Anaesthesia, 1992, Volume 47, pages 343-347 APPARATUS
Resistance and additional inspiratory work imposed by the laryngeal mask airway A comparison with tracheal tubes
S. B. BHATT, A. P. KENDALL, E. S. LIN,
AND
T. E. OH
Summary Laryngeal mask airways and tracheal tubes were studied to determine both their resistance to constant gas flows and additional inspiratory work during simulated inspiration. Laryngeal mask airways imposed less resistance and required lower additional inspiratory work compared with the corresponding sized tracheal tubes. If inspiratory loading during anaesthesia is an important consideration, then the laryngeal mask airway may be preferable to a tracheal tube. Key words Equipment; laryngeal mask, tracheal tube. Airway; resistance.
The laryngeal mask airway (LMA) has recently been introduced into clinical practice as an alternative means of maintaining the airway during anaesthesia. Since its introduction it has gained widespread popularity and has been used in a variety of clinical situations [ I , 21. It is relatively easy to use and may have advantages over a conventional tracheal tube. For example, an LMA can be used in patients who are difficult to intubate [3], and laryngeal injury associated with tracheal intubation may be avoided. The cardiovascular response to LMA insertion is less than that associated with laryngoscopy and tracheal intubation [4, 51. Also the LMA frees the anaesthetist’s hands for performing other tasks. Despite a host of reports on the clinical use of the LMA, little is known of its interaction with the patient’s respiratory system. The LMA has three vertical bars within its lumen at the patient end. These are designed to prevent the epiglottis from obstructing the lumen, but may impede gas flow and lead to increased resistance to breathing. With this in mind, we studied the resistance and additional inspiratory work imposed by the LMA in comparison with tracheal tubes.
Material and methods The resistances of LMAs sizes I 4 and tracheal tubes (Portex, Kent, UK) sizes 4-9 mm internal diameter (ID)
were studied under conditions of constant gas flow and simulated inspiration. All measurements were made using dry air at room temperature. Air flow was measured using a Fleisch no. 2 pneumotachograph (Instrumentation Associates, NY, USA) coupled with a sensitive pressure transducer (MP 45-1, (SD 2) cmH,O Validyne Engineering, Northridge. Ca, USA). Pressure measurements were made using a calibrated Gaeltec 8T 50 differential pressure transducer (Medical Measurements Inc., NJ, USA) which was linear between 0-50 cmH,O. The pressure and flow transducers were calibrated using a n R T 200 calibration analyser (Timemeter Corp, PA, USA). Pressure and flow signals were displayed and recorded on an oscilloscope (DSO 1604, Gould Electronics Ltd, Essex, UK). Digitized data at 200 samp1es.s-’ were then processed using an IBM computer. All tracheal tubes were studied with their connectors in place and cut to various lengths, as shown in Table 1. These lengths have been recommended [6] for use in patients according to patient age, and denote the expected distance from lips to mid-trachea. For constant gas flow measurements, a flowmeter was used to pass air through the pneumotachograph and then through the LMA or tracheal tube (Fig. I(a)). The patient end of the LMA or tracheal tube was interfaced with a model trachea, which was constructed from a transparent plastic hose of 24 mm ID and 10 cm length. The differential pressure between points PI and P2 was measured (Fig. I). Point P2 was
S.B. Bhatt, MD, FFARCS, A.P. Kendall, FFARCS, Lecturers, E.S. Lin, MRCP, FFARCS, T.E. Oh, MD, DA, FFARCS, FFARACS, Professor and Chairman. Department of Anaesthesia and Intensive Care, Chinese University of Hong Kong, Prince of Wales Hospital, Shatin, N.T., Hong Kong. Correspondence should be addressed to Professor T.E. Oh please. 0003-2409/92/040343
+ 05 S03.00/0
@ 1992 The Association of Anaesthetists of G t Britain and Ireland
343
344
S.B. Bhaii et al. Gasflow
4
cm
C
I1
U C
(b)
(a)
Fig. 1. (a) Tracheal tube and LMA during constant flow, and (b) simulated inspiration experiments. Pn, pneumotachograph. pl and p2. differential pressure measuring sites; FG, flow generator; TT. tracheal tube.
located 10 mm downstream of the patient end of the LMA or tracheal tube and the pressure measuring catheter tip was placed midway between the model tracheal wall and the centre of its lumen. Measurements were made at approximately 2.5 I.min- I increments, from &30 I.min-' and thereafter at 5 1.min-I increments up to 60 I.min-I. All measurements were made in duplicate. The means of the two results were used for computing the pressure-flow curves and for the calculation of resistances. A second order polynomial ( P= K; V + K ; Vz, where P is the pressure gradient across the tube , is the flow, and K, and K, are constants) was fitted to the pressure-flow points and appeared to describe the curves adequately. The equation P = K ; V + K;V2 was then solved for flows of 0.5 1.s-l and 1.0 1 . s ~ ' . Resistances were deduced according to the relationship: resistance = pressure gradient/flow. The resistances thus calculated were expressed as kPa 1.s-l at 0.5 IS' and 1.0 1 . s ~ ' . Simulated inspiration studies of additional inspiratory work were performed using a modified Cape T C 50 ventilator (Cape Engineering, UK) as previously described [7]. The LMAs and tracheal tubes were interfaced with the ventilator using the model trachea described earlier. The tracheal tube cuff was inflated to ensure an airtight seal up to a pressure of 20 cmH,O. A similar airtight seal for the LMA was achieved using an interface attached to the model trachea. The interface for the LMA with the model trachea was made from moulded plastic into which the inflated cuff of the LMA fitted snugly. Inspiration was simulated by the vacuum effect created by the Cape ventilator's expiratory phase [7]. The simulated inspiratory flow
during each inspiratory half cycle was an approximate sine wave. The experimental arrangement is as shown in Figure I(b). In measuring additional inspiratory v;ork and dynamic pressure flow characteristics, each LMA and tracheal tube was studied at eight different tidal volumes. The tidal volumes studied ranged from 60 ml for LMA size I to 850 ml for LMA size 3/4. Tidal volumes were selected across a range considered suitable for the patient population in whom these devices were likely to be used. The respiratory rate was held constant at 15 breath.min-' and the I : E ratio was held constant at 1 : 3. Tidal volume was measured by integrating the digitized flow signal during the inspiratory half cycle. Additional inspiratory work was computed from the pressure : flow recordings according to the method of Engstrom and Norlander [8]. The product of pressure (P) and flow (V) a t a time was used to derive power (W) at that time. Integration of the power curve with respect to time (from the onset of to the end of inspiratory flow) was used to yield work over the inspiratory cycle. Additional inspiratory work was expressed as Joulesibreath. For both resistance and additional inspiratory work studies, the data for each size of LMA was compared with tracheal tubes suitable for use in similar sized patients.
Results LMAs size 3 and 4 had identical internal diameters and lengths, and yielded similar values for resistance to gas flow. These data were therefore pooled together.
Table 1. Dimensions and resistances of tracheal tubes and laryngeal mask airways under test.
Device under test Tracheal tube 4 Tracheal tube 5 Tracheal tube 6 Tracheal tube 7 Tracheal tube 8 Tracheal tube 9 LMA I LMA 2 LMA 314
Length (cm)
Internal diameter (mm)
Resistance (kPa.1.s-I)
12
4.00
4.84
14
5.00 6.00 7.00 8.00 9.00 6.00 8.00 11.00
2.12 0.86 0.38 0.26 0.18 i.16 0.26 0.1
15.5 18.5 22 23 10
12 18
at 0.5
Resistance (kPa.1.s-') at 1.0 10.49 4.47
2. I 1.07 0.53 0.33 2.83 0.45 0.15
The LMA and tracheal tubes Table 2. Patient size recommendations for the use of LMA. LMA size
Patient size
One
Neonate, 6.5 kg
Two
6.5-25 kg 25 kg. small adult
Three Four
2-
0 TT 7mm 0 T T 8 mm
ID ID
TT 9 m m I D
0
L M A 3/4
Normal to large adult 0
I -
345
/, /
% 70.
Constant Jon, experiments-resistance Typical differential pressure flow curves for the LMAs and tracheal tubes are shown in Figure 2. The LMA size 1 was grouped with a tracheal tube of 4 mm ID, the LMA size 2 with tracheal tubes of 5 and 6 mm I D and LMAs of sizes 3 and 4 with tracheal tubes of 7, 8 and 9 mm ID. These groupings were made according to the manufacturer's recommendations for patient sizes for LMAs (Table 2). The differential pressure at any flow and the calculated resistances at 0.5 1.s-l and 1.0 I.s-' were less for the LMA compared to the corresponding sized tracheal tube (Fig. 2. Table I ) .
0 0.0
/
I .o
0.5
1.5
T T 6 mm I D
0
TT5mm LMA 2
ID
Simulated inspirat ion exper iments-acidit ional inspira tory work The additional tracheal tube and LMA inspiratory work is shown in Figure 3. At any given tidal volume, the additional inspiratory work increased with decreasing tube diameters. Moreover, at any tidal volume, the additional inspiratory work while inspiring through an LMA was less than that measured when inspiration was simulated through the corresponding-sized tracheal tube (Fig. 3). These differences were especially marked when larger tidal volumes were studied.
0
n I
?
a
2
0 0.0
0.2
0.4
0.6
0.8
I
.o
I .2
Discussion The present study compared the resistance and additional inspiratory loads imposed by LMAs with the corresponding-sized tracheal tubes. Previous reports show varied tracheal tube resistances obtained during constant flow experiments [9, lo]. The differences were attributed to the positioning of the downstream pressure measuring catheter [ 1 I]. Placement of the downstream pressure measuring catheter in the present study was similar to that of Demers and colleagues [l I ] and our results for resistance at 0.5 IS' and 1.0 IS' are comparable to theirs. Demers and colleagues [ 1 I] reported the resistance of an 8.0 mm I D tracheal tube at 0.5 1.s-l as 0.37 kPa.1.s-I. In the present study the resistance of a similar internal diameter tube was found to be 0.26 kPa.1.s-' at 0.5 1 . s ~ ' . Ferguson and colleagues [ 121 measured airway resistance using body plethysmography with and without inspiration through an LMA and concluded that the resistance offered by the LMA was of the same order of magnitude as a tracheal tube. However, in the present study it was found that the resistance of the LMA during constant gas flow was less than with the corresponding size tracheal tube (Table I ) . Ferguson and colleagues d o not specify the size of LMA tested and therefore the present results cannot be compared to theirs.
4 c
/
/
Flow 1 . i '
Fig. 2. Pressure flow relationship of tracheal tubes and LMAs during constant flow experiment. TT-tracheal tube.
The LMA's lower resistance, compared to the corresponding-sized tracheal tube, is a reflection of the shorter length, and more importantly, the larger diameter of the LMA. These differences were especially marked when smaller tracheal tubes were compared with the corresponding-sized LMA. For example, the resistance during
S.B. Bhatt et al.
D /
0 T T 9 m m I0
ip
/
I 0.5 OS6
0.2
0.4
0.6
F
0
T T B m m I0 T T 7 m m 10 Q LMA 3 / 4
-
0.e
0.1
1.2
P
0.4
0 0.3
0
T T 6 mm I D
I T 5 mm I 0 2
LMA
-
0.2
0. I 1
0.4
0.2
I 0.e
0.6
r
0.0
0.I
0.2
0.3
Tidal volume ( I )
Fig. 3. Work-tidal volume relationship of tracheal tubes and LMAs during simulated inspiration experiment. TT, tracheal tube.
constant flow of a 4 mm I D tracheal tube was found to be 4.84 kPa.l.s.-' at 0.5 IK', while that of the correspondingsized LMA (size 1) was only 1.16 kPa.1.s-'. However, when LMAs were compared to tracheal tubes of the same
internal diameter the LMAs, despite their shorter lengths, appeared to have greater resistance. LMA size 1 with a 6 mm I D had a resistance of 1.16 kPa.1.s-' at 0.5 I S ' , while that of a tracheal tube with a 6 mm ID. despite its longer length, had a resistance of only 0.86 kPa.1.s-' at 0.5 I.s-'. The three vertical bars at the patient end of the LMA lumen may contribute to this increased resistance. It has previously been suggested that the additional inspiratory work imposed by tracheal tubes may be a more relevant parameter to study, rather than resistance during constant gas flow [13]. The additional work imposed by tracheal tubes during simulated inspiration has been studied previously [13-15]. Our results for additional work during simulated inspiration are similar to those obtained by Bolder and colleagues [ 131, who measured additional inspiratory work at frequencies of 6, 12 and 20 breath.min-I. In the present study, we simulated respiration at 15 breath.min-' and our results for additional inspiratory work lie between those obtained by Bolder et ul. at 12 and 20 breath.min-'. For example they found thc additional inspiratory work during a 350 ml inspiration through an tracheal tube 7 mm ID was 0.069 Joules at 12 breath.min-l and 0.138 Joules at 20 breath.min-'. In the present study for a 370 ml tidal volume at 15 brcath.min-' we found it to be 0.1 17 Joules. In addition our results confirm that smaller tube diameters and larger tidal volumes increase the additional inspiratory work. (Fig. 3). The additional inspiratory work imposed by LMAs has not been previously reported. In the present study, the relative tidal volume/work relationships for tracheal tubes and LMAs appeared to reflect resistance measured during constant flow experiments. Additional inspiratory work imposed by an LMA was considerably less than that imposed by the corresponding-sized tracheal tube. For example, the additional inspiratory work for a 430 ml tidal volume through an 8.0 mm ID tracheal tube was found to be 0.1 12 Joules, whereas additional work while breathing through an LMA sized 3/4 for a similar tidal volume was 0.012 Joules. Mecklenburgh and colleagues [ 161 estimated the natural work of breathing for a 500 ml tidal volume to be about 0.2 Joules. Thus, while the additional work imposed by an 8.0 mm ID tracheal tube is of the same magnitude as that expended in overcoming the compliance and resistance of the lungs, the work imposed by a corresponding sized LMA is considerably less. Increased resistance of breathing systems and the added inspiratory loads imposed, may affect successful weaning from mechanical ventilation [ 161, in patients recovering from respiratory failure. However, exact consequences of moderate increases in system resistance in an anaesthetised, spontaneously breathing patient are unclear. Nunn and Ezi-Ashi [ 171 found that during anaesthesia respiratory compensations for added inspiratory and expiratory loading was rapid and complete. Freedman and Campbell [ 181, on the other hand, presented evidence to suggest that during inhalational anaesthesia, ventilatory responsiveness to added inspiratory loading may be diminished. Despite this difference of opinion, i t is a good practice to be aware of the respiratory loading effect of different anaesthetic apparatus and to keep these loads to a minimum. The results of the present study indicate that, on the basis of the added inspiratory load of the two tubes, the LMA may be preferable to a tracheal tube. However, an additional source of inspiratory resistance while using an
The LMA and tracheal tubes LMA m a y arise a t mask-larynx interface. A faulty apposition of t h e LMA t o t h e larynx w o u l d increase resistance a n d inspiratory w o r k . T h e methodology of t h e present experimental study did n o t address this question. In conclusion, t h e resistance d u r i n g c o n s t a n t flow a n d additional inspiratory w o r k imposed by t h e LMA w a s f o u n d t o be considerably less t h a n t h a t imposed by t h e corresponding sized tracheal tube. If a choice is available, then in terms of their inspiratory loading effect, it m a y b e preferable t o use a n LMA rather t h a n a tracheal tube.
8. ENCSTROM CG, NORLANDER OP. A new method for analysis of
9. 10.
II. 12.
References I . BRODRICK PM, WESTER NJ, N U N NJF. The laryngeal mask airway. A study of 100 patients during spontaneous breathing. Anaesrhesia 1989; 44: 23841. 2. JOHNSTONDF, WRICLEYSR, ROBH PJ. JONES HE. The laryngeal mask airway in paediatric anaesthesia. Anaesrhesia 1990; 45: 92427. 3. BRAINAIJ. Three cases of difficult intubation overcome by the laryngeal mask airway. Anaesthesia 1985; 40: 353-55. S, CAMERON AE, ASBUKY AJ. Cardiovascular response 4. HICKEY to insertion of Brain’s laryngeal mask. Anaesrhesia 1990; 4 5 629-33. N, CLEMENTS EAF. HOUCESUM, ANDREWS BP. The 5. BRAUDE pressor response and laryngeal mask insertion. A comparison with tracheal intubation. Anae.srhe.sia 1989; 44: 551-54. RK, MILLERRD. Basics of’anesrhesia. New York: 6 STOELTINC Churchill Livingstone. 1984: 157. 7 OH TE, LIN ES, BHATTS. Resistance of humidifiers, and inspiratory work imposed by a ventilator-humidifier circuit. Brirish Journal of Anaesthesiu 1991; 66: 258-63.
347
13. 14.
15.
16.
17. 18.
respiratory work by measurements of the actual power as a function of gas flow, pressure and time. Acra Anaeslhesiologica Scandinavica 1962; 6 49-55. SULLIVAN M, PALIOTTA J, SAKLAD M . Endotracheal tube as a factor in measurement of respiratory mechanics. Journal qf’ Applied Physiology 1976; 41: 59&92. SAHNSA, LAKSHMINARAYAN S, PETTYTL. Weaning from mechanical ventilation. Journal of /he American Medical A.ssociurion 1976; 2 3 5 2208-12. DEMERS RR, SULLIVAN MJ, PALIOTTA J. Airflow resistances of Endotracheal Tubes. Journal of /he American Medical Associarion 1971; 237: 1362. FERCUSON C, HEKDMAN M, EVANSK, HAYESM, COLE PV. Flow resistance of the laryngeal mask in awake subjects. Brirish Journal of Anaesrhesia I99 I ; 66: 400p. BOLDERPM, HEALYTEJ, BOLDERAR, BEATTYPCW. KAYB. The extra work of breathing through adult endotracheal tubes. Anesthesia and Analgesia 1986; 65: 853-59. BERSTENAD, RUTTEN AJ, VEDIC AE, SKOWRONSKI GA. Additional work of breathing imposed by endotracheal tubes, breathing circuits, and intensive care ventilators. Crirical Care Medicine 1989; 17: 671-77. SHAPIRO M, WILSONRK, CASARG . BLOOMK, TEAGUERB. Work of breathing through different sized endotracheal tubes. Crirical Cure Medicine 1986; 1 4 1028-3 I . MECKLENBURCH JS, LATTOIP, AL-OBAIDITAA, SWAIEA. MAPLESONWW. Excessive work of breathing during intermittent mandatory ventilation. Brirish Journal of Anaesthesia 1986; 58: 1048-54. NUNNJF, EZI-ASHITI. The respiratory effects of resistance to breathing in anesthetized man. Anesrhesiology 1961; 2 2 174-85. FREEDMAN S, CAMPBELL EJM. The ability of normal subjects to tolerate added inspiratory loads. Respirarory Physiology 1970; 1 0 213-35.
Resistance and additional inspiratory work imposed by the laryngeal mask airway A comparison with tracheal tubes
S. B. BHATT, A. P. KENDALL, E. S. LIN,
AND
T. E. OH
Summary Laryngeal mask airways and tracheal tubes were studied to determine both their resistance to constant gas flows and additional inspiratory work during simulated inspiration. Laryngeal mask airways imposed less resistance and required lower additional inspiratory work compared with the corresponding sized tracheal tubes. If inspiratory loading during anaesthesia is an important consideration, then the laryngeal mask airway may be preferable to a tracheal tube. Key words Equipment; laryngeal mask, tracheal tube. Airway; resistance.
The laryngeal mask airway (LMA) has recently been introduced into clinical practice as an alternative means of maintaining the airway during anaesthesia. Since its introduction it has gained widespread popularity and has been used in a variety of clinical situations [ I , 21. It is relatively easy to use and may have advantages over a conventional tracheal tube. For example, an LMA can be used in patients who are difficult to intubate [3], and laryngeal injury associated with tracheal intubation may be avoided. The cardiovascular response to LMA insertion is less than that associated with laryngoscopy and tracheal intubation [4, 51. Also the LMA frees the anaesthetist’s hands for performing other tasks. Despite a host of reports on the clinical use of the LMA, little is known of its interaction with the patient’s respiratory system. The LMA has three vertical bars within its lumen at the patient end. These are designed to prevent the epiglottis from obstructing the lumen, but may impede gas flow and lead to increased resistance to breathing. With this in mind, we studied the resistance and additional inspiratory work imposed by the LMA in comparison with tracheal tubes.
Material and methods The resistances of LMAs sizes I 4 and tracheal tubes (Portex, Kent, UK) sizes 4-9 mm internal diameter (ID)
were studied under conditions of constant gas flow and simulated inspiration. All measurements were made using dry air at room temperature. Air flow was measured using a Fleisch no. 2 pneumotachograph (Instrumentation Associates, NY, USA) coupled with a sensitive pressure transducer (MP 45-1, (SD 2) cmH,O Validyne Engineering, Northridge. Ca, USA). Pressure measurements were made using a calibrated Gaeltec 8T 50 differential pressure transducer (Medical Measurements Inc., NJ, USA) which was linear between 0-50 cmH,O. The pressure and flow transducers were calibrated using a n R T 200 calibration analyser (Timemeter Corp, PA, USA). Pressure and flow signals were displayed and recorded on an oscilloscope (DSO 1604, Gould Electronics Ltd, Essex, UK). Digitized data at 200 samp1es.s-’ were then processed using an IBM computer. All tracheal tubes were studied with their connectors in place and cut to various lengths, as shown in Table 1. These lengths have been recommended [6] for use in patients according to patient age, and denote the expected distance from lips to mid-trachea. For constant gas flow measurements, a flowmeter was used to pass air through the pneumotachograph and then through the LMA or tracheal tube (Fig. I(a)). The patient end of the LMA or tracheal tube was interfaced with a model trachea, which was constructed from a transparent plastic hose of 24 mm ID and 10 cm length. The differential pressure between points PI and P2 was measured (Fig. I). Point P2 was
S.B. Bhatt, MD, FFARCS, A.P. Kendall, FFARCS, Lecturers, E.S. Lin, MRCP, FFARCS, T.E. Oh, MD, DA, FFARCS, FFARACS, Professor and Chairman. Department of Anaesthesia and Intensive Care, Chinese University of Hong Kong, Prince of Wales Hospital, Shatin, N.T., Hong Kong. Correspondence should be addressed to Professor T.E. Oh please. 0003-2409/92/040343
+ 05 S03.00/0
@ 1992 The Association of Anaesthetists of G t Britain and Ireland
343
344
S.B. Bhaii et al. Gasflow
4
cm
C
I1
U C
(b)
(a)
Fig. 1. (a) Tracheal tube and LMA during constant flow, and (b) simulated inspiration experiments. Pn, pneumotachograph. pl and p2. differential pressure measuring sites; FG, flow generator; TT. tracheal tube.
located 10 mm downstream of the patient end of the LMA or tracheal tube and the pressure measuring catheter tip was placed midway between the model tracheal wall and the centre of its lumen. Measurements were made at approximately 2.5 I.min- I increments, from &30 I.min-' and thereafter at 5 1.min-I increments up to 60 I.min-I. All measurements were made in duplicate. The means of the two results were used for computing the pressure-flow curves and for the calculation of resistances. A second order polynomial ( P= K; V + K ; Vz, where P is the pressure gradient across the tube , is the flow, and K, and K, are constants) was fitted to the pressure-flow points and appeared to describe the curves adequately. The equation P = K ; V + K;V2 was then solved for flows of 0.5 1.s-l and 1.0 1 . s ~ ' . Resistances were deduced according to the relationship: resistance = pressure gradient/flow. The resistances thus calculated were expressed as kPa 1.s-l at 0.5 IS' and 1.0 1 . s ~ ' . Simulated inspiration studies of additional inspiratory work were performed using a modified Cape T C 50 ventilator (Cape Engineering, UK) as previously described [7]. The LMAs and tracheal tubes were interfaced with the ventilator using the model trachea described earlier. The tracheal tube cuff was inflated to ensure an airtight seal up to a pressure of 20 cmH,O. A similar airtight seal for the LMA was achieved using an interface attached to the model trachea. The interface for the LMA with the model trachea was made from moulded plastic into which the inflated cuff of the LMA fitted snugly. Inspiration was simulated by the vacuum effect created by the Cape ventilator's expiratory phase [7]. The simulated inspiratory flow
during each inspiratory half cycle was an approximate sine wave. The experimental arrangement is as shown in Figure I(b). In measuring additional inspiratory v;ork and dynamic pressure flow characteristics, each LMA and tracheal tube was studied at eight different tidal volumes. The tidal volumes studied ranged from 60 ml for LMA size I to 850 ml for LMA size 3/4. Tidal volumes were selected across a range considered suitable for the patient population in whom these devices were likely to be used. The respiratory rate was held constant at 15 breath.min-' and the I : E ratio was held constant at 1 : 3. Tidal volume was measured by integrating the digitized flow signal during the inspiratory half cycle. Additional inspiratory work was computed from the pressure : flow recordings according to the method of Engstrom and Norlander [8]. The product of pressure (P) and flow (V) a t a time was used to derive power (W) at that time. Integration of the power curve with respect to time (from the onset of to the end of inspiratory flow) was used to yield work over the inspiratory cycle. Additional inspiratory work was expressed as Joulesibreath. For both resistance and additional inspiratory work studies, the data for each size of LMA was compared with tracheal tubes suitable for use in similar sized patients.
Results LMAs size 3 and 4 had identical internal diameters and lengths, and yielded similar values for resistance to gas flow. These data were therefore pooled together.
Table 1. Dimensions and resistances of tracheal tubes and laryngeal mask airways under test.
Device under test Tracheal tube 4 Tracheal tube 5 Tracheal tube 6 Tracheal tube 7 Tracheal tube 8 Tracheal tube 9 LMA I LMA 2 LMA 314
Length (cm)
Internal diameter (mm)
Resistance (kPa.1.s-I)
12
4.00
4.84
14
5.00 6.00 7.00 8.00 9.00 6.00 8.00 11.00
2.12 0.86 0.38 0.26 0.18 i.16 0.26 0.1
15.5 18.5 22 23 10
12 18
at 0.5
Resistance (kPa.1.s-') at 1.0 10.49 4.47
2. I 1.07 0.53 0.33 2.83 0.45 0.15
The LMA and tracheal tubes Table 2. Patient size recommendations for the use of LMA. LMA size
Patient size
One
Neonate, 6.5 kg
Two
6.5-25 kg 25 kg. small adult
Three Four
2-
0 TT 7mm 0 T T 8 mm
ID ID
TT 9 m m I D
0
L M A 3/4
Normal to large adult 0
I -
345
/, /
% 70.
Constant Jon, experiments-resistance Typical differential pressure flow curves for the LMAs and tracheal tubes are shown in Figure 2. The LMA size 1 was grouped with a tracheal tube of 4 mm ID, the LMA size 2 with tracheal tubes of 5 and 6 mm I D and LMAs of sizes 3 and 4 with tracheal tubes of 7, 8 and 9 mm ID. These groupings were made according to the manufacturer's recommendations for patient sizes for LMAs (Table 2). The differential pressure at any flow and the calculated resistances at 0.5 1.s-l and 1.0 I.s-' were less for the LMA compared to the corresponding sized tracheal tube (Fig. 2. Table I ) .
0 0.0
/
I .o
0.5
1.5
T T 6 mm I D
0
TT5mm LMA 2
ID
Simulated inspirat ion exper iments-acidit ional inspira tory work The additional tracheal tube and LMA inspiratory work is shown in Figure 3. At any given tidal volume, the additional inspiratory work increased with decreasing tube diameters. Moreover, at any tidal volume, the additional inspiratory work while inspiring through an LMA was less than that measured when inspiration was simulated through the corresponding-sized tracheal tube (Fig. 3). These differences were especially marked when larger tidal volumes were studied.
0
n I
?
a
2
0 0.0
0.2
0.4
0.6
0.8
I
.o
I .2
Discussion The present study compared the resistance and additional inspiratory loads imposed by LMAs with the corresponding-sized tracheal tubes. Previous reports show varied tracheal tube resistances obtained during constant flow experiments [9, lo]. The differences were attributed to the positioning of the downstream pressure measuring catheter [ 1 I]. Placement of the downstream pressure measuring catheter in the present study was similar to that of Demers and colleagues [l I ] and our results for resistance at 0.5 IS' and 1.0 IS' are comparable to theirs. Demers and colleagues [ 1 I] reported the resistance of an 8.0 mm I D tracheal tube at 0.5 1.s-l as 0.37 kPa.1.s-I. In the present study the resistance of a similar internal diameter tube was found to be 0.26 kPa.1.s-' at 0.5 1 . s ~ ' . Ferguson and colleagues [ 121 measured airway resistance using body plethysmography with and without inspiration through an LMA and concluded that the resistance offered by the LMA was of the same order of magnitude as a tracheal tube. However, in the present study it was found that the resistance of the LMA during constant gas flow was less than with the corresponding size tracheal tube (Table I ) . Ferguson and colleagues d o not specify the size of LMA tested and therefore the present results cannot be compared to theirs.
4 c
/
/
Flow 1 . i '
Fig. 2. Pressure flow relationship of tracheal tubes and LMAs during constant flow experiment. TT-tracheal tube.
The LMA's lower resistance, compared to the corresponding-sized tracheal tube, is a reflection of the shorter length, and more importantly, the larger diameter of the LMA. These differences were especially marked when smaller tracheal tubes were compared with the corresponding-sized LMA. For example, the resistance during
S.B. Bhatt et al.
D /
0 T T 9 m m I0
ip
/
I 0.5 OS6
0.2
0.4
0.6
F
0
T T B m m I0 T T 7 m m 10 Q LMA 3 / 4
-
0.e
0.1
1.2
P
0.4
0 0.3
0
T T 6 mm I D
I T 5 mm I 0 2
LMA
-
0.2
0. I 1
0.4
0.2
I 0.e
0.6
r
0.0
0.I
0.2
0.3
Tidal volume ( I )
Fig. 3. Work-tidal volume relationship of tracheal tubes and LMAs during simulated inspiration experiment. TT, tracheal tube.
constant flow of a 4 mm I D tracheal tube was found to be 4.84 kPa.l.s.-' at 0.5 IK', while that of the correspondingsized LMA (size 1) was only 1.16 kPa.1.s-'. However, when LMAs were compared to tracheal tubes of the same
internal diameter the LMAs, despite their shorter lengths, appeared to have greater resistance. LMA size 1 with a 6 mm I D had a resistance of 1.16 kPa.1.s-' at 0.5 I S ' , while that of a tracheal tube with a 6 mm ID. despite its longer length, had a resistance of only 0.86 kPa.1.s-' at 0.5 I.s-'. The three vertical bars at the patient end of the LMA lumen may contribute to this increased resistance. It has previously been suggested that the additional inspiratory work imposed by tracheal tubes may be a more relevant parameter to study, rather than resistance during constant gas flow [13]. The additional work imposed by tracheal tubes during simulated inspiration has been studied previously [13-15]. Our results for additional work during simulated inspiration are similar to those obtained by Bolder and colleagues [ 131, who measured additional inspiratory work at frequencies of 6, 12 and 20 breath.min-I. In the present study, we simulated respiration at 15 breath.min-' and our results for additional inspiratory work lie between those obtained by Bolder et ul. at 12 and 20 breath.min-'. For example they found thc additional inspiratory work during a 350 ml inspiration through an tracheal tube 7 mm ID was 0.069 Joules at 12 breath.min-l and 0.138 Joules at 20 breath.min-'. In the present study for a 370 ml tidal volume at 15 brcath.min-' we found it to be 0.1 17 Joules. In addition our results confirm that smaller tube diameters and larger tidal volumes increase the additional inspiratory work. (Fig. 3). The additional inspiratory work imposed by LMAs has not been previously reported. In the present study, the relative tidal volume/work relationships for tracheal tubes and LMAs appeared to reflect resistance measured during constant flow experiments. Additional inspiratory work imposed by an LMA was considerably less than that imposed by the corresponding-sized tracheal tube. For example, the additional inspiratory work for a 430 ml tidal volume through an 8.0 mm ID tracheal tube was found to be 0.1 12 Joules, whereas additional work while breathing through an LMA sized 3/4 for a similar tidal volume was 0.012 Joules. Mecklenburgh and colleagues [ 161 estimated the natural work of breathing for a 500 ml tidal volume to be about 0.2 Joules. Thus, while the additional work imposed by an 8.0 mm ID tracheal tube is of the same magnitude as that expended in overcoming the compliance and resistance of the lungs, the work imposed by a corresponding sized LMA is considerably less. Increased resistance of breathing systems and the added inspiratory loads imposed, may affect successful weaning from mechanical ventilation [ 161, in patients recovering from respiratory failure. However, exact consequences of moderate increases in system resistance in an anaesthetised, spontaneously breathing patient are unclear. Nunn and Ezi-Ashi [ 171 found that during anaesthesia respiratory compensations for added inspiratory and expiratory loading was rapid and complete. Freedman and Campbell [ 181, on the other hand, presented evidence to suggest that during inhalational anaesthesia, ventilatory responsiveness to added inspiratory loading may be diminished. Despite this difference of opinion, i t is a good practice to be aware of the respiratory loading effect of different anaesthetic apparatus and to keep these loads to a minimum. The results of the present study indicate that, on the basis of the added inspiratory load of the two tubes, the LMA may be preferable to a tracheal tube. However, an additional source of inspiratory resistance while using an
The LMA and tracheal tubes LMA m a y arise a t mask-larynx interface. A faulty apposition of t h e LMA t o t h e larynx w o u l d increase resistance a n d inspiratory w o r k . T h e methodology of t h e present experimental study did n o t address this question. In conclusion, t h e resistance d u r i n g c o n s t a n t flow a n d additional inspiratory w o r k imposed by t h e LMA w a s f o u n d t o be considerably less t h a n t h a t imposed by t h e corresponding sized tracheal tube. If a choice is available, then in terms of their inspiratory loading effect, it m a y b e preferable t o use a n LMA rather t h a n a tracheal tube.
8. ENCSTROM CG, NORLANDER OP. A new method for analysis of
9. 10.
II. 12.
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