High-volume, Low-pressure Cuffs

High-volume, Low-pressure Cuffs· Are They Always Low Pressure? Douglas Guyton, M.D.;t Michael J Bannet; Ph.D.;+ and Robert R. Kirby, M.D.§

Ischemic tracheal complications due to the ETT cuff occur in approximately 10 percent of mechanically ventilated

critically ill patients despite the use of high-volume, lowpressure ETT cuffs. Using a laboratory model, we studied the eR'ects of airway pressure on three different ETT cuR' designs, including two "low pressure" designs. Positive airway pressure acted on the "low pressure" cuR's to create a "self-sealing" eR'ect that maintained tracheal occlusion despite airway pressures that exceeded cuR' inflation pressure. Increases in airway pressure caused by decreased lung compliance resulted in higher cuR' inflation pressures in all three groups, with the smallest increase occurring in the design that bad the longest tracheal contact length. We

In the 1960s, ischemic tracheal complications such as tracheal stenosis and tracheoesophageal fistula occurred in up to 20 percent of intubated and mechanically ventilated patients. Endotracheal tubes were typically constructed with low-volume cuffs which had little or no resting volume and a diameter smaller than tracheal diameter. Such cuffs frequently required inflation pressures of 160 to 300 mm Hg to seal the trachea l -8 and deformed the trachea from its normal HC" shape into an expanded circular shape. The pressure exerted upon the tracheal mucosa (CT pressure) by inflating a low-volume, high-pressure cuff was impossible to estimate; however, directly measured CT pressures ranged up to 200 mm Hg or more. 4-7 Furthermore, small increments (1 to 3 ml) in cuff volume beyond the MOV produced increases in the CT pressure of up to 100 mm Hg or more.5-8 Such high CT pressures severely restricted tracheal wall blood flow and caused mucosal ulceration, tracheomalacia with tracheal dilation, perforation and tracheal stenosis as scar formation occurred with healing. In the early 1970s, large volume cuffs with a diameter greater than that of the trachea largely replaced the low-volume, high-pressure cuffs. 8 When used properly, these large cuffs sealed the trachea by draping themselves freely along the contours of the tracheal wall without altering the normal "c" shape of the trachea. As a result, these cuffs did not need to be inflated beyond their resting diameter and required *From the University of Florida College of Medicine, Gainesville. t Assistant Professor of Anesthesiology. iAssistant Professor of Anesthesiology and Physiology. §Professor of Anesthesiolo~ Manuscript received August 20; revision accepted March 11

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conclude that the current high-volume, low-pressure ETT cuff design currently used does not guarantee low cuff pressure when high airway pressures occur, and an alternative design should be developed. (Chest 1991; 100:1076-81)

ANOVA=analysis of variance; CL=lung compliance; Cf= cuffed-tracheal; ETT = endotracheal tube; HI = hildl volume, low pressure; ID = inside diameter; LO = low vofume, high pressure; MED=medium volume, low pressure; MOV=minimal occlusive volume; Patm = atmospheric pressure; Paw = airway pressure; Pcuff= ETT intracuff pressure; PIP = peak inflation pressure; SICU = surgical intensive care unit

lower inflation pressures than the earlier low-volume, high-pressure design. Furthermore, cuffinflation pressures approximately equalled CT pressure because no pressure was generated by stretching the cuff to fill the trachea. Thus, the high-volume, low-pressure design operated with less CT pressure and reduced the overall incidence of ischemic tracheal complications. In 1981, Stauffer et al9 reported findings in a group of 150 patients who were mechanically ventilated with tracheal intubation in a SICU. Despite the use of high-volume, low-pressure endotracheal cuffs, at follow-up 11 percent (3 of 27) had tomographic evidence of tracheal stenosis at the cuff site, an incidence strikingly similar to previous reports involving lowvolume, high-pressure cuffs. They also found that 19 percent of the 150 SICU patients required excessive cuff pressure (>25 mm Hg) to seal the trachea, while an inability to seal the airway was noted in 11 percent despite intracuff pressures up to 60 mm Hg. The reasons for the excessive intracuff pressure requirements were not clear. Recently we had occasion to review two cases in which patients with ARDS sustained ischemic tracheal complications following a prolonged period ofmechanical ventilation with high PIP above 50 cm H 20. One ofthese patients developed a tracheoesophageal fistula requiring tracheal reconstruction and prolonged hospitalization. The other patient suffered massive tracheal dilation before succumbing to multiple organ failure. In both cases, ETTs with high-volume, lowpressure cuffs had been continuously employed. Proper cuff inflation (minimal leak) techniques had been utilized, together with appropriate humidificaHigh-volume. Low-pressure Cuffs (Guyton, Banner, Kirby)

tion, tidal volume and minute ventilation. As in the report of Stauffer et ai, we were unable to explain the extensive tracheal damage associated with the use of high-volume, low-pressure ETI cuffs and created a laboratory model to examine intracuff pressure and volume characteristics. METHODS

\\e studied the effects of airway pressure on the performance of current E1T cu&' designs by using a laboratory model to simulate the extremes of clinically employed positive pressure mechanical ventilation, ie, under conditions ofhigh CL with low PIP vs marlced1y reduced CL with high PIP. Three E1T cu&' designs, LO, MED and HI were evaluated using 7.0- and 8.o-mm ID ETTs (Fig 1). Resting cu&' volumes (the performed just inflated volume) were measured using a calibrated syringe. A mechanical lung (Vent Aid TIL, Michigan Instruments, Inc), which was connected to a lubricated (lightweight household oil, WD-40) 19 x 22-mm model trachea (Imatrach, Mallinckrodt) was used to simulate changes in CL from 100 to 15 mUcm H.O. A volume-cycled ventilator (Bear 2) delivered a tidal volume (VT) of 1,000 ml; inspiratory and expiratory volumes were measured with two rotating vane-type spirometers (Wright) connected in series just proximal to the ETr. Ten ETTs of each size (7.0- and 8.o-mm ID) and cu&' type (LO, MED, HI) were inflated in the trachea at 21"<: using a calibrated syringe until a "minimal" leak of 10 percent (100 ml) and then 5 percent (SO ml) of the 1,000 mL VT occurred (American National Standards Institute deBnes a minimal leak as 5 percent of the delivered volume.'O) Airway and ETT intracuft' pressures were measured at both the 10 and 5 leak volumes using pressure transducers (Gould-Statham NiO) and a multichannel

polygraph recorder (Grass modeI7B). Leak volume was ascertained by the difference between the inspiratory and expiratory volumes. Complete occlusion of the trachea was demonstrated by lack of bubble formation in a soap solution coating the proximal tracheacu&' interface and showed this method of leak measurement to be accurate ± 2 percent. Intracuft' pressure measurements were divided into four groups according to leak volume and CL, Ie, 10 percent, high CL; 5 percent, high CL: 10 percent, decreased CL: and 5 percent, decreased CL. Within each group, the pressure readings for each ETT cu&' type were compared using ANOVA and a Duncan's multiple comparisons test. Alpha was set at 0.05 for statistical signi6cance. RESULTS

Resting cuff volumes ranged from 5.2 ml for the 7.o-mm ID LO, to 20.6 mL for the 8.o-mm ID HI ETT cuff (Table 1). When CL was 100 mVcm H20, a PIP of approximately 15 cm H 20 (11 mm Hg) was generated; reducing CL to 15 mVcm H 20 resulted in a PIP of approximately 80 cm H20 (60 mm Hg). After cuff inflation to the predetermined leak volumes we noticed that the airway pressure was transmitted to the cuff, causing intracuffpressure to rise and fall with inspiration and exhalation (Fig 3). The intracuff pressure measurements presented in Figure 2 were recorded at end-exhalation their lowest point (referred to as the baseline cuff inHation pressure). Figure 2A shows that in the highly compliant lung (CL =100 mUcm H20) there is little difference be-

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FICURE 1. 1bp, Schematic of testing apparatus and picture of the three endotracheal tube cu&' designs studied: Bottom left, LO; bottom center; MED: and bottom right, HI. CHEST 1100 I" 1 OCTOBER. 1991

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FIGURE 2. Baseline cuff inHation pressures for each cuff type under conditions of (A) high lung compliance (100 mllcm H.O) and (8) reduced lung compliance (15 mllcm H.O), with a 10 and 5 percent leak in delivered VT. A reference line is drawn at 25 mm Hg, indicating the maximum safe cuff pressure, ie, one that will not cause significant interference with tracheal wall blood Bow.

tween the baseline cuffin8ation pressures ofthe MED and HI cuff groups with either a 10 or 5 percent leak. The 7.0 LO cuffed ETT required markedly high baseline cuff in8ation pressures, whereas the 8.0 LO which had a resting diameter slightly larger than that of the trachea required baseline in8ation pressures closer to those of the MED and HI groups. When CL decreased to 15 mVcm H20 (Fig 2B), every cuff required higher baseline in8ation pressures, and differences among the three cuffgroups became evident. To achieve identical performance, especially under conditions of reduced CL, the 7.0 LO required the highest pressures, the MED cuffs required intermediate pressures and the HI cuffs required the lowest Table l-Remng Intra-cuff Volumes for Thrw TfIPft of Endotracheal Tube.· ETIType ETIsize 7.0mm 8.0mm ·See Figure 1.

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FIGURE 3. Typical airway and intracuff pressure tracings for a 7.0 HI cuff during two cycles of ventilation, under conditions of high compliance (CL= 1(0) and reduced compliance (CL= 15). Above each pressure tracing is a photograph ofthe model trachea containing the 7.0 HI cuff at the PIP. Note that under conditions of reduced compliance and high PIp, the cuff is deformed into a cone shape as the airway pressure compresses the distal end of the cuff, "millcing" the gas within the cuff to fill and distend the proximal end. Under conditions of high compliance and low PIp, the cuff retains a cylindrical shape.

pressures (p<0.05). Differences in performance also were discernible between the 7.0 and 8.0 HI cuffs. Figure 3 shows a typical pressure tracing from the testing of a 7.o-mm ID ETT with a HI cuff. In the highly compliant lung (eL = !OO mVcm H 20), the cuff had to be in8ated to approximately 20 mm Hg in order to seal with a 5 percent VT leak. Intracuff pressure exhibited little change with ventilation, sometimes rising slightly during inspiration. Under conditions of decreased compliance, the cuff had to be in8ated to a baseline of approximately 40 mm Hg in order to seal with a 5 percent leak. Intracuff pressure was now greatly affected by ventilation, increasing by 50 percent during inspiration. Intracuff pressure remained constant at 40 mm Hg as the airway pressure rose from zero to 40 mm Hg. Intracuff pressure began to increase as the airway pressure exceeded the baseline cuff inflation pressure. Thereafter, intracuff pressure equaled airway pressure as both rose simultaneously to the PIP of 60 mm Hg. DISCUSSION

This in vitro experiment utilized a plastic tracheal model specifically recommended by the American National Standards Institute for the testing of ETT cuffs designed for prolonged intubation. lo Plastic tracheal models avoid the problems ofrapid deterioration of freshly excised tracheas and poor correlation between animal species. II While our mechanical model High-voIume. l.ow-pIessu1'8 Cuffs (Guyton, Banner, Kirby)

allowed strict control of test conditions, the absolute pressures reported may not accurately reflect in viooresults. Unlike the poorly compliant trac~eal model, the human trachea can expand significantly with increases in airway pressure,12 impacting negatively on ETT cuff seal. When softened by warmth and humidity ETT cuffs are more likely to collapse as airway pressure increases, further jeopardizing tracheal occlusion. Therefore, the absolute in vioo intracuffpressures were likely to be underestimated by our model, but the values obtained in this study are reasonable estimates of in vivo pressures. Apart from its in vivo relevance, the model certainly is accurate enough to establish the relative performance of one cuff design to another under similar conditions, especially when using three cuff designs from a single manufacturer that possess similar physical characteristiCS. 13.14 Although we designated 25 mm Hg as a reference point for safe cuff pressures, the limits of safe CT pressures have not been firmly established. In the early 1970s many investigators reported reductions in tracheal damage when CT pressure was limited to approximately 30 mm Hg, the estimated capillary perfusion pressure. 15-19 Measurements by radioactive microsphere and micropuncture techniques have confirmed tracheal capillary pressure to lie between 20 and 30 mm Hg.00.21 Endoscopic studies have shown impaired tracheal blood flow at 22 mm Hg and total obstruction at 37 mm Hg, suggesting that CT pressures should not exceed a critical value of 20 mm Hg.22.23 This study demonstrates several different properties of ETT cuff design: (1) the influence of cuff diameter to tracheal diameter on intracuff pressure, (2) the dependence of cuff inflation pressure on airway pressure, (3) the self-sealing action of large diameter cuffs and (4) the differences in performance between different high-volume, low-pressure designs.

Cuff vs Tracheal Diameter Although designed to operate in similar fashion to the 7.o-mm LO cuff (a high pressure design), the 8.0mm LO cuff actually possessed pressure characteristics more like the MED and HI cuffs. This unexpected low pressure performance is attributable to the fact that the 8.0 LO cuff resting diameter was slightly larger than that of the tracheal model and did not have to be stretched to fill the trachea, instead passively assuming tracheal contour as it was inflated. The 7.0 LO cuff has the same shape as the 8.0 LO cuff, but is slightly smaller in diameter than the tracheal model. Consequentl~ the 7.0 LO cuff had to be stretched in order to occlude the trachea, leading to high intracuff pressures and alteration of normal tracheal contour. The marked difference in 7.0 and 8.0 LO intracuff pressures results from slight differences in cuff vs

tracheal diameter. This example shows that the relationship of CT diameter has been the key principle to the reduction of intracuff (hence CT) pressures in the change from low-volume, small-diameter cuffs to highvolume, large-diameter cuffs.

The Effect ofAinoo.y Pressure on Cufflnjlation Pressure Regardless of cuff design, raising the PIP by reducing CL necessitated additional cuff inflation and resulted in higher intracuff pressure to maintain the same level of ventilation. This in turn increases CT pressure that interferes with tracheal wall blood flow and predisposes to ischemic necrosis. The magnitude of the increase in cuff inflation pressure varied with the cuff design. In the less compliant lung (Fig 2B), the MED cuff design consistently required higher pressures than the HI cuff, and both designs required pressures bordering on or exceeding the upper limit n of "safe cuff pressure. In no case did the 7.0 LO n design function within the range of "safe pressure (the 8.0 LO is considered to be a unique example of ideal CT diameter, as discussed previously). The absolute intracuff pressures reported in this study are estimates and are subject to error; however, the fact that cuff inflation pressure must be increased in order to seal the trachea against high PIPs is clear. Nonetheless, it is not generally appreciated that current high-volume, low-pressure cuff designs cannot effectively seal the trachea with low pressures when the PIP is high. The problem does not revolve around improper cuff inflation techniques but is instead an inherent limitation of cuff design as outlined in the next two sections.

Self-Sealing Action When sealing the trachea during positive pressure ventilation, the ETT cuff is suspended between atmospheric pressure on one side and airway pressure on the other. The cuff resists leakage of gas from the airway by pressing itself outward into the tracheal mucosa (the CT pressure). Because the CT pressure is equal to the intracuff pressure in large diameter cuffs, a leak can develop whenever airway pressure exceeds intracuff pressure. However, our experiment showed that in such cuffs intracuff pressure begins to rise as the airway pressure exceeds the baseline cuff inDation pressure (Fig 3). By increasing its intracuff (hence CT) pressure, the cuff is automatically compensating for the increases in airway pressure without additional inflation. This self-sealing action was described by Carroll et al24 who wrote " ... we discovered in 1968 that any large diameter, large residual volume cuffcan be inflated to a baseline pressure just adequate to prevent aspiration, and that this resting intracuff pressure then rises in synchrony with the airway n pressure. CHEST I 100 I 4 I OCTOBER, 1991

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The self-sealing nature of large volume cuffs is explained by the dynamic effects of airway pressure on cuff size and shape. Increasing airway pressure is freely transmitted to the cuff across the flexible cuff wall, equalizing intracuff pressure and airway pressure. As airway pressure continues to rise, the gas contained within the cuff moves away from the area of high pressure (the distal cuff) and toward an area of lower pressure (the proximal cufl). Thus, the gas within the cuff is redistributed, or "milked" from the distal end of the cuff to fill the proximal end, resulting in a cone-shaped cuff (Fig 3) as the cuff collapses distally and bulges proximally. In 1971 Lomholt25 described " . . . this transformation of the cuff shape from cylindrical to conical during the inspiratory phase . . ." and estimated the extent to which such cuffs could compensate for volume changes while maintaining their seal. Differences between High-Volume, Low-Pressure De~gns

This study examines the performance of two different large-diameter cuff designs, both of which claim to be low pressure. While no differences are apparent at low PIPs, significant differences appear as the PIP increases (Fig 2). It is obvious from the difference in cuff inDation pressures shown in Figure 2 for the MED and HI cuff designs that all high-volume, lowpressure designs are not created equal. A reconsideration ofthe principles governing the self-sealing nature of these cuffs will expose their operational differences. As airway pressure increases, tracheal diameter increases. In order to maintain tracheal occlusion at the lowest possible cr pressure, the resting cuff diameter must be greater than the tracheal diameter at the point of maximal expansion (at the PIP). As the trachea is expanded by rising airway pressure, gas within the cuff is compressed into a conical shape as described previously. A leak will develop when the gas within the cuff is compressed to the point that the diameter of the proximal end of the cuff is smaller than the diameter of the expanded trachea. At that point, providing more gas to the proximal end of the compressed cuff will abolish the leak. Unfortunatel); additional cuff inDation will elevate the baseline cuff inflation pressure. Providing a larger reservoir of gas within the cuff allows additional gas to be "milked" into the proximal end of the cuff without increasing baseline cuff inDation pressure (Fig 4). Thus, the selfsealing action ofthe cuffis improved while maintaining the lowest possible cr pressure and preserving tracheal mucosal blood flo~ Because cuff inflation is limited by the tracheal diameter when cuff diameter is greater than tracheal diameter, further increases in cuff diameter will not change operational cuff volume. In practice, the only 1080

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FIGURE 4. IntraculT pressure equalizes with Paw as the latter rises. The gas within the culT moves proximally toward the end of the culT that is exposed to Palm. As a result, the distal end of the cuff collapses while the proximal end balloons proximally and laterally, changing the culT shape from cylindrical to conical (A --+ B). This redistribution of the gas within the culT creates a self-sealing action that allows the cuff to maintain tracheal occlusion despite increases in aiJWay pressure above the baseline culT inflation pressure. The amount of air available for redistribution determines the degree to which airway pressure may exceed the baseline culT inflation pressure before a leak will develop. Increases in culT length provide additional gas that can be redistributed to maintain inflation of the proximal culT and preserve tracheal occlusion (B --+ C).

means of increasing cuff volume without additional inflation is construction of a longer cuff The improvement in performance gained by an increase in cuff length is exemplified in our experiment. The 8.0 MED cuff dimensions have been measured as 28.4 (diameter) x 33.5 mm (length), and the 8.0 HI cuff as 29.5 X 43.3 mm. l " The two cuffs have nearly identical diameters, both of which exceed the 19 x 22-mm internal diameter of the model trachea. The HI cuff is about 1 cm longer and provides a larger volume of gas that can be "milked" to fill the proximal end during inspiration. The result is an improvement in the 8.0 HI cuff's self-sealing action that is manifested by achieving tracheal occlusion with a 5 percent leak at a lower baseline cuff inDation pressure than the 8.0 MED cuff (Fig 2B). From the foregoing discussion it follows that as long as resting cuff diameter is greater than that of the trachea at maximal expansion, cuff length is the primary factor in maintaining a selfsealing effect in the face of rising airway pressure (or, all else being equal, the longer cuff wins). CONCLUSION

The confounding problem of excessive intracuff pressure, which damages the trachea but is necessary to provide a seal at high PI~ has not been resolved. We recently determined that 55 percent of a group of mechanically ventilated SICU patients required cuff inDation pressures in excess of 25 mm Hg despite the High-volume, Low-pressure Cuffs (Guyton, Banner, Kirby)

use of high-volume, low-pressure cuffs (unpublished data, E.A. Radson, T.E. Banner, M.J. Banner, et al). This observation and the report of Stauffer et al9 suggest that current cuff design is inadequate. When high PIP is necessary during mechanical ventilation, tracheal damage is predictable, even with meticulous attention to cuff inflation. Such requirements are incompatible with maintenance of low intracuff and CT pressures if satisfactory ventilation is to be maintained. Intermittent cuff inflation synchronized with the ventilator's inspiratory phase, with partial deflation during exhalation, can resolve some ofthese problems. The ultimate solution awaits a new cuff design. REFERENCES 1 Steen JA. Impact of tube design and materials on complications of tracheal intubation. In: Bishop MJ, (guest ed). Physiology and consequences of tracheal intubation: problems in anesthesia, vol 2, No.2, April-June 1988. Philadelphia: JB Lippincott Co, 1988: 211-24 2 Cooper JD, Grillo HC. Analysis of problems related to cuffs on intratracheal tubes. Chest 1972; 62:21S-27S 3 Gemn B, Pontoppidan H. Reduction of tracheal damage by the prestretching of inftatable cuffs. Anesthesiology 1969; 31:462-63 4 Wen-Hsien ~ I1-Taik L, Simpson FA Jr, Turndorf H. Pressure dynamics of endotracheal and tracheostomy cuffs. Crit Care Moo 1973; 1:197-202 5 Carroll R, Hedden M, Safar e Intratracheal cuffs: performance characteristics. Anesthesiology 1969; 31:275-80 6 McGinnis GE, Shively JG, Patterson RL, Macgovern GJ. An engineering analysis of intratracheal tube cuffs. Anesth Analg Curr Res 1971; 50:557-64 7 Knowlson GTG, Bassett HFM. The pressures exerted on the trachea by endotracheal inflatable cuffs. Br J Anaesth 1970; 42:834-37 8 MacKenzie CF, Klose S, Browne DRG. A study of inflatable cuffs on endotracheal tubes. Br J Anaesth 1976; 48:105-09 9 Stauffer JL, Olson DE, Petty TL. Complications and consequences of endotracheal intubation and tracheotomy. Am J Med 1981; 70:65-76

10 American National Standards Institute. American national standard for anesthetic equipment-cuffed oral tracheal and nasal tracheal tubes for prolonged use. ANSI Z79.16-1983. New York: American National Standards, 1983:6-20 11 ECRI. Artificial airways. Health Devices 1978; 7:67-91 12 Griscom NT, Wohl MEB. Tracheal size and shape: effects of change in intraluminal pressure. Radiology 1983; 149:27-30 13 Bernhard WN, Yost L, Joynes D, Cothalis S, Turndorf H. Intracuff pressures in endotracheal and tracheostomy tubes. Chest 1985; 87:720-25 14 Bernhard WN, Yost L, TurndorfH, Danziger F. Cuffed tracheal tubes-physical and behavioral characteristics. Anesth Analg 1982; 61:36-41 15 Cooper JL, Grillo HC. Experimental production and prevention of injury due to cuffed tracheal tubes. Surg Gynecol Obstet 1969; 129:1235-41 16 Grillo HC, Cooper JD, Gemn B, Pontoppidan H. A lowpressure cuff for tracheostomy tubes to minimize tracheal injury. J Thorac Cardiovasc Surg 1971; 62:898-907 17 Ching N~ Ayres SM, PaegIe ~ Linden JM, Nealon TF. The contribution of cuff volume and pressure in tracheostomy tube damage. J Thorac Cardiovasc Surg 1971; 52:402-10 18 Gemn B, Grillo HC, Cooper JD. Stenosis following tracheostomy for respiratory care. JAMA 1971; 216:1984-88 19 Lewis FR Jr, Schlobohm RM, Thomas AN. Prevention of complications from prolonged tracheal intubation. Am J Surg 1978; 135:452-57 20 Nordin U, Undholm CE, Wolgast M. Blood flow in the rabbit tracheal mucosa under normal conditions and under the influence of tracheal intubation. Acta Anaesth Scand 1977; 21:81-94 21 Bjorkerud S, Ekedahl C, Hansson PG, Jeppsson PH, Undstrom J. Experimental tracheal wall injury. Acta Otolaryng 1973; 75:387-88 22 Ching N, Nealon TF Jr. Cuffpressure measurements (Communication to the Editor). Chest 1974; 66:604-05 23 Seegobin RD, van Hasselt GL. Endotracheal cuff pressure and tracheal mucosal blood Bow: endoscopic study of effects of four large volume cuffs. Dr Moo J 1984; 288:965-68 24 Carroll RG, McGinnis GE, Grenvik A. Performance characteristics oftracheal cuffs. International Anesthesiology Clinics 1974; 12:111-41 25 Lomholt N. A new tracheostomy tube. Acta Anesth Scand Suppl 1971; 44:6-39

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