Pulmonary air trapping during two-lung and one-lung ventilation

Pulmonary Air Trapping During Two-Lung and One-Lung Ventilation Laurent Ducros, MD, Marc Moutafis, MD, Marie-H616ne Castelain, MD, Ngai Liu, MD, and Marc Fischler, MD Objective: Evaluation of the magnitude of pulmonary air trapping during routine thoracic surgery and single-lung transplantation. Design: Prospective study on consecutive patients. Setting: Single institution, university hospital. Participants: Sixteen patients with no or moderate obstructive lung disease undergoing routine thoracic surgery (group 1), six patients with severe emphysema (group 2), and six patients with severe fibrosis (group 3) undergoing singlelung transplantation. Interventions: Occlusion maneuver timed at the end of expiration to measure auto-positive end-expiratory pressure (auto-PEEP) and trapped volume (A FRC). The maneuver was performed during two-lung ventilation in supine (2LV supine) and lateral decubitus (2LV lateral) positions and during one-lung ventilation (OLV} in lateral decubitus position. At the same time, airway pressures and PaO2 measurements were performed. Measurements and Main Results: In group 1, consistent values of auto-PEEP and A FRC occurred only during OLV:

4.8 -- 2.5 cm H20 and 109 +_ 61 ml_ (mean +-standard deviation). In group 2, auto-PEEP and A FRC values were 11.7 -+ 6.9 cm H20 and 355 +_ 125 mL during 2LV supine, 8.8 +_ 5.7 cm H20 and 320 -+ 122 mL during 2LV lateral, and 15.9 -+ 3.9 cm H20 and 284 +_ 45 mL during OLV. In group 3, pulmonary air trapping was low. For the three groups together, auto-PEEP and A FRC (p < 0.0001) related inversely to the ratio of forced expired volume in 1 second (FEV1) to forced vital capacity (FVC) expressed in percent (FEV1/FVC%) during OLV. In contrast, there was no correlation between PaO2 and auto-PEEP or A FRC. Conclusion: Pulmonary air trapping must be suspected in patients with no or moderate obstructive lung disease during OLV and in those with severe obstructive disease as soon as 2LV is initiated.

p

lung ventilation (2LV) and OLV in patients undergoing thoracic surgery or lung transplantation.

ULMONARY air trapping or dynamic pulmonary hyperinflation (DPH) is an occult phenomenon, referred to as intrinsic or auto-positive end-expiratory pressure (auto-PEEP), that may occur during mechanical ventilation. 1,2The magnitude of auto-PEEP depends mostly on the patient's respiratory mechanics, and DPH is almost invariably present in patients with chronic obstructive pulmonary disease. 3 Furthermore, additional external flow resistance (endotracheal tube, tubing, valves, etc) and inappropriate ventilatory settings can considerably worsen DPH. 2 One-lung ventilation (OLV) during thoracic surgery is a potentially hazardous situation that often associates the abnormal pulmonary function of the patient with the presence of a high-resistance double-lumen tube. In such a case, the lack of detection and treatment of an occult DPH may result in severe complications. It is widely accepted that auto-PEEP may decrease cardiac output2,4; air trapping-induced cardiac arrest has even been reported. 5 Deliberate hypoventilation may be mandatory to prevent a considerable decrease in cardiac output and systemic hypotension during open-chest OLV for lung transplantation procedures. 6 Conversely, increasing levels of auto-PEEP and trapped volume (A FRC) will shift the tidal volume toward the upper inflection point and flat portion of the pressure-volume curve of the lung, exposing overdistended alveoli to the risk of rupture (barotrauma). 2 In a nonhomogenous lung, DPH is most pronounced in the lung regions with the greatest expiratory time constant. These regions may be selectively exposed to overdistension and barotranma. Finally, auto-PEEP is a factor of arterial pulmonary occlusion pressure 1 and lung compliance 7 mismeasurements. During thoracic surgery, the presence of auto-PEEP has been reported using flow-vohime loops 8 and measured using an end-expiratory port occlusion method. 9 However, auto-PEEP association with A FRC and auto-PEEP values in patients with various pulmonary diseases remains to be evaluated. In the present study, auto-PEER A FRC, intraoperative mechanical ventilation parameters, and PaO2 were measured during two-

Copyright© 1999by W.B. Saunders Company KEY WORDS: anesthetic techniques, one-lung ventilation, lung hyperinflation, positive end-expiratory pressure, thoracic surgery

PATIENTS AND METHODS After ethical committee approval and written, informed consent were obtained, auto-PEEP and A FRC were measured prospectively in 28 patients. All patients had preoperative pulmonary function tests to measure, in particular, forced expired volume in 1 second (FEV1, in milliliters), forced vital capacity (FVC, in milliliters), and total lung capacity (TLC, in percent of predicted values). The ratio of FEVJFVC was calculated and expressed as a percentage (FEVjFVC%). Obstructive disease was defined as a less than 75% FEVI/FVC% with a normal TLC, and restrictive disease was defined as a less than 80% TLC% with a normal FEV1/FVC%. Sixteen consecutive patients with no or moderate obstructive lung disease (FEVjFVC% >45%; group 1) were studied while undergoing routine thoracic surgery requiring OLV (pneumonectomy, lobectomy, thoracoscopic surgery). Six patients with severe emphysema (group 2) and six patients with severe fibrotic lung disease (group 3) were studied while undergoing single-lung transplantation. Group 1 patients were premedicated with 5 mg of intramuscular midazolam. Patients undergoing single-lung transplantation (groups 2 and 3) did not receive may premedication. All patients were monitored with a five-lead electrocardiogram, pulse oximetry and capnometry. In the case of single-lung transplantation, a femoral artery catheter and a 7.5F fiberoptic pulmonary artery catheter were inserted. Before anesthetic induction, preoxygenation was performed for 5 minutes with 100% oxygen. General anesthesia was induced with propofol, 2 mg/kg; fentanyl, 3 ~g/kg, and vecuronium, 0.1 mg/kg, in group 1 or with sufentanil, dose as required, and vecuronium for groups 2 and 3. Anesthesia was maintained with an infusion of the same drugs and myorelaxation was monitored. After induction, a left-sided double-

From the D~partement d'Anesth~sie-Rdanimation, Centre HospitaloUniversitaire Lariboisi~re, Paris; and the Ddpartement d'Anesthdsie, H@ital Foch, Suresnes, France. Address reprint requests to Marc Fischler, MD, Service d'Anesthdsie, Htpital Foch, 40 rue Worth, Suresnes, 92150, France. Copyright © 1999 by W.B. Saunders Company 1053-0770/99/1301-0008510. 00/0

Journal of Cardiothoracic and Vascular Anesthesia, Vo113, No 1 (February),1999:pp 35-39

35

36

DUCROS ET AL

lumen endobronchial tube (red rubber Carlens tube, Crasch, Waiblingen, Germany) was inserted; the tubes varied in size from 37F to 39F in women and from 39F to 41F in men. Complete lung separation was confirmed before and after turning the patient to the lateral decubitus position by auscultation after alternate clamping and by the absence of leaking from the nondependent lung using the bubble technique)° Finally, effective collapse of the lung was assessed by the surgeon after chest opening. All patients were mechanically ventilated with an Evita ventilator (Dr~ger, Lubeck, Germany), which is usually used in critical care units. The deadspace and compressible volume of both external tubing devices and ventilator were constant. Ventilator settings were identical in all groups during 2LV and OLV at 10 mL/kg tidal volume, 10 to 12 breaths/rain, zero end-expiratory pressure, and no plateau pause, with a 100% inspired oxygen fraction. The inspiratory time was 33% for control and fibrotic patients and 25% for emphysematous patients. The measurement of auto-PEEP and A FRC is a computer-controlled automated function implemented in the ventilator monitor. After a manual triggering of the function, both the inspiratory and expiratory valves are automatically occluded at the end of expiration. The steady-state plateau pressure obtained at this time is the auto-PEER The expiratory valve is opened at the end of the pause and the total volume released by the lung, until flow becomes zero, is the A FRC. This automated auto-PEEP determination was validated by comparison with the reference method (external computer-controlled valve and measurement of pressure in the inspiratory limb of the circuit)./I DPH can be different between the dependent and nondependent lungs because of different mechanical properties. Because auto-PEEP and A FRC were not measured for each lung separately, but at the airway opening, a statistical comparison of 2LV to OLV auto-PEEP and A FRC values would not be relevant. Expired tidal volume, peak inspiratory pressure, mean airway pressure, PaO2, auto-PEER and A FRC were monitored during the following three anesthetic phases: (1) after a few minutes of equilibration of 2LV in the supine position (2LV supine), (2) 10 minutes after turning the patient in the lateral decubitus position (2LV lateral), and (3) a few minutes after the beginning of OLV with the chest open but not submitted to any surgical manipulation (OLV lateral). In case of hypotension (systolic blood pressure <90 mmHg), which is considered an adverse hemodynamic effect of DPH, it was planned to lower both tidal volume and rate of ventilation concurrently with ephedrine infusion. A chest radiograph was performed postoperatively to check for pneumothorax contralateral to the operated side. Values are expressed as mean + standard deviation. Preoperative data were compared using unpaired t-tests. Measurements taken during 2LV supine, 2LV lateral, and OLV lateral were compared using paired t-tests (intragroup comparison) and two-way analysis of variance (intergroup comparison). Statistical comparisons between 2LV and OLV auto-PEEP and A FRC have not been elicited because dependent and nondependent lungs can have different mechanical properties and because auto-PEEP and A FRC were not measured from each lung separately, but at the airway opening. Correlations between OLV auto-PEEP or A FRC values and FEV1, FEV1/FVC% indices, or PaO2 were determined using a simple linear regression; p less than 0.05 was considered statistically significant. RESULTS

The preoperative characteristics of the patients are listed in Table 1. There was no difference among groups regarding height, weight, and age. Emphysematous and fibrotic patients were at the end-stage of their disease. The average FEV1/FVC% of group 1 patients (no or moderate obstructive disease) was 69% -+ 14%. This index was significantly less in the emphysema group (p < 0.0001) than in the other groups. In fibrotic

Table 1. Preoperative Characteristics and Pulmonary Function Tests in the Three Groups of Patients Studied

Age (yr) Height (cm) Weight (kg) FEVl (mL) FVC (mL) TLC% FEV1/FVC%

Group 1

Group 2

Group 3

56 _+ 12 171 _+ 10

54 _+ 8 172 _+ 5

4 2 -+ 14 169 _+ 8

69 _+ 15

69 _+ 7

66 _+ 8

2,939 _+ 954 4,186 _+ 1,113

583 _+ 162 2,236 -- 397

1,247 _+ 417 1,573 _+ 546

105 ± 13 69 _~ 14

104 _+ 26 25 _+ 5f

43 + 9* 80 -+ 7

*Significantly different (p < 0.0001) from groups 1 and 2. 1-Significantly different (p < 0.0001) from groups 1 and 3.

patients, TLC% values were significantly less (p < 0.0001) than TLC% values in the two other groups. Expired tidal volumes remained unchanged from 2LV to OLV (10 mL/kg), and no hemodynamic adverse effect or barotrauma was observed during the study. Tidal volumes remained identical throughout the study in the three groups. Patients in group 3 developed greater peak airway pressures than patients in groups 1 and 2 (p < 0.001). Mean airway pressures were significantly greater in group 3 compared with groups 1 and 2 (p < 0.0001). In all groups, peak and mean airway pressures increased significantly from 2LV lateral to OLV lateral (p < 0.05; Table 2). PaO2 values were identical among the groups throughout the study; they decreased significantly during OLV (p < 0.0001 in group 1; p < 0.05 in groups 2 and 3; Table 2). Individual values of auto-PEEP and A FRC measured during 2LV supine, 2LV lateral, and OLV lateral are shown in Fig 1. In group 1 (no or moderate obstructive disease), auto-PEEP and A FRC increased during OLV (Table 2); A FRC during OLV was 17% _+ 12% of tidal volume. In group 2 (severe emphysema), auto-PEEP and A FRC values were elevated throughout the study; A FRC during OLV was 41% _+ 8% of tidal volume. The levels of auto-PEEP and A FRC in emphysematous patients were significantly greater than those measured in the two other groups (Table 2). In group 3 (severe fibrosis), auto-PEEP and A FRC values were decreased whatever the situation. During OLV, 2x FRC was 6% -+ 5% of tidal volume. Two patients had low A FRCs (47 and 80 mL) despite auto-PEEP values of 6.9 and 12.1 cm H20, respectively (Fig 1). For the three groups together, the magnitude of auto-PEEP and A FRC during OLV were inversely correlated to the preoperative F E V J F V C % ratio (Fig 2) but not to the preoperative FEV1. In contrast, the level of PaO2 during OLV was not correlated to the magnitude of auto-PEEP or A FRC (Fig 3). DISCUSSION

The present study confirms that consistent pulmonary hyperinflation may occur during thoracic surgical procedures requiring OLV. DPH is correlated to theintensity of the obsguctive syndrome. In patients with severe obstructive diseases, DPH was important during both 2LV and OLV. In cases of moderate obstructive disease, pulmonary hyperinflation was shown only during high-volume ventilation, ie, OLV. Finally, there was no consistent pulmonary hyperinflation, even during OLV, when patients had no underlying obstructive disease. End-expiratory lung volume corresponds to the relaxation

PULMONARY AIR TRAPPING DURING 2LV AND OLV

37

Table 2. Mean Values of the Intraoperative Parameters of Mechanical Ventilation, PaOz, Auto-PEEP, and A FRC Measured During the Three Situations Group 1 2LV Supine

2LV Lateral

Group 2

OLV

2LV Supine

Lateral

Group 3

2LV Lateral

VT(mL) 699 -+ 111 690 -+ 113 693 ± 108 743 + 110 733 Ppeak(cm H20) 26.6 ± 4.4 28 ± 4.1 46.4 _+ 7.9* 42.7 ± 13.5t 45,5 Pm~ao(cm H20) 5.6 ± 1.1 6.4 -+ 1.2 10.8 +_ 1.9" 9.2 -+ 3.31" 9,2 PaO2 (mmHg) 436 ± 64.3 429 ± 64.8 128.9 +_ 53.9¢ 500.7 ± 84.2 468.3

-± ± ±

OLV Lateral

2LV Supine

84 653 ± 114 637 11.4¢ 57.7 ± 13.3"¢ 52.8 2.81" 12.7 _+ 3.3"1" 18.7 78,9 181.7 _+ 117.8" 404.3

± ± -

2LV Lateral

97 612 8.21" 58.8 41" 21.3 149.8 444.7

OLV Lateral

-+ 86 ± 9.71± 5.61" + 144.7

598 69.5 29.2 113

± 101 ± 6.4"1" _+ 5.4"1" ± 45.4*

Auto-PEEP (cm H20)

1.5 _+ 0.7

1.5 ± 0.8

4.8 ± 23

11.7 ± 6.91"

8.8 _+ 5.71-

15.9 ± 3.9f

2.7 ± 2.3

2.7 ± 2.3

4.5 _+ 4.3

59 ± 34

64 -+ 39

109 ± 61

355 + 125t

320 ± 1221"

284 _+ 451"

36 ± 30

33 ± 32

34 ± 29

A FRC (mL)

Abbreviations: VT, expired tidal volume; Ppeak, inspiratory airway pressure; Pmean, mean airway pressure; A FRC, trapped volume. *lntragroup comparison, significantly different (p < 0.05) from 2LV lateral or 2LV supine. tlntergroup comparison, significantly different (p < 0.0001) from group 1. $1ntragroup comparison, significantly different (p < 0.0001) from 2LV lateral or 2LV supine.

Auto-PEEP ( cm H20 )

Trapped Volume (mL) 500

20

40015300Group

volume of the respiratory system, ie, the lung volume at which the sum of elastic recoil pressures of the respiratory system is null, and is called the functional residual capacity. An increase of the end-expiratory lung volume to greater than the elastic equilibrium volume of the respiratory system is a well-known feature in mechanically ventilated patients with severe obstructive pulmonary diseaseJ 2,13 This implies a remaining occult alveolar positive pressure that can be measured during an end-expiratory pause) The magnitude of DPH is related to the

1 10200-

2o. 2LV Supine

2LV Lateral

1000

OLV

15-

i 2LV Supine

2LV Later~

OLV

Auto-PEEP (cm H20 )

500 -

20 18 16 14 12 lO

p < 0,0001

/k

o

8

o?O O

6 4

400 300-

Group

2 1o2005"

0

0

10 20

30

100 -4

40

50 60

70

80 90 100

FEV1/FVC% 40O

I

2 LV Supine

I

1

2 LV

OLV

0

I

I

i

2 LV

2 LV Lateral

OLV

Supine

Lateral

350

a

300

Q

15o 300 -

0

200 -

50

lO0 -

o

0 2LV Supine

2LV Lateral

OLV

°ofo

lOO

3 105-

B

A

A FRC (mL) 20o

400 -

15-

/S

p < 0.0001

250

500 -

20-

Group

A

0 , , 9 2LV Supine

1

1~ ~ 2LV Lateral

OLV

Fig 1. individual values of auto-PEEP (left side, expressed in centimeters of water) and trapped volume (right side, expressed in milliliters) during the following periods: 2LV supine, 2LV lateral, and OLV lateral. Data are presented for the three groups: group 1 (top), group 2 (middle), group 3 (bottom).

10 20

30

40

50 60

70

80 90 100

FEV 1/FVC% Fig 2. Relationship during OLV between (A) auto-PEEP and FEVd FVC% ratio and between (B) 3, FRC and FEV1/FVC% ratio. Data are presented for the three groups together. Auto-PEEP (y = -0,2 x + 19.8; r2 = 0 . 6 9 ; p < 0.0001) and A FRC ( y = - 4 x +380; r z = 0.83; p < 0.0001) are related inversely to FEV1/FVC% ratio. D, Group 1; A, group 2; ©, group 3.

38

DUCROS ET AL

400

A

350 300 250

PaO 2

(m~g)

200

0 [] n

U

[]

100 50

0 O0

150

Cb

o

o

[]

A

~'

0

Auto-PEEP (cm H20 ) 400

B

350 300 []

250

PaO2

(mmHg)

200 [3

150 100 50 0

0

0

0

~3

n m mOO

5;

~nrl

[3 A

1;o

1;o

I

~oo ~o

3;o

l

3so 4;0

A FRC (mL) Fig 3. Relationship between (A) PaO2 and auto-PEEP and between (B) PaO2 and A FRC during OLV, Data are presented for the three groups together. PaO2 is not related to auto-PEEP or A FRC. [~, Group 1; A, group 2; ©, group 3.

patient and/or external factors. DPH tends to occur under conditions in which expiratory air flow is reduced (increased expiratory resistance) and is facilitated by a low expiratory driving pressure when pulmonary compliance is high (low elastic recoil pressure). This is observed in such obstructive pulmonary diseases as emphysema. Conversely, DPH is prevented when compliance is low (pulmonary fibrosis). DPH is also facilitated by a short expiratory time (high-frequency ventilation) or a high minute ventilation (high tidal volume).2J4 Finally, external factors, such as the resistance of tracheal and external tubes, may add to the intrinsic resistance and facilitate hyperinflation. 2 In the present work, three groups of patients were studied who showed highly different respiratory functions and who were ventilated with the same additional resistances, the same ventilator settings, and the largest double-lumen tube. The increase in peak and mean airway pressures from 2LV to OLV, as well as the decrease in PaO2 during OLV observed in the three groups, are well-documented phenomena in the literature. 9J5 Increased values of both auto-PEEP and A FRC were measured in patients with severe emphysema during 2LV and OLV despite the deliberate reduction in the inspiratory time from 33% to 25%. All these emphysematous patients had severe respiratory insufficiency leading to the lung transplantation decision. In such obstructive diseases, both bronchial wall inflammation thickening and passive expiratory bronchial collapse caused by the destruction of the elastic tissue explain the marked increase in expiratory resistance. Conversely, fibrotic disease prevents DPH even if expiratory resistance is increased

because of a low pulmonary compliance, resulting in a high driving expiratory pressure and a high expiratory flow. Finally, in patients scheduled for routine thoracic surgical procedures who have an FEV1/FVC% ratio ranging between 93% and 48% (ie, with no or moderate obstructive lung disease), auto-PEEP and A FRC were low during 2LV but increased during OLV. During OLV, some auto-PEEP values were close to 10 cm H20, whereas A FRC reached 200 mL. Part of the interest of the present study lies in A FRC measurements that give information about the intensity of the DPH. In the case of high lung compliance (emphysema), a small change in transpulmonary pressure may result in a high degree of inflation and, even for low levels of auto-PEER the presence of a high A FRC will confirm the intensity of the hyperinflation phenomenon. In contrast, in the presence of low lung compliance (fibrosis), higher auto-PEEP is required to create even a low A FRC. Diagnosis of DPH requires the A FRC measurement. In the present study, two patients with fibrotic lung disease did not show a significant A FRC despite auto-PEEP values of 6.9 and 12.1 cm H20. Bardoczky et al8 detected auto-PEEP by recording the remaining positive expiratory flow just before the beginning of inspiration in 67% of the patients undergoing routine thoracic surgery. They confirmed the presence of auto-PEEP during OLV with the end-expiration occlusion reference method and showed that auto-PEEP correlated with the functional residual capacity, a volume that also increased in the presence of chronic obstructive lung diseases, but did not correlate with FEV1.16 However, as in the study of Yokota et al,9 an inverse correlation was found between auto-PEEP or A FRC and FEV1/FVC% ratio, which is a more specific index of obstructive disease than FEV1 because FEVt may decrease also in restrictive lung diseases. The guidelines for ventilation in thoracic surgery requiring OLV are to maintain the same 10-mL/kg tidal volume from 2LV to OLV and to adjust the rate of ventilation to maintain PaCO2 near 35 mmHg. 17 This is relevant for patients without any severe obstructive pulmonary disease in whom these settings promote no or moderate hyperinflation. Conversely, in severely emphysematous patients, the level of hyperinflation may be so high that the risk for cardiac arrest becomes threatening. 5,6 There is no numeric tolerable level for auto-PEEP; it depends mostly on the hemodynamic consequences of DPH, which are related to several parameters, especially the circulating blood volume. In the presence of a low arterial pressure and a low cardiac output, a high degree of auto-PEEP must be suspected. In case of emergency, the deliberate withdrawal of the ventilator tubing is the way to diagnose and treat hemodynamic hyperinflation consequences and leads to reduced tidal ventilation and respiratory rate. During ventilation through a double-lumen tube, the clinician must also be aware of a too-far intubation resulting in a selective hypervenfilation (lower left lobe ventilation with a left-sided tube; lower and middle lobe ventilation with a right-sided tube). Finally, large tidal volumes (10 to 15 mL/kg) may also shift the ventilation to the volume-pressure curve flat portion and increase the risk for terminal airway rupture. As reported recently, 9 the authors did not find any correlation between the degree of auto-PEEP and the PaO2 value during

PULMONARY AIR TRAPPING DURING 2LV AND OLV

39

OLV. However, the relation between intrinsic or applied positive end-expiratory pressure and PaO2 during OLV must be discussed. Maintaining a high tidal volume from 2LV to OLV contributes to the prevention of lateral decubitus-induced dependent lung atelectasis. Is Hedenstierna et a119 have shown that applied positive end-expiratory pressure on the dependent lung may improve oxygenation during ventilation in the lateral position. However, applied PEEP to the dependent ventilated lung may either keep the alveoli distended or divert blood flow to the nonventilated nondependent lung; this maneuver has been reported to increase 2° or to decrease PaO221 during OLV. The effects of auto-PEEP on pulmonary gas exchange appear to be similar to those of applied PEEP despite a possibly less homogenous distribution of auto-PEEP and inspired gas between lung units. 4 It can be difficult to predict the influence of external applied PEEP and auto-PEEP during OLV because PaO2 depends on many parameters: the lung perfusion ratio

between the operated nonventilated lung and the contralateral lung, 22 the use of anesthetic agents that can decrease hypoxic pulmonary vasoconstriction, the hemodynamic status, the presence of a patent foramen ovale, and so on. In conclusion, during thoracic surgery, DPH must be suspected in those with severe obstructive diseases during 2LV and OLV and those with mild obstructive lung diseases during OLV. This clinical problem is particularly relevant when these patients are submitted to new surgical techniques, such as single-lung trausplantation, 23 lung-volume reduction, 2425 and some thoracoscopic procedures. 26 In such cases, it is important to routinely monitor auto-PEEP and 2x FRC to prevent and/or treat pulmonary hyperinflation. Despite the usual measures (the largest tracheal tube possible, frequent suction of mucosal secretions, a long expiratory time, and a low rate of ventilation), severe DPH may occur, requiring dramatic reductions in tidal volume and respiratory rate.

REFERENCES

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ment of dynamic hyperinflation during lung transplantation. J Cardiothorac Vasc Anesth 11:100-104, 1997 14. Broseghini C, Brandolese R, Poggi R, et al: Respiratory resistance and intrinsic positive end-expiratory pressure (PEEP) in patients with the adult respiratory distress syndrom (ARDS). Eur Respir J 1:726-733, 1988 15. Szegedi L, Bardoczky G, Engelman E, et al: Airway pressure changes during one-lung ventilation. Anesth Analg 84:1034-1037, 1997 16. Bardoczky G, Yemanlt JC, Engelman E, et al: Intrinsic positive end-expiratory pressure during one-lung ventilation for thoracic surgery. Chest 110:180-184, 1996 17. Benumof JL: Conventional and differential lung management of OLV, in Benumof JL (ed 2): Anesthesia for Thoracic Surgery. Philadelphia, PA, Saunders, 1995, pp 406-431 18. Larsson A, Malmkvist G, Weruer O: Variations in lung volume and compliance during pulmonary surgery. Br J Anaesth 59:585-591, 1987 19. Hedensfierna G, Baehrendtz S, Klingstedt C, et al: Ventilation and perfusion of each lung during differential ventilation with selective PEEP. Anesthesiology 61:369-376, 1984 20. Cohen E, Eisenkraft JB, Thys DM, et al: Oxygenation and hemodynamic changes during one-lung ventilation: Effects of CPAP10, PEEP10, and CPAP10/PEEP10. J Cardiothorac Anesth 2:34-40, 1988 21. Capan LM, Turudoff H, Patel C, et al: Optimization of arterial oxygenation during one-lung anesthesia. Anesth Analg 59:847-851, 1980 22. Hurford WE, Kolker AC, Strauss HW: The use of ventilationperfusion lung scans to predict oxygenation during one-lung anesthesia. Anesthesiology 67:841-844, 1987 23. Mal H, Andreassian B, Pamela F, et al: Unilateral lung transplantation in end-stage pulmonary emphysema. Am Rev Respir Dis 140:797-802, 1989 24. Barker SJ, Clarke C, Trivedi N, et al: Anesthesia for thoracoscopic laser ablation of bullous emphysema. Anesthesiology 78:44-50, 1993 25. Cooper JD, Trulock EP, Triantafillou AN, et al: Bilateral pneumectomy (volume reduction) for chronic obstructive pulmonary disease. J Thorac Cardiovasc Surg 109:106-116, 1995 26. Wakabayashi A: Thoracoscopic laser pneumoplasty in the treatment of diffuse bnllous emphysema. Ann Thorac Surg 60:936-942, 1995