among other things by constriction ofthe airways by secretion on account of insufficientsuctioning or aspiration.

If both peak pressure (Ppeak) and plateau pressure(Pplat) increase, then this indicates falling compliance.

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44

Capnography – keeping an eye on the details

Besides the uptake of O2, the removal of CO2 is themost important function of respiration, and thereforethe proportion of CO2 in the exhaled air is closelyrelated to the physiological processes in the humanbody. The size and development over time of theproportion of CO2 in the gas mixture flowing past theY-piece is thus a meaningful parameter with regard topatient status and the success of therapy.

The graphical representation of the proportion ofCO2 in the breathing air and its development overtime is called a capnogram. Normally, this curve isdisplayed as CO2 partial pressure or is given in volumepercent. During inspiration, the CO2 partial pressureof the inhaled air is virtually zero, assuming that theopen breathing systems normally used in long-termventilation are used. At the onset of expiration, theproportion of CO2 in the exhaled gas mix is initiallylow, and it is only after exhalation of the dead spacevolume not involved in the process of gas exchangethat an increase in the proportion of CO2 can beobserved. The capnogram finally reaches a plateauphase during which the CO2 proportion rises onlyslowly. When the next inspiration starts, the curvequickly drops back to the value of the inhaled gasmixture.

However, it is not only the capnogram itself whichis of interest, but also the parameters which can bederived from it. The endtidal CO2 proportion ofbreathing air is shown by the height of the plateauphase at the end of expiration. The integration of theexhaled CO2 over time, i.e. the exhaled CO2 volumeper unit time, is described as CO2 elimination.Furthermore, capnography allows a calculation of thedead space volume which does not take part in the gasexchange process during a breathing cycle. In this


context, the dead space volume per breath as aproportion of the overall tidal volume is of particularinterest.

Typical curve forms and changes in the parametersderived from the capnograms provide informationabout physiological processes and apparatus factorssuch as the ventilation settings. Due to the fact that itcan be used to reliably detect incorrect intubation,capnography has already become a standard elementof monitoring, particularly in anaesthesia. In addition,capnograms provide information about metabolic andcirculatory parameters, gas exchange and lung venti-lation, and about ventilator settings. The effects ofventilator settings can be seen from the change in CO2elimination before blood gas values change. Using amainstream sensor positioned close to the patient, theCO2 monitoring system of the Evita 4 and Evita 2 duraprovides capnograms in real time, and calculates and monitors the derived parameters. It thereforerepresents an effective and rapid-reaction monitoringtool for hospital personnel.

The following pages include a physiological cap-nogram and a selection of deviating CO2 curve shapesand trend displays with a short interpretation andexamples of possible causes.

CO2-Monitoring on Evita 4

Capnography – keeping an eye on the details46

The physiological capnogram

A - B: Emptying of the upper dead space of the airwaysThe CO2 concentration in this section of the curveequals zero, as this is the first phase of expirationduring which air from the upper airways, which hasnot been involved in the process of gas exchange, isanalysed.

B - C: Gas from the lower dead space and alveoliThe CO2 concentration increases continuously, as theair being analysed comes partly from the upperairways and partly from the alveoli which are rich inCO2.

C - D: Alveolar gasThis phase is described as the «alveolar plateau». Thecurve rises very slowly. The air being analysed comesmainly from the alveolar area.

D: Endtidal CO2 partial pressureRepresents the highest possible concentration ofexhaled CO2 and is reached at the end of expiration.This point is described as endtidal CO2 (etCO2) andrepresents the final portion of air which was involvedin the exchange of gases in the alveolar area. It thusrepresents under certain conditions a reliable index ofCO2 partial pressure in the arterial blood.

Normal values for endtidal CO2 concentration approx. 5.0-5.3 %, 5.1-5.3 kPa or 38-40 mmHg

D - E: InspirationThe CO2 concentration falls rapidly, as fresh gas notcontaining CO2 forces its way into the airways at thebeginning of inspiration.


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Exponential fall in pCO2

Possible causes:– Cardiopulmonary bypass– Cardiac arrest– Pulmonary embolism– Large loss of blood– Sudden drop in blood pressure

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Interpretations ofcapnogram


A persistently low pCO2

Possible causes:– Hyperventilation as a result of high minute volume– Low body temperature– Following shock

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A persistently low pCO2 without plateau

Possible causes:– Insufficient alveolar ventilation– COPD– Obstruction of upper airways– Tube partly closed

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Sudden drop in pCO2 to around zero

Possible causes:– Accidental extubation– Complete airway stenosis– Disconnection– Oesophageal intubation

(drop after 1-2 tidal volumes)

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Gradual increase in pCO2

Possible causes:– Increase in metabolism and body temperature

(with MV=const.)– Beginning of hypoventilation– Reduction in effective alveolar ventilation

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Sudden drop in pCO2, but still above zero

Possible causes:– Leaks in hose system (tube)– Partial airway stenosis– Tube in laryngopharynx

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pCO2 plateau not horizontal

Possible causes:– Asthma– Ventilatory distribution problems

(asynchronous emptying)

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A constantly high pCO2

Possible causes:– Respiratory depression due to drugs– Metabolic alkalosis (respiratory compensation)– Insufficient minute ventilation

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56

Literature

[1] A. Nahum, Use of Pressure and Flow Waveforms toMonitor Mechanically Ventilated Patients,Yearbook of Intensive Care and EmergencyMedicine 1995, 89-114

[2] Sydow M.,Burchardi H.,Zinserling J., Ische H.,Crozier Th.A., Weyland W. Improved determinationof static compliance …; Intensive Care Med (1991)17:108-114

[3] Marco Ranieri, Rocco Giuliani, Tommaso Fiore,Michele Dambrosio, Joseph Milic-Emili. Volume-Pressure Curve of the Respiratory System PredictsEffects of PEEP in ARDS: «Occlusion» versus«Constant Flow» Technique. Am J Respir Crit CareMed.; Vol 149. pp 19-27, 1994

[4] Michael Shapiro, MD; R. Keith Wilson, MD;Gregorio Casar, MD; Kim Bloom, MD; Robert B.Teague, MD. Work of breathing through differentsized endotracheal tubes. Critical Care Medicine,Vol. 14, No. 12

[5] Jurban A, Tobin MJ (1994) Use of Flow-Volumecurves in detecting secretions in ventilatordependent patients. Am J Respir Crit Care Med150:766-769

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Curves and Loops in Mechanical Ventilation · 5 Contents Ventilation curve patterns 6 Pressure-time...

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