effects of fiberoptic bronchoscopy (2000)

7/29/2019 Effects of Fiberoptic Bronchoscopy (2000)

1/8

Effects of Fiberoptic BronchoscopyDuring Mechanical Ventilation in a LungModel*

R. Wayne Lawson, MS, RRT; Jay I. Peters, MD, FCCP; andDavid C. Shelledy, PhD, RRT

Study objectives: To evaluate the effects of fiberoptic bronchoscopy (FOB) on delivered volumesand pressures during mechanical ventilation, utilizing a lung model.Design: Bench study.Setting: Laboratory.Materials and methods: Using varying-sized endotracheal tubes (ETTs), we ventilated a lungmodel at two levels of compliance utilizing different modes and parameters of ventilation. Afterestablishing baseline measurements, the bronchoscope was inserted and measurements re-peated.Measurements and results: During controlled mechanical ventilation (CMV) with a presethigh-pressure limit (HPL), tidal volumes (VTs) were reduced from 700 mL to 0 to 500 mL

following insertion of the bronchoscope. Increasing the HPL to 120 cm H2O resulted in a VT of40 to 680 mL. Changing from square to decelerating flow waveform resulted in no consistentdifference in VT. Auto-positive end-expiratory pressure (auto-PEEP) of 0 to 41 cm H2O waspresent under most conditions. Higher rates and lower peak inspiratory flows were associated

with higher levels of auto-PEEP. In the pressure-control (PC) mode, using a preset inspiratorypressure level (IP), VT was reduced from 700 mL to 40 to 280 mL following insertion of thebronchoscope. Maximum IP (100 cm H2O) increased VT to 260 to 700 mL. Auto-PEEP was less inthe PC mode.Conclusions: Extreme care must be taken when bronchoscopy is performed on a patient receivingmechanical ventilation. Extremely low VT and significant auto-PEEP may develop unless flow,respiratory rate, mode, and ETT size are carefully selected. The PC mode delivered more volumethan did the CMV mode. When performing FOB during mechanical ventilation, the insidediameter of the ETT should be > 2.0-mm larger than the outside diameter of the bronchoscope

to maintain volume delivery and minimize the development of auto-PEEP.(CHEST 2000; 118:824 831)

Key words: auto-positive end-expiratory pressure; bronchoscopy; mechanical ventilation; peak inspiratory pressure

Abbreviations: CMV controlled mechanical ventilation; ETT endotracheal tube; FOB fiberoptic bronchos-copy; ID inside diameter; I:E inspiratory to expiratory; OD outside diameter; Palv alveolar pressure;PC pressure control; PEEP positive end-expiratory pressure; PIF peak inspiratory flow; PIP peak inspiratorypressure; Te expiratory time; Ti inspiratory time; Vt tidal volume

Fiberoptic bronchoscopy (FOB) is performed on

both spontaneously breathing patients and pa-tients receiving mechanical ventilation to accomplishboth diagnostic and therapeutic objectives.14 Many

retrospective studies have been done examining the

adverse effects and complications of bronchoscopyduring the procedure, almost all focusing directly onalterations in arterial blood gas values and hemody-namics, occurring secondary to BAL and transbron-chial biopsy procedures.515 Tension pneumothoraxand death have been reported as complications ofbronchoscopy in patients receiving mechanical ven-tilation.1,8,14 Ricou et al5 compared BAL results, tidalvolume (Vt) reduction, and intratracheal pressurechanges in a group of 20 adult patients receivingmechanical ventilation, in whom paired bronchosco-pies were performed using an adult (5.9-mm outside

*From the Department of Respiratory Care (Mr. Lawson and Dr.Shelledy), and Department of Medicine, Division of Pulmonaryand Critical Care Medicine (Dr. Peters), University of TexasHealth Science Center at San Antonio, San Antonio, TX.Support was provided by departmental funds.Manuscript received February 17, 1999; revision accepted March22, 2000.Correspondence to: R. Wayne Lawson, MS, RRT, Department ofRespiratory Care, MSC-6248, University of Texas Health ScienceCenter at San Antonio, 7703 Floyd Curl Dr, San Antonio, TX78229-3900; e-mail: [email protected]

824 Laboratory and Animal Investigations

wnloaded From: http://journal.publications.chestnet.org/ on 02/11/2013


2/8

diameter [OD]) vs a pediatric (3.4-mm OD) bron-choscope. BAL recovery compared favorably be-tween the two bronchoscopes, Vt was reduced 46%with the adult bronchoscope and 38% with thepediatric bronchoscope, while intratracheal pressureincreased 16% with the adult bronchoscope vs 0%with the pediatric bronchoscope.

Aside from Ricou et al,5 only two prospective

studies examined complications during bronchos-copy on patients receiving mechanical ventilation.6,7

Lindholm et al6 evaluated the use of FOB in 55patients, 38 of whom were receiving mechanicalventilation, to assess diagnostic and therapeutic im-plications as well as complications, and influence oncardiorespiratory function. They found no mortalityor serious complications related to FOB, but didobserve mild complications, including cardiac ar-rhythmias in two patients, and tachycardia as acommon finding. In a follow-up analysis of theoriginal work, Lindholm et al7 discussed the cardio-

respiratory effects of FOB on those 38 patients. Allwere receiving mechanical ventilation using volume-targeted ventilation with Vt set at 10 to 15 mL/kgand ventilator frequencies between 12 and 18breaths/min, via orotracheal or tracheostomy tubes.Tube size varied between 7.5-mm inside diameter(ID) and 9.5-mm ID (mean, 8.7-mm ID). Intratra-cheal pressure was monitored in 25 of these patientsat the distal tip of the bronchoscope by using amanometer connected to the suction channel. Dur-ing the procedure, peak tracheal pressures variedbetween 18 and 60 cm H2O, while peak inspiratory

pressure (PIP) varied between 28 and 80 cm H2O.These values were limited by the fact that the safetypop-off valve of the ventilator was set at 60 to 80 cmH2O. Exhaled Vt fell dramatically in all cases duringbronchoscopy and returned to prebronchoscopic val-ues immediately on its removal. Even though posi-tive end-expiratory pressure (PEEP) was discontin-ued during the procedure, auto-PEEP was presentin all but five patients immediately on insertion ofthe bronchoscope through the tracheal airway, aver-aging 10.4 cm H2O for those 25 patients. Arterialblood gas analysis was performed on a small sub-group of the patients receiving mechanical ventila-tion. Although the mean arterial oxygen tensionremained stable, the mean Paco2 rose during theprocedure. In their conclusions, Lindholm et al7

listed a series of recommendations for bronchoscopyin patients receiving mechanical ventilation. Theseincluded use of a tracheal tube no smaller than8.0-mm ID, discontinuance of PEEP, increasing thefraction of inspired oxygen to 1.0, monitoring foradequate chest excursion, suctioning for short periodsonly, frequent arterial blood gas analysis, and ruling outmediastinal emphysema and pneumothorax by chest

radiograph after FOB. This study did not examine theeffects of ventilator frequency, ventilatory mode, in-spiratory flow rate, or inspiratory flow waveform ondelivered volumes and pressures. And while they re-ported that exhaled Vt decreased drastically in allcases, numbers were not reported.

Because so little clinical information is knownabout the effect of bronchoscopy on mechanical

ventilation, we used a lung model to address thisissue. This model is designed to realistically simulatethe mechanics of the adult ventilatory system. It usestwo elastomer bellows-type lung compartments witha total residual volume of 1.84 L. Lung compliance isindependently adjustable for each lung utilizing steelsprings with a range of 0.01 to 0.15 L/cm H2O foreach lung, while airway resistance can be alteredusing fixed orifice resistors calibrated to simulate 5,20, or 50 cm H2O/L/s for each lung. These featuresallow the lung model to simulate a wide variety ofpulmonary conditions. Our study examines the effect

of FOB on peak pressure and alveolar pressure(Palv), auto-PEEP, and exhaled Vt during mechan-ical ventilation, and how they can be effectively dealtwith during the procedure to ensure the adequacy ofventilation. The objectives were as follows: (1) todetermine the effect of FOB on delivered volumes,PIP, Palv, and auto-PEEP during mechanical venti-lation of a lung model utilizing three common-sizedadult endotracheal tubes (ETTs) and two levels oflung compliance, and (2) to determine how changesin respiratory rate, peak inspiratory flow (PIF), andmode of ventilation would affect these parameters.

Materials and Methods

Using three different-sized ETTs, 7.5-, 8.0-, and 8.5-mm ID,we ventilated a two-chambered lung (Dual Adult TTL, Model2600i; Michigan Instruments; Grand Rapids, MI) with an adult

ventilator (7200ae; Nellcor Puritan Bennett; Carlsbad, CA).Experiments were done using two levels of compliance: a normal

value of 100 mL/cm H2O and a reduced compliance value of 50mL/cm H2O. Airway resistance was held constant at a value of 5cm H2O/L/s for all experimental circumstances by utilizing afixed orifice resistor with a calibrated pressure drop of 5 cm

H2O/L/s for each lung. We chose not to look at changes in airwayresistance and their effect on delivered volumes and pressures forthis study. However, increasing airway resistance is associated

with decreased delivered volumes and air trapping leading to thedevelopment of auto-PEEP. All experimental conditions arereflective of nonspontaneously breathing patients under theinfluence of heavy sedation. Two modes of ventilation were used:controlled mechanical ventilation (CMV) and pressure control(PC). A Vt of 700 mL was utilized as a baseline setting for allexperimental conditions, at respiratory rates of 12 breaths/minand 24 breaths/min. In the CMV mode, PIFs of 40 L/min and 80L/min were used under each experimental condition, as weresquare and decelerating inspiratory flow waveforms. The high-pressure limit was initially set at 15 cm H2O above the averagePIP. In the PC mode, the inspiratory pressure level was adjusted

CHEST / 118 / 3 / SEPTEMBER, 2000 825



3/8

to deliver a Vt of 700 mL at an inspiratory to expiratory (I:E)ratio of 1:3.5, which corresponded to the I:E ratio obtained in theCMV mode under the following conditions: Vt, 700 mL; respi-ratory rate, 12 breaths/min; and PIF, 40 L/min. After establishingand recording baseline values, the 5.9-mm OD bronchoscope(Olympus B2 Fiberoptic Bronchoscope; Olympus; Tokyo, Japan)

was introduced through a bronchoscope adapter at the yconnector to a distance of 3 cm past the distal tip of the ETT. Atime period of 10 to 15 s was allowed for equilibration of volumes

and pressures prior to any measurements. Measurements wererepeated three times, and the highest value was recorded. Palvwas measured at end-inspiration while auto-PEEP was measuredat end-expiration. Following insertion of the bronchoscope, in theCMV mode, the high-pressure limit was increased to the maxi-mum value of 120 cm H2O. In the PC mode, the inspiratorypressure level was increased to its maximum value of 100 cm H2Oin an effort to return delivered volumes to prebronchoscopicbaseline values, and the resulting measurements were recorded.

Exhaled volumes were measured at the y connector using arespirometer (Wright Mark 8; Ferraris Medical Limited; LondonUK), with an accuracy of 2% at a continuous flow of 16 L/min,and 5 to 10% at 60 L/min. PIP was recorded as measured bythe ventilator system, while simulated Palv, including auto-PEEP, was measured by the two-chambered adult test lung

(Dual Adult TTL, Model 2600i; Michigan Instruments). As partof its standard configuration, the two-chambered adult test lungis equipped to directly measure pressure within each lungutilizing a pressure manometer built into each chamber.

Results

Postbronchoscopic values for Vt, PIP, Palv, andauto-PEEP are reported in Tables 13. Table 1shows the CMV and PC modes at a compliance of100 mL/cm H2O prior to readjusting the high-pressure limit upward from the baseline setting of 15

cm H2O above average PIP in the CMV mode, andprior to increasing the inspiratory pressure level inthe PC mode. In the CMV mode, at a compliance ofeither 100 cm H2O or 50 cm H2O, the delivered Vtfollowing insertion of the bronchoscope ranged from0 to 500 mL (0 to 71% of baseline), with PIP from 8to 24 cm H2O and auto-PEEP from 0 to 15 cm H2O,depending on ETT size, PIF, and waveform settings. Inthe PC mode at a compliance of either 100 mL/cmH2O or 50 mL/cm H2O, the delivered Vt followinginsertion of the bronchoscope ranged from 30 to 340mL (4 to 49% of baseline), with PIP between 11 to 35cm H2O and auto-PEEP from 0 to 4 cm H2O.

In the CMV mode at a compliance of 100 mL/cmH2O, comparing the difference among the threetubes with the bronchoscope in place, moving froman 8.5-mm to an 8.0-mm to a 7.5-mm ETT resultedin an average loss of delivered Vt of 10, 20, and 52%,respectively. In the PC mode, comparing the differ-ence among the three tubes with the bronchoscope inplace, moving from an 8.5-mm to an 8.0-mm to a7.5-mm ETT in the airway resulted in an average lossof delivered Vt of 4, 14, and 43%, respectively. Whenlung compliance was reduced to 50 mL/cm H2O, using

the 7.5-mm ETT, the average loss of delivered Vt roseto 87% in the CMV mode and 79% in the PC mode.

The loss of delivered Vt using the 8.5-mm or8.0-mm ETT can be compensated for by increasingthe high-pressure limit or, in the case of the PCmode, by increasing the inspiratory pressure level.With the exception of experimental conditions doneat a respiratory rate of 12 breaths/min, compliance of100 mL/cm H2O, and decelerating inspiratory flowwaveform, the loss of volume using the 7.5-mm ETT

Table 1CMV and PC Modes Prior to Readjusting theHigh-Pressure Limit Upward From the Baseline Setting

in the CMV Mode, and Prior to Increasing theInspiratory Pressure Level in the PC Mode*

Variables

Respiratory Rate,Breaths/min

12 24

CMV modeSquare inspiratory flow waveform8.5-mm ETT

40 L/min PIF 290 (41) 290 (41)80 L/min PIF 90 (13) 90 (13)

8.0-mm ETT40 L/min PIF 70 (10) 70 (10)80 L/min PIF 50 (7) 50 (7)


Decelerating inspiratory waveform8.5-mm ETT

40 L/min PIF 500 (71) 340 (49)80 L/min PIF 60 (9) 60 (9)



PC mode8.5-mm ETT

Vt, mL (%Vt) 280 (40) 260 (37)PIP, cm H2O 11 23Palv/Pauto 4 /0 5 /2

8.0-mm ETTVt, mL (%Vt) 200 (29) 200 (29)PIP, cm H2O 11 24

Palv/Pauto 4 /0 5 /37.5-mm ETT

Vt, mL (%Vt) 40 (6) 90 (13)PIP, cm H2O 13 29Palv/Pauto 2 /1 5 /4

*Data are presented as No. (%) unless otherwise indicated.Pauto auto-PEEP.Vt is 700 mL baseline, bronchoscope in place. High-pressure limitis 15 cm H2O above PIP prior to insertion of the bronchoscope,delivered Vt with percentage.The effect of rate on delivered Vt, PIP, and Palv using PC modewithout readjustment of the inspiratory pressure level; Vt is 700 mLbaseline; I:E 1:3.5, bronchoscope in place.




4/8

represents a critical loss in delivered Vt that cannotbe compensated for by manipulating ventilator pa-rameters.

Table 2 shows the CMV mode at a compliance of100 mL/cm H2O, after readjusting the high-pressurelimit from the baseline setting of 15 cm H2O aboveaverage PIP to its maximum value of 120 cm H2O.Delivered Vt following insertion of the broncho-scope ranged from 140 to 680 mL (20 to 97% of

baseline), with PIP from 21 to 120 cm H2O, andauto-PEEP from 1 to 23 cm H2O, depending on ETTsize, PIF, and waveform settings. Changing ETT sizedemonstrates an average 10% drop in delivered Vtwith the 5.9-mm bronchoscope inserted through the8.5-mm tube (resulting in a 2.6-mm lumen), an average20% drop when inserted through the 8.0-mm tube(resulting in a 2.1-mm lumen), and an average 52%drop when inserted through a 7.5-mm tube (resultingin a 1.6-mm lumen).

The results at a compliance of 50 cm H2O dem-onstrate that delivered Vt following insertion of the

bronchoscope ranged from 40 to 650 mL (6 to 93% ofbaseline), with PIP between 23 to 120 cm H2O, andauto-PEEP from 0 to 41 cm H2O, depending on ETTsize, PIF, and waveform settings. Changing ETT sizedemonstrates an average 12% drop in delivered Vtwith the 5.9-mm bronchoscope inserted through the8.5-mm tube (resulting in a 2.6-mm lumen), an average30% drop when inserted through the 8.0-mm tube(resulting in a 2.1-mm lumen), and an average 87%

drop when inserted through a 7.5-mm tube (resultingin a 1.6-mm lumen). Comparing delivered Vt betweenthe two compliance values using the 7.5-mm ID ETTin the CMV mode, at a respiratory rate of 12 breaths/min, using the square inspiratory flow waveform at aPIF of 40 L/min, the delivered Vt was 500 mL at acompliance of 100 mL/cm H2O and 100 mL at acompliance of 50 mL/cm H2O. With the deceleratinginspiratory flow waveform at a PIF of 40 L/min,delivered Vt was 450 mL at a compliance of 100mL/cm H2O, and 200 mL at a compliance of 50mL/cm H2O. Using a square wave inspiratory flow

Table 3Effect of Rate on Delivered VT, PIP, and Palv During PCV With the Inspiratory Pressure Level Increasedin an Attempt to Achieve 100% Volume Recovery*

Variables

Respiratory Rate, Breaths/min

12 24

Vt (% Vt), mL PIP, cm H2O Palv/Pauto V t (% Vt), mL PIP, cm H2O Palv/Pauto

8.5-mm ETT 700 (100) 32 9/2 640 (91) 88 14/98.0-mm ETT 700 (100) 45 10/3 500 (71) 90 13/97.5-mm ETT 540 (77) 95 15/10 260 (37) 95 14/1

* Vt is 700 mL baseline; I:E, 1:3.5, bronchoscope in place. See Table 1 legend for abbreviation.

Table 2CMV Mode After Readjusting the High-Pressure Limit From the Baseline Setting to Its Maximum Value*

Variables

Respiratory Rate, Breaths/min

12 24

Vt(%Vt), mL PIP, cm H2O Palv/Pauto

Vt(%Vt), mL PIP, cm H2O Palv/Pauto

Peak flow 40 L/min square wave8.5-mm ETT 630 (90) 34 8/1 680 (97) 47 20/14

8.0-mm ETT 580 (83) 45 8/3 600 (86) 60 23/187.5-mm ETT 500 (71) 120 14/9 380 (54) 120 26/23

Peak flow 40 L/min decelerating wave8.5-mm ETT 610 (87) 21 8/3 NVE NVE NVE8.0-mm ETT 540 (77) 27 9/4 NVE NVE NVE7.5-mm ETT 450 (64) 59 15/11 NVE NVE NVE

Peak flow 80 L/min square wave8.5-mm ETT 610 (87) 95 7/1 640 (91) 103 14/98.0-mm ETT 550 (79) 120 8/2 510 (73) 120 13/97.5-mm ETT 140 (20) 120 4/2 160 (23) 120 9/7

Peak flow 80 L/min decelerating wave8.5-mm ETT 600 (86) 45 7/1 650 (93) 55 16/118.0-mm ETT 550 (79) 54 8/2 600 (86) 68 20/157.5-mm ETT 500 (71) 115 12/8 200 (71) 120 11/9

* Vt is 700 mL baseline, bronchoscope in place. NVE no volume equilibration; see Table 1 legend for abbreviation.

CHEST / 118 / 3 / SEPTEMBER, 2000 827



5/8

waveform at a PIF of 80 L/min, delivered Vt was 140mL at a compliance of 100 mL/cm H2O, and 0 mL ata compliance of 50 mL/cm H2O. With the deceleratinginspiratory flow waveform at a PIF of 80 L/min,delivered Vt was 500 mL at a compliance of 100mL/cm H2O, and 40 mL at a compliance of 50 mL/cmH2O. Comparing delivered Vt between the two com-pliance values using the 7.5-mm ID ETT in the PC

mode, at a respiratory rate of 12 breaths/min, deliveredVt was 540 mL at a compliance of 100 mL/cm H2O,and 190 mL at a compliance of 50 mL/cm H2O. Figure1 shows the average delivered Vt from all experimentalcircumstances in the CMV mode for each ETT size atrespiratory rates of 12 breaths/min and 24 breaths/min,after the high-pressure limit was increased in an effortto recover volume delivery.

Table 3 shows the PC mode at a compliance of 100mL/cm H2O, after readjusting the inspiratory pres-sure level in an attempt to compensate for volumeloss caused by the presence of the FOB in the

airway. At a compliance of 100 mL/cm H2O, deliv-ered Vt following insertion of the bronchoscoperanged from 260 to 700 mL (37 to 100% of baseline),with PIP between 32 to 95 cm H2O and auto-PEEPfrom 2 to 10 cm H2O. At a compliance of 100 mL/cmH2O, changing the ETT size demonstrates an average4% drop in delivered Vt with the 8.5-mm tube, an

average 14% drop when inserted through the 8.0-mmtube, and an average 43% drop when inserted througha 7.5-mm tube.

At a compliance of 50 mL/cm H2O, delivered Vtfollowing insertion of the bronchoscope ranged from100 to 700 mL (14 to 100%) of baseline), with PIPbetween 41 to 100 cm H2O and auto-PEEP from 0to 6 cm H2O. At a compliance of 50 mL/cm H2O,

changing ETT size demonstrates an average 6% dropin delivered Vt with the 8.5-mm tube, an average19% drop when inserted through the 8.0-mm tube,and an average 79% drop when inserted through a7.5-mm tube. Figure 2 shows the effect of ETT sizeon delivered Vt in the PC mode at respiratory ratesof 12 breaths/min and 24 breaths/min and a compli-ance of both 100 mL/cm H2O and 50 mL/cm H2Ofollowing insertion of the bronchoscope.

Discussion

In this study on the effects of FOB on deliveredvolumes and pressures during mechanical ventilationon a lung model, significant changes in various param-eters were observed following insertion of a 5.9-mmOD bronchoscope. PIP was increased, delivered Vtwas reduced under most experimental circumstances,

Figure 1. The average delivered Vt (VT) from all experimental circumstances in the CMV mode foreach ETT size at respiratory rates (f) of 12 breaths/min and 24 breaths/min, after the high-pressurelimit was increased in an effort to recover volume delivery. C compliance.




6/8

and under conditions of higher respiratory rates andlower inspiratory flows, significant auto-PEEP wasobserved. These problems were further aggravated bydecreasing ETT size. Our findings also demonstratedthat volume delivery was slightly higher using the PCmode at a respiratory rate of 12 breaths/min, comparedto the CMV mode at the same rate.

At a given set of ventilatory parameters, actualpatient-delivered volume from a mechanical ventila-tor during bronchoscopy is limited by the size of theETT and the bronchoscope. The obstruction offeredby the bronchoscope limits flow and, as a conse-

quence, delivered Vt into the lung, causing PIP torise as volume is delivered to the ventilator circuitrather than the lung. Poiseuilles law states that flowis directly proportional to the pressure gradient, butin the case of a mechanical ventilator, the availablepressure gradient necessary to maintain flow in theface of the obstruction is limited by the high-pressure limit setting. The increased resistance of-fered by the bronchoscope in the airway limitsexpiratory flow as well as inspiratory flow, resultingin volume trapped within the lung (auto-PEEP).

ETT size had a significant impact on volume

delivery. The reduction in delivered Vt was a directresult of obstruction of the airway by the broncho-scope and, if the high-pressure limit setting isreached, the ventilator prematurely cycles from theinspiratory to the expiratory phase, resulting in afurther decrease in machine-delivered Vt. While thefirst of these phenomena was present to a greater orlesser extent in all of our experimental conditions,the second phenomenon was more closely associatedwith the 7.5-mm ETT, as evidenced by the maxi-mum high-pressure limit being reached in six of theeight experimental conditions in the CMV mode

using the 7.5-mm ETT.Meduri and Chastre,16 in an effort to standardize

how a patient receiving mechanical ventilationshould be managed during the peribronchoscopyperiod to ensure the maintenance of adequate ven-tilation and oxygenation while minimizing the risk ofbarotrauma and other untoward physiologic effects,recommended a series of guidelines designed tostandardize ventilator management during that time.They include the use of an ETT 1.5-mm largerthan the OD of the FOB, and ventilator settings toinclude a fraction of inspired oxygen of 1.0, respira-

Figure 2. The effect of ETT size on delivered Vt in the PC mode at respiratory rates of 12 breaths/minand 24 breaths/min and a compliance of both 100 mL/cm H2O and 50 mL/cm H2O, following insertionof the bronchoscope. See Figure 1 legend for abbreviations.

CHEST / 118 / 3 / SEPTEMBER, 2000 829



7/8

tory rate between 15 and 20 breaths/min, PIF 60L/min, and high-pressure limit set to a level thatallows adequate ventilation. Ventilator settings werethen titrated to maximize exhaled Vt. These recom-mendations were not accompanied by supportingdata. On the basis of our findings, to ensure 10mL/kg-delivered Vt, we would recommend a mini-mum 2.0-mm difference between the ID of the

bronchoscope and the OD of the ETT, rather thanthe 1.5-mm minimum difference recommended byMeduri and Chastre.16 This recommendation is sup-ported by the substantial fall observed in deliveredVt when going from a 8.0-mm to a 7.5-mm ETT.The size difference between the ID of the 8.0-mmETT and the OD of the 5.9-mm bronchoscope was2.1 mm, while the size difference in the ID of the7.5-mm ETT and the OD of the 5.9-mm broncho-scope was 1.6 mm.

When the high-pressure limit was increased to itsmaximum value in an effort to recover patient-

delivered Vt, dramatic increases in PIP were ob-served, the higher PIPs being the direct result ofincreased resistance due to the partial obstructioncaused by the presence of the bronchoscope. How-ever, despite PIPs that averaged 82 cm H2O at theproximal airway in the CMV mode, Palv remainedlow. The increased resistance offered by the pres-ence of the bronchoscope in the airway protected thealveolar space from the excessively high PIPs. Mea-sured within the lung model, Palv averaged 11 cmH2O at a compliance of 100 mL/cm H2O, and 17 cmH2O at a compliance of 50 cm H2O, compared to an

average Palv of 8 cm H2O prior to insertion of thebronchoscope

Decreasing ETT size was also associated withincreasing levels of auto-PEEP, as shown in Table 2.Auto-PEEP is the product of an inappropriate com-bination of expiratory flow, expiratory time (Te), andexpiratory resistance, resulting in a lack of completeexhalation prior to the beginning of the next inspira-tory phase. A portion of the previous Vt remainstrapped in the lung on which the next Vt is deliv-ered. Expiratory flow is directly proportional to theelastic recoil potential of the respiratory system andinversely proportional to the resistance offered bythe combination of the respiratory system and theartificial airway. With regard to Te, at a given inspira-tory flow, because the length of one complete respira-tory cycle decreases as ventilator rate increases, Tedecreases as well. Also, depending on how the ventila-tor functions in terms of volume delivery and phasevariables, Te may be fixed at a given respiratory rate ormay change inversely with ventilator inspiratory flow.By way of explanation, some ventilators, at a given rate,time the length of each phase of the respiratory cycleindependently, resulting in fixed inspiratory time (Ti)

and Te. Other ventilators, however, time only thelength of the total respiratory cycle, not its individualcomponents, meaning that at a given respiratory rateand Vt, Ti is controlled by inspiratory flow, with Tecomprising the time remaining in that cycle. ThePuritan Bennett 7200 series ventilator functions in thismanner. Because higher respiratory rates and lowerinspiratory flows are both associated with shorter Tes,

they are associated with an increased prevalence ofauto-PEEP. Careful attention, therefore, must be givento the matching of PIF with ventilator rate. The partialobstruction caused by the presence of the broncho-scope in the airway can prolong the active componentof the expiratory phase into the next inspiratory phase.

Changing from square to decelerating inspiratoryflow waveform had no consistent effect on deliveredVt and auto-PEEP. At low respiratory rates, thischange had little effect on delivered volume andauto-PEEP, while at high respiratory rates, thedecelerating waveform delivered more volume but at

the expense of higher levels of auto-PEEP. With thecombination of a respiratory rate of 24 breaths/minand PIF of 40 L/min, decelerating flow waveformwas attempted and equilibration within the lungmodel failed to occur, causing the model to contin-ually gain both volume and pressure until the volu-metric capacity of the lung model was exceeded. Thecombination of low inspiratory flow and resultantshort Te in the face of the resistance offered by thebronchoscope led to stacking of volume within thelung model. These findings suggest that when per-forming FOB on a patient who is receiving mechani-

can ventilation, careful adjustment of PIF and flowwaveform is necessary. To prevent Ti from becom-ing too great a portion of respiratory cycle, squareflow waveform at an appropriate PIF should be usedin an effort to minimize volume trapping.

In the PC mode, the presence of the broncho-scope in the airway resulted in a reduction indelivered Vt similar to that observed in the CMVmode when the high-pressure limit was not read-justed. Delivered Vt ranged from 6 to 40%.

One strategy to maintain adequate ventilationduring bronchoscopy is to increase the ventilatorrate. We examined this strategy in an attempt todetermine if it was effective based on our lungmodel. Assuming a constant dead space of 150 mLfor the purpose of determining an estimated minutealveolar ventilation, this strategy results in an in-crease in estimated alveolar ventilation in all cases,except with a 7.5-mm ID ETT or when using adecelerating inspiratory flow waveform and a PIF of40 L/min. While increasing the ventilator rate to 24breaths/min was effective in increasing alveolar ven-tilation, the resultant calculated minute alveolar ven-tilation was often at a level that would result in




8/8

hyperventilation for most patients. At a ventilatorrate of 24 breaths/min and PIF of 40 L/min, using adecelerating inspiratory flow waveform resulted inTes that were too short to allow for adequateexhalation. This caused stacking of volume and sub-sequent overdistention of the lung model for all sizeETTs. We recommend using a square inspiratoryflow waveform with an increased PIF whenever the

ventilator rate is increased in an effort to maintainventilation during the peribronchoscopy period.

A common practice among bronchoscopists per-forming FOB on patients receiving mechanical ven-tilation is to transiently withdraw the bronchoscopefor brief periods to allow the patient unobstructedventilation, and therefore reduced physiologic stress.If the PC mode is used during the procedure, it iscritically important that the inspiratory pressure levelbe reduced to prebronchoscopic values before thebronchoscope is removed from the ETT. Failure todo so would expose the lung to very high pressures,

and thus the potential for barotrauma.Comparing our findings to that of Lindholm et

al,6,7 we found similar increases in PIP and reduc-tions in delivered Vt, as well as the development ofauto-PEEP immediately on insertion of the bron-choscope, especially with the 7.5-mm ID ETT. Ourdirectly measured Palvs were slightly lower than thetracheal pressures measured by Lindholm et al.6,7

Limitations to our study include the fact thatventilatory parameters were arbitrarily chosen, thestudy was done on a lung model, and only one brandof mechanical ventilator was utilized. This study does

demonstrate that extremely low Vt and significantauto-PEEP will develop if an inappropriate-sized ETTand ventilatory settings are used during bronchoscopyon a patient receiving mechanical ventilation. Clinicalstudies should be undertaken to establish the optimalsettings for both spontaneously breathing patients andthose paralyzed during the procedure. Further studiesare essential to validate recommendations already pub-lished in the literature.

Conclusion

FOB during mechanical ventilation should not beperformed prior to careful readjustment of the high-pressure limit in the CMV mode or the inspiratorypressure level in the PC mode. The PC mode delivereda greater percentage of baseline Vt, compared to theCMV mode. Despite extremely high PIPs, Palv re-mained low (from 1 to 23 cm H2O) during broncho-scopic occlusion of the ETT. Increasing the respiratory

rate in either the CMV or PC modes resulted insignificantly higher auto-PEEP. When performingbronchoscopy on a patient receiving mechanical venti-lation using an adult bronchoscope, the ID of the ETTshould be 2.0-mm larger than the OD of the bron-choscope to maintain volume delivery and minimizethe development of auto-PEEP.

References

1 Dreisin RB, Albert RK, Talley PA, et al. Flexible fiberopticbronchoscopy in the teaching hospital: yield and complica-tions. Chest 1978; 74:144149

2 Jolliet P, Chevrolet JC. Bronchoscopy in the intensive careunit. Intensive Care Med 1992; 18:160169

3 Turner JS, Willcox PA, Hayhurst MD, et al. Fiberopticbronchoscopy in the intensive care unit: a prospective studyof 147 procedures in 107 patients. Crit Care Med 1994;22:259264

4 Adolf J, Bartels H, Feussner H, et al. Fiberoptic bronchos-copy in intensive care medicine-functional efficacy and meth-odological side effects. Langenbecks Arch Chir 1985; 1:37 46

5 Ricou B, Grandin S, Nicod L, et al. Adult and paediatric sizebronchoscopes for bronchoalveolar lavage in mechanically

ventilated patients: yield and side effects. Thorax 1995;50:290293

6 Lindholm CE, Ollman B, Snyder JV, et al. Flexible fiberopticbronchoscopy in critical care medicine: diagnosis, therapy andcomplications. Crit Care Med 1974; 2:250261

7 Lindholm CE, Ollman B, Snyder JV, et al. Cardiorespiratoryeffects of flexible fiberoptic bronchoscopy in critically illpatients. Chest 1978; 74:362368

8 Pereira W Jr, Kounat DM, Snider GL, et al. A prospectivecooperative study of complications following flexible fiberop-tic bronchoscopy. Chest 1978; 73:813816

9 Pue CA, Pacht ER. Complications of fiberoptic bronchoscopyat a university hospital. Chest 1995; 107:430432

10 Papazian L, Colt HG, Scemama F, et al. Effects of consecu-tive protected specimen brushing and bronchoalveolar lavageon gas exchange and hemodynamics in ventilated patients.Chest 1993; 104:15481552

11 Montravers P, Gauzit R, Dombret MC, et al. Cardiopulmo-nary effects of bronchoalveolar lavage in critically ill patients.Chest 1993; 104:15411547

12 Hertz MI, Woodward ME, Gross CR, et al. Safety ofbronchoalveolar lavage in the critically ill, mechanically ven-tilated patient. Crit Care Med 1991; 19:15261532

13 Trouillet JL, Guiguet M, Gibert C, et al. Fiberoptic bron-choscopy in ventilated patients: evaluation of cardiopulmo-

nary risk under midazolam sedation. Chest 1990; 97:92793314 Humbert M, Robinson DS, Assoufi B, et al. Safety offiberoptic bronchoscopy in asthmatic and control subjects andaffects on asthma control over two weeks. Thorax 1996;51:664669

15 Suratt PM, Smiddy JF, Gruber B. Deaths and complicationsassociated with fiberoptic bronchoscopy. Chest 1976; 69:747751

16 Meduri GU, Chastre J. The standardization of bronchoscopictechniques for ventilator-associated pneumonia. Chest 1992;102:557S564S

CHEST / 118 / 3 / SEPTEMBER, 2000 831

effects of fiberoptic bronchoscopy (2000)

Documents