Case SeriesClinical application of continuous spirometry with apitot-based flow meter during equine anaesthesiaY. P. S. MoensClinic of Anaesthesiology and Perioperative Intensive Care, Vienna, Austria.

Keywords: horse; anaesthesia; monitoring; spirometry; respiratory mechanics

Summary

This report documents the feasibility and clinicalinformation provided by a new method for spirometricmonitoring adapted for equine anaesthesia. Monitoring ofventilatory function was done with continuous spirometryduring general anaesthesia of client-owned horsespresented for various diagnostic and surgical procedures.An anaesthetic monitor with a spirometry unit for humananaesthesia was used. To allow the measurement of largetidal volumes, a remodelled larger version of the pitot tube-based flow sensor was used. This technology providedreliable spirometric data even during prolongedanaesthesia when water condensation accumulated in theanaesthetic circuit and the sensor. In addition to flow andvolume measurement and respiratory gas analysis, thecontinuous display of flow-volume and pressure-volumeloops offered visually recognisable information aboutcompliance, airway resistance and integrity of the circuit.Continuous spirometry with this monitoring system washelpful in evaluating the efficacy of spontaneousventilation, in adjusting intermittent positive pressureventilation and detecting technical faults in theanaesthetic apparatus and connection with the patient.This adapted spirometry method represents a practicaland reliable measuring system for use during equineanaesthesia. The variety of information provides anopportunity to optimise anaesthetic management in thisspecies.

Introduction

General anaesthesia in horses is often characterised by thedevelopment of respiratory depression, hypoxaemia andhypercapnia (Gillespie et al. 1969; Hall 1972). Therefore,monitoring of ventilation together with other vital functionsis an important aspect of anaesthetic management (Hall

and Clarke 1991; Trim 1998). Spirometry during anaesthesiaallows accurate measurement of tidal and minute volumes.This is important information during spontaneous breathingand during mechanical ventilation. Even when a mechanicalventilator is equipped with a graduated breathing bag thetidal volume effectively delivered to the patient will bedifferent because of volume loss caused by expansion ofbreathing hoses and possible leaks in the system.

During clinical large animal anaesthesia the use ofspirometry to monitor ventilation is not routine because ofthe lack of a reliable and practical method adapted tolarge animals. In human anaesthesia, technology referredto as side stream spirometry1 has been used since 1991and is now commonly applied (Hufmann 1991; Bardoczkyet al. 1993). This spirometer uses a pitot tube-based flowsensor with an integrated respiratory gas sample port anda dedicated host monitor. The sensor has no moving partsand in essence combines the measurement of gasvelocity by pitot tubes with the principle of a fixedresistance. The pressure difference generated isconverted to flow and volume. The values areautomatically corrected for the effect of changing gasconcentrations. The flow sensor is placed between theendotracheal tube and the breathing circuit. Additionallyto check measurement of tidal volume and minutevolume, the system calculates dynamic compliance(Cdyn) of the respiratory system and visually displays thepressure-volume (PV) and flow-volume (FV) loop ofeach breath, representing the compliance (PV) andresistance (FV) of the respiratory system. The inspiration/expiration time (I/E) ratio is calculated as well as thevolume exhaled in first second (V1.0, %). The latterparameter describes the airway resistance duringexpiration. When used in man this parameter depends onpatient cooperation to provide expiratory effort; inanimals no surrogate has yet been standardised.

To allow the use of this technology in large animalanaesthesia, the original sensor was remodelled on alarger scale (Moens et al. 1994). The accuracy of flow andvolume measurements using this remodelled sensorCorresponding author email: yves.moens@vetmeduni.ac.at

354 EQUINE VETERINARY EDUCATIONEquine vet. Educ. (2010) 22 (7) 354-360

doi: 10.1111/j.2042-3292.2010.00066.x

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combined with the original host monitor has been studiedin vitro and found to be adequate for use in large animalanaesthesia (Moens et al. 2009).

The author routinely uses this monitoring system duringequine inhalation anaesthesia including intermittentpositive pressure ventilation (IPPV) with different types oflarge animal ventilators. The purpose of this paper is toreport the feasibility of continuous spirometry in horses usingthe equipment described and to illustrate the usefulness ofthe information with some clinical examples of PV andFV loops.

Materials and methods

The standard equipment

Two types of anaesthetic monitors equipped with identicalside stream spirometry technology were used: theCapnomac Ultima2 or the S/5 Compact Monitor1. Thespirometry unit uses the remodelled large animal flowsensor, henceforth called H-lite, instead of the sensor forhuman use (D-lite). It is a bi-directional, pressure-based flowsensor that combines the principle of the resistiveflowmeter with the principle of the pitot tube (Fig 1). A

diagram of the complete measurement system is shown inFigure 2.

Due to the fact that the monitor algorithm was originallydeveloped for measuring tidal volumes of up to 2 l usingthe smaller, D-lite, sensor, a recalibration procedure(Capnomac Ultima) or the calculation of a conversionfactor (S/5) is necessary. This procedure necessitates acalibration syringe3 of a volume of 3–5 l. After performingthis procedure, the displayed values for volumes must bemultiplied by 5 (Capnomac Ultima) or by the determinedconversion factor (S/5) to give the actual values for tidaland minute volume.

Details of the operating principle, accuracy andcalibration procedure of the sensor-monitor combinationare described elsewhere (Moens et al. 2009).

For the purpose of documentation the host monitor wasconnected with a standard videoprinter producing aprintout of the actual display of the monitor.

Anaesthetic apparatus and techniques

Spirometric monitoring was done in client-owned horsesanaesthetised for various diagnostic and surgicalprocedures. Following induction and tracheal intubationthe horses were connected to large animal circle systems.The H-lite coupled to the host monitor was insertedbetween the endotracheal tube and the Y-piece of thecircle system (Fig 3). Care was taken to position the sensorin a way that the gas sampling line and the 2 airwaypressure lines faced upwards. Anaesthesia was maintainedwith isoflurane and oxygen sometimes supplemented withdifferent drugs such as ketamine, benzodiazepines, a2

agonists and guiaphenesin. Procedures lasted up to6 h. If IPPV was used it was done with a large animalventilator operated in volume or pressure controlled mode(Smith respirator LA and LA21004; GT ventilator)5.

P1 P2a

Fig 1: Design of the pitot tube based sensor. A) Lateral view of theD-lite (left) for human use and of the H-lite for use in large animals(right). The 2 pressure sensing ports (large arrows) and the gassampling port (small arrow) are shown for each sensor. B) Design ofthe sensor head (D-lite). End view through the tube (left) showingthe star shaped resistor element and a cross-sectional view (right)showing the 2 pitot tubes (P1 and P2) for pressure measurementand the gas sampling port (a).

Display

SignalProcessing

PressureSensor

#1 #2

GasAnalysis

MONITOR

SENSORExp. Insp.

Fig 2: Complete diagram of the combination of the pitottube-based flow sensor (2 pressure pick-ups and one gas samplingport) and the host monitor. Sensor 2 is used for flow measurement.Sensor 1 is used to perform corrections related to varying airwaypressures.

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Y. P. S. Moens 355

Results and case examples

With the H-lite sensor, spirometric data were obtained in allpatients, including a prolonged case (6 h) wherecondensed water accumulated in the anaesthetic circuit.Flow, volume, pressure, and compliance measurementswere displayed numerically. The I/E ratio and V1.0were also displayed numerically. Pressure-volume loopsand flow-volume loops were displayed graphically,representing compliance and resistance, respectively. PVloops allow easy identification of compliance changes inthe respiratory system and were in principle selected forthe default display during IPPV (Figs 4–8), whereas FV loopswere in general used during spontaneous ventilation(Figs 9–10). For comparison with the patient displays,

schematic diagrams of a typical PV loop and FV loop areshown in Figures 11 and 12, respectively.

Using spirometric data, the anaesthesiologist was ableto document appropriate settings and identify and correctinadequate settings during IPPV. Changes in ventilationand anaesthetic depth, problems with the endotrachealtube, the breathing circuit and the anaesthesia machinewere also discovered using both PV and FV loops.Changes in compliance were also identified and measurestaken when necessary. Explanation of examples illustratedby figures and other instances where spirometry washelpful is included in the following paragraphs.

Pressure-volume loops during manual or mechanicalIPPV were elliptical in shape and plotted mostly at about a45° angle on the x-y graph (Fig 4). However, a ‘fish-like’distortion of the normally epileptically PV loop occurredalways when horses were mechanically ventilated in theassisted mode (Fig 5). This pattern was also seen wheneverthe horse started to breath against the ventilator, e.g.when anaesthesia became too light or the effect of

Fig 3: The pitot tube based sensor (H-lite) is interposed between theendotracheal tube and the Y-piece of a circle system duringequine anaesthesia. The pressure sensing and gas sampling linesare facing upwards.

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Fig 4: Printout of the complete display of the Capnomac Ultimaduring inhalation anaesthesia of a 550 kg bwt horse using IPPVshowing spirometric parameters with a typical pressure-volumeloop and results of respiratory gas analysis. Tidal volume (TV),Minute volume (MV), Dynamic Compliance (C) are flow-deriveddata to be multiplied by the correction factor 5. Peak pressure(Ppeak), plateau pressure (Pplat), positive end expiratory pressure(PEEP), Respiratory frequency (RF), percent of expired volumeexhaled in the first second (V1.0), inspired: expired time ratio (I : E).

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Fig 5: Spirometric data and a pressure-volume loop recordedduring inhalation anaesthesia of a 450 kg bwt horse using IPPV inthe assisted mode. Inspiratory efforts that trigger the ventilatorcreate a negative pressure and disturb the normal appearance ofthe pressure-volume loop creating a typical ‘fish’-like pattern(flow-derived data to be multiplied by 5).

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Fig 6: Spirometric data and loops recorded following induction ofinhalation anaesthesia of 700 kg bwt horse. Hypoventilation duringspontaneous respiration because of an insufficient tidal volume(stored loop, dotted) was diagnosed. IPPV with an adequate tidalvolume (actual PV loop) restored normoventilation with normalend-tidal CO2 (flow-derived data to be multiplied by 5).

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356 Spirometry during equine anaesthesia

chemically induced neuromuscular blockade dissipated.The PV loops during spontaneous ventilation had theshape of an inverted teardrop on the y-axis of a X(pressure)-Y (volume) graph. The typical shape of a PVloop commonly recorded during spontaneous respirationcan be compared with the shape during IPPV in Figure 6.The PV loop is shifted to the right (Fig 8) in a case wherecontinuous positive airway developed accidentally duringspontaneous respiration.

Typical shapes of an FV loop during spontaneousrespiration and during IPPV are shown in Figure 13. DuringIPPV the shape of the FV loop differs because thecharacteristics of the ventilator and its operation modusinfluence inspiratory and expiratory flow patterns.Ventilators can deliver tidal volume in a pressure-controlled

or volume-controlled modus and generate accelerating,decelerating or constant inspiratory flow patterns. Theexpiratory flow pattern can be influenced by addedexpiratory resistance to generate positive end expiratorypressure.

Spirometry allowed continuous measurement of tidaland minute volume on a breath-to-breath basis and quickdetection of any changes. An example of unexpectedabrupt changes in tidal volume during spontaneousrespiration that occurred during a castration procedureand suggesting inadequate anaesthetic depth is given inFigure 9.

Spirometry continuously indicated the volumedelivered by the ventilator at the patient side. This wasuseful in choosing initial ventilator settings (tidal volume,frequency, pressure limit) when IPPV was needed to treathypoventilation and to identify reasons for the settingsconsidered appropriate for the patient not resulting in

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Fig 7: Spirometric data and pressure-volume loops recorded duringinhalation anaesthesia and IPPV for exploratory laparatomy in a400 kg bwt colic horse. A loop was stored before opening of theabdomen (dotted loop, white arrow). The actual loop (black arrow)was registered 1.5 h later when a right dorsal displacement of thecolon was surgically corrected and intestines were decompressed.The increased compliance of the respiratory system is visible asan increased angle of the actual pressure-volume loopdemonstrating delivery of a similar tidal volume at a lower airwaypressure (flow-derived data to be multiplied by 5).

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Fig 8: Spirometric data and pressure-volume loops recorded duringinhalation anaesthesia of a spontaneously ventilating 450 kg bwthorse illustrating the effects of an inadvertently closed pop-offvalve creating continuous positive pressure in the circuit.In comparison with the stored loop (dotted) the actual loopis shifted to the right and tidal volume is greatly reduced(flow-derived data to be multiplied by 5).

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Fig 9: Spirometric data and flow-volume loops recorded duringcastration of a 500 kg bwt stallion under inhalation anaesthesiaand spontaneous ventilation. Following a steady state period withadequate tidal volumes (stored loop, dotted), tidal volumesuddenly decreased during emasculation of the spermatic cord(actual loop) (flow-derived data to be multiplied by 5).

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Fig 10: Spirometric data and flow-volume loops recorded duringinhalation anaesthesia and spontaneous ventilation of a 500 kgbwt horse illustrating the correction of a leak between H-lite sensorand patient. The stored loop (dotted) presents a large gap(inspiratory/expiratory tidal volume difference 260 ml). When thecuff of the endotracheal tube was further inflated the actual loopalmost closed and the tidal volume difference decreased to 103 ml(flow-derived data to be multiplied by 5).

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Y. P. S. Moens 357

expected respiratory parameters. On some occasions alarge discrepancy between inspired and expired volumeexisted and loops presented a large gap. This was oftencaused by a leak due to insufficient inflation of the cuff ofthe endotracheal tube (Fig 10). From time to time amarked discrepancy between tidal volume selected onthe volume-controlled ventilator and the tidal volumeeffectively delivered to the horse was noticed. It was foundto be due to accumulation of water in the concertina bagof the ventilator. When the concertina bag is an ascendingbellows type this can occur and remain unnoticed.

During IPPV, the dynamic compliance (Cdyn) of therespiratory system (lungs, chest wall, endotracheal tube)is continuously measured. Dynamic compliance variedwidely among clinical anaesthetic cases and ranged from80–900 ml/cmH2O. Compliance was often low in horsesanaesthetised with signs of abdominal distension. Analmost 2-fold increase of Cdyn (from 389–875 ml/cmH2O)was noticed following successful correction ofhypoxaemia in a colic horse during IPPV by using analveolar recruitment manoeuvre and the use of positiveend expiratory pressure (Levionnois et al. 2006). A very lowcompliance was seen in 2 horses where a previouslyunrecognised diaphragmatic hernia was discovered atsurgery. Changes in compliance occurred followingpostural changes during anaesthesia (e.g. dorsal vs. lateralrecumbency, head down tilt) but were generally small.More pronounced changes often occurred following

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Fig 11: The pressure-volume or compliance loop demonstrates therelationship between pressure and volume. The ascending lowerlimb represents the increasing inspiratory pressure to inflate thelung and the descending upper limb represents the decreasingpressure during deflation of the lungs. The slope of the curverepresents the dynamic compliance. The compliance iscalculated by dividing the tidal volume by the difference betweenthe end-inspiratory pressure and the end-expiratory pressure.

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Fig 12: The flow-volume (FV) or resistance loop demonstrates therelationship between flow and volume. The inspiratory flow is thelower part of the curve and the upper part represents expiratoryflow. The loop A has a high peak flow indicating typical resistance.The loop B has a decreased peak flow suggesting increasedairway resistance.

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Fig 13: Spirometric data and flow-volume loops recorded duringinhalation anaesthesia of a 550 kg bwt horse. A: duringspontaneous ventilation. B: during volume-controlled IPPV withconstant inspiratory flow. The shape of the FV loops is influencedby the characteristics and operation modus of the ventilator inuse (flow-derived data to be multiplied by 5).

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358 Spirometry during equine anaesthesia

surgical exteriorisation and decompression of distendedintestines (Fig 13) or during laparascopic procedures whenabdominal insufflation/deflation was used. A suddensignificant decrease in Cdyn was caused by obstruction ofthe airway due to kinking of the endotracheal tube in onecase and by herniation of the cuff of the endotrachealtube in another.

Discussion

The use of the H-lite/monitor combination allowedcontinuous and reliable monitoring of ventilator volumes,airway pressures and respiratory mechanics. The process ofmoving gas volumes to and from the alveoli has not beenmeasured routinely during equine anaesthesia largely dueto the absence of a reliable and practical method.Difficulties in obtaining accurate flow and volumemeasurement during anaesthesia are caused by the factthat respiratory gas density and viscosity are affectedby changing airway pressure, water vapour content,temperature and the varying composition of therespiratory gases. This will lead to errors in flow and volumemeasurement (Jones 1980; Habre et al. 2001). Satisfactorycompensation for such changes has not been establishedfor the types of flowmeters that could be used in largeanimal anaesthetic systems, e.g. ultrasonic flowmeters orFleish pneumotachographs (Marlin and Roberts 1998). If apneumotachograph is to be used during inhalationanaesthesia it needs at least to be calibrated with therespiratory gas mixture that is used if volume measurementerrors up to 40% are to be avoided (Filmyer et al. 1999).Further error is introduced by baseline drift andaccumulation of condensed water in the sensorhead requiring frequent recalibration (Rolly andRenders-Versichelen 1974; Marlin and Roberts 1998). This isin line with the findings of Ionita and Moens (2007) whofound errors in tidal volume measurement from 14–43% witha Fleish pneumotachograph during clinical inhalationanaesthesia of horses. The H-lite/monitor combination hasthe advantage that it uses the results of simultaneousrespiratory gas analysis for breath-to-breath compensationof flow measurement during anaesthesia. In vitro validationof pitot-based spirometry with the H-lite/monitorcombination using different volumes, gas mixtures and flowrates, demonstrated excellent accuracy of respiratoryvolume measurement (Moens et al. 2009). The error in tidalvolume measurement was substantially less than �10%,which is generally regarded as acceptable for clinicalpurposes. These results suggest that when the H-lite/monitor combination is used on clinical patients there is noneed for recalibration when gas composition changesduring anaesthesia. Also, in the author’s experience andunlike other spirometric methods, humidity and fluidaccumulation in the sensor did not interfere withcontinuous measurement. Accumulation of water in theairway pressure sensing lines is avoided when these lines

face upwards (Fig 2). The continuous measurement ofinhaled and exhaled gas volumes was helpful in assessingventilatory function during both spontaneous ventilationand manual or mechanical IPPV. It allowed alsoidentification, treatment or prevention of complicationssuch as airway obstruction and hypoxaemia and problemsrelated to the anaesthetic machine and the connectionwith the patient. The functional result of pulmonaryventilation can to a certain extent, be noninvasivelymonitored in horses with other techniques such aspulseoximetry and capnography (Trim 1998). However,correct interpretation of end-tidal CO2 values and theirchange is only possible if the contribution of minute volumeof ventilation is recognised. By placing the H-lite at the levelof the endotracheal tube and not at another place in thecircuit, the effect of gas compression in the ventilator andbreathing hoses is removed and thus real patientventilation is measured.

With pressure controlled ventilation mode thedelivered volume depends on generated airway pressureand the latter in turn depends on the compliance of therespiratory system. Especially low compliance is a concernwhen IPPV is used. In cases of pressure controlled IPPVand low compliance the selected peak pressure may bereached before the appropriate tidal volume is deliveredand so lead to hypoventilation and hypercapnia. In caseof volume controlled IPPV low compliance can lead tohigh intrathoracic pressures. This will exacerbate thenegative haemodynamic effects of IPPV especially incardiovascularly compromised patients. Low compliancecan find its origin in mechanical obstruction of the airway,lung disease, space occupying processes in the thorax, orhigh intra-abdominal pressure. A decreased complianceduring anaesthesia has been documented duringlaparascopic procedures with head-down tilt andabdominal insufflation both in man (Oikkonen andTallgren 1995; Tanskanen et al. 1997) and in horses(Donaldson et al. 1998; Filzek et al. 2001). In humanmedicine measurement of Cdyn is used to optimise lungrecruitment strategies in patients with compromised lungfunction and atelectasis (Almarakbi et al. 2009; Caramezet al. 2009).

The loops registered in horses resembled in essence theloops registered during human anaesthesia (Bardoczkyet al. 1993). Unlike in human anaesthesia loops registeredduring equine anaesthesia regularly failed to closecompletely despite an airtight seal of the cuff. This isthought to be due to a lack of sensitivity when expiratoryflow becomes minimal at the end of the expiratory phasein horses. However, this was not considered an obstacle tocorrect interpretation of the loops.

The fact that to originally set up the H-lite/monitorcombination, a calibration procedure is necessary andthat numerical displays of volumes and compliance mustbe multiplied by a default correction factor (here 5) or bya determined conversion factor to yield exact values isconsidered an acceptable disadvantage.

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Y. P. S. Moens 359

Conclusion

The use of a pitot-based technology with an adaptedsensor and a dedicated monitor is a practical and reliableway of performing continuous spirometry during routineequine anaesthesia. The information obtained aboutrespiratory flow and volume combined with measurementof airway pressures can provide valuable informationabout basic pulmonary function and airway integrityduring both spontaneous and mechanical ventilation. Inaddition to numerical values the generation of PV and FVloops on the graphic display provides another waveformto evaluate, a visual method, of assessing compliance andresistance. Much like other waveforms, e.g. capnogramsor electrocardiograms, such a system allows rapiddetection of abnormal conditions creating the possibility ofrapidly executing corrective action. As the monitoringsystem performs continuous respiratory gas analysis and,optionally, pulseoximetry, a noninvasive monitoring systemfor all aspects of ventilation and gas exchange isavailable. This allows the veterinary anaesthetist tooptimise anaesthetic management in equids.

Manufacturers’ addresses

1Datex-Ohmeda, Helsinki, Finland.2GE Healthcare, Little Chalfont, Buckinghamshire, UK.3Hans Rudolph Inc, Wyandotte, Kansas, USA.4Vet Technics, IJmuiden, The Netherlands.5Stephan GmbH, Gackenbach, Germany.

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