ORIGINAL ARTICLES

Respiratory Function and Ribcage Contribution to Ventilation in Body Positions Commonly Used During Anesthesia Andrew €3. Lumb, MB, BS, FCAnaes, and John F. Nunn, MD, PhD, FFARCS Division of Anaesthesia, Clinical Research Centre, Middlesex, England

Lung function tests are normally performed in the upright position, whereas anesthesia is usually administered with the patient in the supine position, and occasionally in other postures. We therefore compared forced vital capacity (FVC), forced expiratory volume in 1 s (FEV,), functio~lresidual capacity (FRC), and ribcage contribution to ventilation by respiratory inductive plethysmography in 13 conscious healthy male volunteers, sitting and in four horizontal positions used during anesthesia. Forced vital capacity and FEV, were similar in all positions, except for a significant mean increase in FVC of 300 mL (SD 213) when sitting compared with when supine (P < 0.001). The mean decrease in FRC was 806 mL (SD 293) between the sitting and supine positions (P < 0.001). A significant increase in FRC occurred (252 mL, SD 329, P < 0.01) when supine

hereas preoperative lung function tests are usually performed in the upright position, most anesthesia is administered to supine patients, and to a lesser extent to patients in the prone and lateral positions. For example, in thoracic anesthesia the lateral position is very mmmon and the lung function of the patients is often poor. Also of interest is the supine position with the arms raised above the head. We do not know of this position being used in routine anesthesia, but it is essential in studies of anesthesia on the respiratory system that include computed tomography (0 scanning of the chest (1). The average contribution of the ribcage (RC) to total ventilation decreases from 60% to 35% on lying down (2), probably due to increased fiber length of the diaphragm which results in a stronger contraction. Assessment of RC contribution by measurement

Accepted for publication June 6, 1991. Address correspondence to Dr.Lumb, Division of Anaesthesia, Clinical Research Centre, Watford Road, Harrow, Middlesex, HA1 3UJ, England.

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subjects raised their arms above their heads as required for computed tomography. Functional residual capacity in the prone and lateral positions was significantly larger than in the supine position (mean chan e 350 mL, P < 0.001),but was still some 450 mL less t an in the sitting position. Mean ribcage contribution was similar in all horizontal positions (32%%%), whereas supine values were significantly different from those of the sitting position (mean 70%, SD 11, P < 0.001). In conclusion, the various horizontal postures studied have no effect on FVC, FEV,, or ribcage contribution to ventilation. However, FRC in the prone, lateral, and arms-up positions is on average 250 mL larger than in the supine position, an observation that may affect gas exchange during anesthesia in these positions.

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of cross-sectionalareas of the chest and abdomen has not previously been performed in positions such as the prone and lateral ones. This may provide more accurate results than measuring anteroposterior or lateral diameters because of the body cavity distortion that may occur in these positions. Functional residual capacity (FRC) has been studied in many positions, including the prone position (3), but not in the lateral and arms-up positions. Similarly, though vital capacity has been studied in postures such as sitting, supine, and prone (3), there is a paucity of information on the effect of posture on "bedside" lung function tests such as forced vital capacity (FVC) and forced expiratory volume in 1 s (FEVJ. We therefore studied respiratory function including FRC and the RC contribution to ventilation in these different positions using awake volunteers.

Methods Thirteen healthy male subjects were studied with their informed consent after approval had been ob01991 by the International Anesthesia Research Society 0003-2999/9l/$3.50

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Table 1. Subject Data Age

Subject 1 2 3 4 5 6 7 8 9 10 11 12 13 Mean SD

(Yd 64 27 27 51 34 28 27 24 27 28 35 28 44 34.1 11.8

Sitting

Height (cm)

Weight (kg)

170 178 188 169 183 185 192 180 169 174 185 183 182

68 72 67 64 72 70 86 67 68 67 67 94 76

FVC (LBTPS) 3.85 5.77 6.08 4.86 4.91 4.15 5.94 4.90 3.96 3.97 6.09 6.08 4.56

179.8 7.1

72.1 8.3

5.01 0.89

2.31 4.64 4.48 4.10 4.46 3.73 2.98 4.25 3.32 3.91 2.77 4.66 3.62

FRc (LBTrs) 2.56 4.16 3.92 2.55 2.20 3.11 3.96 2.30 1.72 1.56 3.21 2.40 4.17

3.79 0.76

2.91 0.92

FEV,

(LBTPS)

%RC 88 67 66 74 87 66 57 59 66 77

58 81 61 69.7 10.6

BTPS, converted to body temperature and pressure saturated; FEV,, forced expiratory volume in 1 s; FRC,functional residual capacity; FVC,forced vital capacity; ZRC, percentage contribution of ribcage to ventilation.

tained from the local ethical committee. The subjects had no history of respiratory disease and were not obese (obesity defined as a body mass index of >29). All were staff members of the hospital or research departments of anesthesia and so were familiar with breathing through a mouthpiece, but only three were familiar with the aims of the study. Their physical characteristics are shown in Table 1. Each subject was studied in the morning at least 2 h after consuming a light breakfast without beverages containing stimulants. Throughout the study the subjects were distracted by headphones playing music of their choice. All ventilatory measurements were made with a valveless closed circle system (Figure 1)incorporating a circulating fan (35 Wmin) and an 8-L wet spirometer. The subject could be connected to the circuit with the tap open either to room air or the spirometer. Gas was continuously sampled at 500 mL/min from the inspiratory limb and returned to the circuit via a katharometer to measure the helium concentration and a paramagnetic oxygen analyzer. The katharometer displayed the helium concentration digitally and was corrected for changes in the oxygen concentration before each reading. Forced vital capacity and FEV, were measured by instructing the subjects to breathe in to total lung capacity and then perform a forced expiration to residual volume. Subjects unfamiliar with this technique practiced in the sitting position before the study until consistent values were obtained. The maneuver was then undertaken once in each position, allowing the FVC and FEV, to be read from the spirometer. Functional residual capacity was mea-

m 4

\

l

+ II

Figure 1. Apparatus used for measurement of FRC.

sured with a closed-circuit helium dilution method (4), all measurements being carried out in 40%-50% oxygen and 8%-12% helium in nitrogen. Partitioning of ventilation into RC and abdomendiaphragm components was by respiratory inductive plethysmography (RIP) (5-7). The RIP system consists of two elastic belts approximately 8-10 cm wide placed around the chest and abdomen and secured to the subject using adhesive tape. Each belt contains a zig-zag-patterned single loop of wire through which an alternating current at 1 MHz is passed. Selfinductance of the wire is continuously measured and is proportional to the cross-sectional area within the belt (5). In this study, calibration was by the first stage of the isovolume technique, which is the preferred method when RC partitioning is required (8).

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LUMBANDNUNN RESPIRATORY FUNCTION AND BODY POSITION

FwMon

h Sing

-Supine

h u p

ANESTH ANALG 1991;73:422-6

vPrww,

lateral

Figure 2. Example of RIP signals in different positions (subject 4).

The RC and abdomen signals are displayed on a paper chart recorder (Figure 2). Each subject was studied in five positions in random order. These positions were sitting with arms resting on the chair arms, supine with arms by the side, supine with both arms above the head, prone with one arm by the side and one arm in front, and left lateral with the right arm by the side and left arm in front (Figure 2). In each position the following protocol was followed: 1. Calibration of the RIP by the isovolume maneuver 2. “Run-in” period with no mouthpiece until regular respiration returned 3. Recording of RIP signals for 1 min with no mouthpiece, allowing the subjects to breathe via their natural airway 4. Mouthpiece and noseclip applied and a further run-in period allowed while breathing air 5. Recording of RIP signals with mouthpiece/ noseclip 6. Subject connected to spirometer and FRC measured 7. A forced vital capacity maneuver performed.

All stirometer volumes were converted to body tempera’tureand pressure saturated, and the average contribution of the RC to tidal volume for each period was calculated as a percentage of total tidal volume (%RC). Data were analyzed by two-way analysis of variance except for the %RCdata, which were pooled for all positions to compare data with and without a mouthpiecehoseclip using a paired t-test. Comparisons of %RC between different positions were made using the data obtained without the mouthpiece and noseclip. All comparisons between positions were made in the supine posture, which is the most relevant to anesthetic practice.

Results Forced vital capacity and FRC for the individual subjects while seated were all within the normal

range (Table 1). There was a significant decrease in FVC when the subject was supine compared with when sitting (P < 0.001), individual changes ranging from an increase of 51 mL to a decrease of 645 mL with a mean differenceof 300 mL (SD213). There were no differences in FVC between any of the other postures and supine, nor any significant differences in FEV, or FEV,/FvC% between any positions (Table 2) Functional residual capacities decreased by a mean value of 806 mL (SD293) when supine was compared with sitting (P < 0.001). Functional residual capacities in the lateral, prone, and arms-up positions were not significantly different from each other, but the mean values of all were sigruficantly greater than in the supine position by approximately 300 mL (mean differences 341 mL (SD 318) when lateral, 351 mL (SD 216) when prone, and 252 mL (SD 329) with arms-up). All were sisluficantly less than the sitting FRC (P < 0.001) by approximately 400 mL (Table 2). In each position %RCwas consistently less with a mouthpiece and noseclip but not significantly so for any individual position. When the data were pooled (i.e., 65 pairs of data) there was a significant (P < 0.05) decrease of 1.8%in the %RC when breathing changed from the natural airway to a mouthpiece and noseclip. Data for %RC without the mouthpiece showed a highly sigru6cant decrease (P < 0.001) in the %RCbetween sitting and supine postures, but no difference between supine and any of the other positions (Figure 2, Table 2).

-

Discussion Respiratory inductive plethysmography measures the cross-sectionalareas of the chest and abdomen (5) and is now accepted as the most accurate assessment of ventilation available from body surface recordings (6). Respiratory inductive plethysmography has been widely used in respiratory physiology because of its minimal effect on the subject (who usually forgets it is there) and it has also been used as a noninvasive respiratory monitor during sleep studies (7). The use of a noseclip has no effect on the measurement of FVC and FEVl (9). However, use of a mouthpiece and noseclip in another study resulted in an increased tidal volume, decreased respiratory rate, and a slightly increased inspiratory time (10). These respiratory changes were thought to be a result of changing from M A to oral breathing. We have shown a small but consistent and significant decrease in the RC contribution to tidal volume associated with the use of a mouthpiece and noseclip, but this change is too small to be of practical importance. Regardless of the airway, we have confirmed that breathing is predominantly RC in the sitting position and abdom-

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Table 2. Mean Results Position

Sitting Supine Arms-up Prone Lateral

FVC (Lm) 5.01 & 0.89" 4.71 f 0.86 4.59 f 0.96 4.63 f 1.02 4.87 f 0.99

FEVl (LBTPS)

3.79 f 0.76 3.70 f 0.66 3.27 f 0.80 3.49 0.84 3.67 f 0.71

*

FEv,IFvc

FRC

(%)

(LBTPS)

77

f 16 79 f 9 74 & 20 76 f 13 77 f 13

2.91 f 0.92" 2.10 f 0.83 2.36 f 0.76b 2.45 f 0.779 2.44 f 0.79

%RC natural

%RC +MP/NC

69.7 f 10.6" 32.3 f 12.0 33.0 2 14.7 32.6 f 11.5 36.5 f 16.8

65.1 f 14.2' 31.5 f 10.9 32.5 f 12.7 32.1 f 12.1 32.7 f 15.7

FEVI, forced expiratory volume in 1 s; FRC, functional residual capaaty; FVC, forced vital capaaty; %RC, percentage contribution of ribcage to ventilation; MPNC, mouthpiece and nosedip.

Values are mean f SD. Significance of difference relates to the supine position. 'P c 0.001. bP c 0.01.

inal while in the supine position (69.7% RC sitting; 32.3% RC supine). This agrees with the findings of Mannix et al. (2)who also used RIP (60.4% RC sitting; 36.2% RC supine). Sharp et al. (11) used two pairs of magnetometers to measure anteroposterior diameters of chest and abdomen and obtained essentially similar findings (70% RC sitting; 25% RC supine). Vellody et al. (12) used magnetometers to measure both anteroposterior and lateral diameters, from which they calculated cross-sectional areas. Their values for %RCare lower than those of other workers (48% RC sitting; 20% RC supine), and this may be a consequence of assumptions in the calculation of crosssectional area. Vellody et al. is the only group to have examined partitioning in the prone and lateral positions, and they found larger values for %RC (32%) in these positions compared with the supine position. Thus, our findings agree with their %RC values for prone and lateral positions, but not for supine or sitting. Our data for FVC and FEV, show surprisingly little change with different postures. A slightly increased FVC and unchanged FEV, for the sitting position suggest either a greater total lung capacity at the start of expiration or a more complete expiration and hence a lower residual volume, although the latter is unlikely considering the larger FRC when sitting. An increased total lung capacity may result from the subject's arms (and so shoulders)being supported by the chair, thus enhancing the use of the pectorals as accessory respiratory muscles. This study confirms the well-known change in FRC between upright and supine. Functional residual capacity was almost identical in the prone and lateral positions, being on average 350 mL greater than when supine. Studies using plain radiographs in the lateral position (13,14)have shown that the lower diaphragm is displaced much further cephalad than the upper, which may not move cephalad at all (14). Thus perhaps only the lower diaphragm (in our case the left) contributes to the decrease in FRC, thereby

causing only approximately 50% of the reduction seen from sitting when compared with supine. Moreno and Lyons (3)found that subjects in the prone position failed to show a significant change in FRC as compared with the supine position, despite a mean change of 149 mL. Rehder et al. (15) used a nitrogen washout technique to measure FRC in 10 volunteers in the prone position, but unfortunately did not compare their data with either the supine or upright postures. They obtained a mean FRC of 3.57 L, which is much larger than that of our subjects even when differences in weight are taken into account (FRC for our subjects, 34 mwkg; those of Rehder et al., 43 mL/kg). In dogs, the costal part of the diaphragm is less compliant than the crural part (16),but this difference in diaphragmaticmorphology has yet to be confirmed in humans. Again, radiographic studies help to explain the changes in FRC when prone. Krayer et al. (17) used CT scanning to reconstruct a three-dimensional image of the diaphragm in three supine and three prone subjects. They observed that two of the three prone subjects still used the posterior part of their diaphragm for ventilation. This, coupled with the possible noncompliant costal diaphragm, may explain the relatively high FRC when prone. In the Rehder et al. study (15), five of the ten volunteers were anesthetized (with paralysis) in the prone position, resulting in a nonsigruficant reduction of 400 mL in their FRC and also a reduction in closing capacity. Similarly, Krayer et al. (17)found a consistent cephalad displacement of the diaphragm during prone anesthesia, which he calculated to represent a mean volume displacement of 615 f 466 mL. Our data, along with these two studies, indicate that for anesthesia in the prone position, the FRC is initially larger than in the supine position, the reduction in FRC with anesthesia is similar to that seen when supine, and the closing capacity may also be reduced. Thus less disturbance of gas exchange probably will occur during anesthesia in the prone position when compared with su-

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pine, but the alveolar to arterial oxygen difference [AP(A-a)O,] during anesthesia in different positions has not been studied. Many of the most sigruficant studies aimed at explaining the increased AP(A-a)O, during anesthesia have been performed in CT scanners. Complex changes in diaphragmatic shape have been described (17), and lung opacities have been demonstrated and their extent correlated with the AP(A-a)0, (1). To perform any chest CT scan, the arms of the subject must be raised above the head to avoid artifacts. We have shown that although this maneuver does not alter the %RC contribution, it does increase FRC by approximately 250 mL, which represents almost half of the usual FRC decrease seen during anesthesia. Once again the effect this position has on closing capacity or the reduction of FRC with anesthesia is unknown. However, if these factors are the same as in the supine arms-down position, then the larger initial FRC may cause studies of anesthesia carried out in CT scanners to underestimate the changes that take place during anesthesia in the n o d supine position with the patient's arms by his side.

References 1. Hedenstiema G. Gas exchange during anaesthesia. Br J Anaesth 1990;64.5U7-14. 2. Mannix SE,Bye P, Hughes JMB, Cover D, Davies EE.Effect of posture on ventilatory response to steady-state hypoxia and hypercapnia. Respir Physiol19&4;58:87-99. 3. Moreno F, Lyons HA. Effect of body posture on lung volumes. J Appl Physiol 1%1;16:27-9.

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4. Hewlett AM, Hulands GH, NUM JF, Minty KB. Functional residual capaaty during anaesthesia I: methodology. Br J Anaesth 1974;46:475c85. 5. Milledge JS, Stott FD. Inductive plethysmography-a new respiratory transducer. J Physiol (Lond) 1979;26745P. 6. McCool FD,Kelly KB, bring SH, Greaves IA, Mead J. Estimates of ventilation from body surface measurements in unrestrained subjects. J Appl Physiol 1986;61:1114-9. 7. Staats BA, Bonekat HW,Harris CD, Offord KP. Chest wall motion in sleep apnoea. Am Rev Respir Dis 1984;130:59-63. 8. Chadha TS,Watson H, Birch S, et al. Validation of respiratory inductive plethysmograph using different calibration procedures. Am Rev Respir Dis 1982;125:6449. 9. Verrall AB, Julian JA, Muir DCF, Haines AT. Use of noseclips in p u l m o ~ r yfunction tests. J Occup Med 1989;31:29-31. 10. Rodenstein DO, Mercenier C, Stanescu DC.Influence of the respiratory route on the resting breathing pattern in humans. Am Rev Respir Dis 1985,131:1634. 11. Sharp JT, Goldberg NB, huz WS, Danon J. Relative contribution of rib cage and abdomen to breathing in normal subjects. J Appl Physiol1975;39:60&18. 12. Veflody VP, Nassery M,huz WS, Sharp JT.Effects of body position change on thoracoabdominal motion. J Appl Physiol 1978;45:581-9. 13. Froese AB, Bryan AC. Effect of anesthesia and paralysis on diaphragmatic mechanicsin man. Anesthesiology 1974;41:24255. 14. NUM JF.Applied respiratory physiology. 3rd ed. London: Buttenvorths, 1987103. 15. Rehder K, Knopp TJ,Sessler AD. Regional intrapulmonary gas distribution in awake and anaesthetized-paralysed prone man. J Appl Physiol197845:52845. 16. Road J, Newman S, Derenne JP, Grassino A. In vivo lengthforce relationship of canine diaphragm. J Appl Physiol 1986; 6063-70. 17. Krayer S, Rehder K, Vettennann J, Didier EP, Ritman EL. Position and motion of the human diaphragm during anesthesia-paralysis. Anesthesiology 1989;70:891-8.

Respiratory function and ribcage contribution to ventilation in body positions commonly used during anesthesia.

Lung function tests are normally performed in the upright position, whereas anesthesia is usually administered with the patient in the supine position...
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