Peri-Operative Atelectasis and Alveolar Recruitment Manoeuvres

Respiratory complications are a significant cause of postoperative morbidity and mortality. Perioperative atelectasis, in particular, affects 90% of surgical patients and its effects can be prolonged due to changes in respiratory mechanics, pulmonary circulation and hypoxaemia. Alveolar collapse is caused by certain predisposing factors, mainly due to compression and absorption mechanisms. Several therapeutic strategies have been proposed to prevent or treat this atelectasis, such as alveolar recruitment manoeuvres, which has become widely used in the last few years. Its application in patients with alveolar collapse, but without a previous significant acute lung lesion has some unusual features. Its use is, therefore, not free of uncertainties and complications. This review describes the frequency, pathophysiology, importance and treatment of perioperative atelectasis. Special attention is paid to treatment with recruitment manoeuvres, so as to provide a basis for their rational and appropriate use.     

Effects of Fentanyl on Respiratory Pressure-Volume Relationship in Supine Anesthetized Children

The effects of fentanyl on total respiratory pressure-volume relationship were studied in 12 supine, anesthetized (thiopental, succinylcholine, halothane, N₂O/O₂) children, 5, 10 and 15 min after administration of fentanyl, 3 μg/kg (Group F) and at similar intervals in 11 control, anesthetized children who did not receive fentanyl (Group C). Lung and chest wall pressure-volume relationships were measured in five children from Group F and in six from Group C. Compliance values were estimated from the linear part of PVR curves. In Group F, a significant decrease was observed at 5, 10 and 15 min (maximum 29%) for total compliance and at 15 min (maximum 26 %) for lung compliance. Chest wall compliance was reduced (maximum 45 %) at 15 min, but not significantly. In Group C., no change in compliances was noted. The comparison between the two groups for total, chest wall and lung compliances showed a significant decrease in these three parameters at 15 min in children receiving fentanyl. The decrease of chest wall compliance can be explained by increased muscle tone, but the mechanism of decreased lung compliance awaits further study.     

Morphometric Effects of the Recruitment Maneuver on Saline-lavaged Canine Lungs: A Computed Tomographic Analysis

Background: In the face of widespread use of lung-protective, low-volume ventilation in patients with acute lung injury, interest in the recruitment maneuver (RM) is growing. Little is known about lung-morphometric effects of the RM as compared with positive end-expiratory pressure (PEEP) titration (PT) without the RM.

Methods: RM was defined as a stepwise change in PEEP from baseline to 10, 20, 30, and 20 cm H2O every 30 s, after which PEEP was reset at the lower inflection point + 2 cm H2O. For PT, PEEP was simply increased from baseline to the lower inflection point + 2 cm H2O. Both maneuvers were performed in 10 lung-lavaged dogs. Computed tomography of the lung was performed before and 30 s and 30 min after the maneuver.

Results: Thirty seconds after the maneuver, the decrease in the amount of nonaerated plus poorly aerated lung was greater and decreases in Hounsfield units in the caudal and dorsal lung regions were greater with the RM than with the PT. The hyper-aerated lung volume after the RM tended to be greater than that after the PT. At 30 s and 30 min after the maneuver, gas plus tissue volume, gas-only volume, and gas-tissue ratio of the lung were greater with the RM than with the PT. At both time points after the maneuver, the coefficient of variation of regional Hounsfield units, an index of regional heterogeneity of aeration, was lower with the RM than with the PT.     

Influence of Gas Composition on Recurrence of Atelectasis after a Reexpansion Maneuver during General Anesthesia

Background: Atelectasis, an important cause of impaired gas exchange during general anesthesia, may be eliminated by a vital capacity maneuver. However, it is not clear whether such a maneuver will have a sustained effect. The aim of this study was to determine the impact of gas composition on reappearance of atelectasis and impairment of gas exchange after a vital capacity maneuver.

Methods: A consecutive sample of 12 adults with healthy lungs who were scheduled for elective surgery were studied. Thirty minutes after induction of anesthesia with fentanyl and propofol, the lungs were hyperinflated manually up to an airway pressure of 40 cmH2 O. FI sub O2 was either kept at 0.4 (group 1, n = 6) or changed to 1.0 (group 2, n = 6) during the recruitment maneuver. Atelectasis was assessed by computed tomography. The amount of dense areas was measured at end-expiration in a transverse plane at the base of the lungs. The ventilation-perfusion distributions (V with dot A/Q with dot) were estimated with the multiple inert gas elimination technique. The static compliance of the total respiratory system (Crs) was measured with the flow interruption technique.

Results: In group 1 (FIO2 = 0.4), the recruitment maneuver virtually eliminated atelectasis for at least 40 min, reduced shunt (V with dot A/Q with dot < 0.005), and increased at the same time the relative perfusion to poorly ventilated lung units (0.005 < V with dot A/Q with dot < 0.1; mean values are given). The arterial oxygen tension (PaO2) increased from 137 mmHg (18.3 kPa) to 163 mmHg (21.7 kPa; before and 40 min after recruitment, respectively; P = 0.028). In contrast to these findings, atelectasis recurred within 5 min after recruitment in group 2 (FIO2 = 1.0). Comparing the values before and 40 min after recruitment, all parameters of V with dot A/Q with dot were unchanged. In both groups, Crs increased from 57.1/55.0 ml *symbol* cmH2 O sup -1 (group 1/group 2) before to 70.1/67.4 ml *symbol* cmH2 O sup -1 after the recruitment maneuver. Crs showed as low decrease thereafter (40 min after recruitment: 61.4/60.0 ml *symbol* cmH2 O sup -1), with no difference between the two groups.     

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Use of a Vital Capacity Maneuver to Prevent Atelectasis after Cardiopulmonary Bypass: An Experimental Study

Background: Respiratory failure secondary to cardiopulmonary bypass (CPB) remains a major complication after cardiac surgery. The authors previously found that the increase in intrapulmonary shunt was well correlated with the amount of atelectasis. They tested the hypothesis that post-CPB atelectasis can be prevented by a vital capacity maneuver (VCM) performed before termination of the bypass.

Methods: Eighteen pigs received standard hypothermic CPB (no ventilation during bypass). The VCM was performed in two groups and consisted of inflating the lungs during 15 s to 40 cmH2 O at the end of the bypass. In one group, the inspired oxygen fraction (FIO2) was then increased to 1.0. In the second group, the FIO2 was left at 0.4. In the third group, no VCM was performed (control group). Ventilation-perfusion distribution was measured with the inert gas technique and atelectasis by computed tomographic scanning.

Results: Intrapulmonary shunt increased after bypass in the control group (from 4.9 +/- 4% to 20.8 +/- 11.7%; P < 0.05) and was also increased in the vital capacity group ventilated with 100% oxygen (from 2.2 +/- 1.3% to 6.9 +/- 2.9%; P < 0.01) but was unaffected in the vital capacity group ventilated with 40% oxygen. The control pigs showed extensive atelectasis (21.3 +/- 15.8% of total lung area), which was significantly larger (P < 0.01) than the proportion of atelectasis found in the two vital capacity groups (5.7 +/- 5.7% for the vital capacity group ventilated with 100% oxygen and 2.3 +/- 2.1% for the vital capacity group ventilated with 40% oxygen.     

Response to Recruitment Maneuver Influences Net Alveolar Fluid Clearance in Acute Respiratory Distress Syndrome

Background: Alveolar fluid clearance is impaired in the majority of patients with acute respiratory distress syndrome (ARDS). Experimental studies have shown that a reduction of tidal volume increases alveolar fluid clearance. This study was aimed at assessing the impact of the response to a recruitment maneuver (RM) on net alveolar fluid clearance.

Methods: In 15 patients with ARDS, pulmonary edema fluid and plasma protein concentrations were measured before and after an RM, consisting of a positive end-expiratory pressure maintained 10 cm H2O above the lower inflection point of the pressure-volume curve during 15 min. Cardiorespiratory parameters were measured at baseline (before RM) and 1 and 4 h later. RM-induced lung recruitment was measured using the pressure-volume curve method. Net alveolar fluid clearance was measured by measuring changes in bronchoalveolar protein concentrations before and after RM.

Results: In responders, defined as patients showing an RM-induced increase in arterial oxygen tension of 20% of baseline value or greater, net alveolar fluid clearance (19 +/- 13%/h) and significant alveolar recruitment (113 +/- 101 ml) were observed. In nonresponders, neither net alveolar fluid clearance (-24 +/- 11%/h) nor alveolar recruitment was measured. Responders and nonresponders differed only in terms of lung morphology: Responders had a diffuse loss of aeration, whereas nonresponders had a focal loss of aeration, predominating in the lower lobes.     

Dose-Response of Rocuronium Bromide in Children Anesthetized with Propofol: A Comparison with Succinylcholine

Background: The aim of this study was to determine the potency of rocuronium during propofol/fentanyl/N2 O anesthesia in children and to compare the time course of action of rocuronium at doses of two and three times the ED95 with that of succinylcholine.

Methods: Rocuronium (120, 160, 200, or 240 micro gram/kg) was administered to 48 children aged 2-10 yr. Neuromuscular block was assessed by monitoring the electromyographic response of the adductor digiti minimi to supramaximal stimulation of the ulnar nerve at 2 Hz for 2 s every 10 s. Potency was determined by log-probit transformation and least-squares linear regression analysis of dose and response. In a second group of 30 children, the onset and recovery profile of rocuronium at doses of two and three times the ED95 was compared with that of succinylcholine (2 mg/kg).

Results: Values for ED50 and ED95 were 210 +/- 24 and 404 +/- 135 micro gram/kg, respectively. The time to 90% neuromuscular block after 1.2 mg/kg rocuronium (three times the ED95), 33 +/- 5 s (mean +/- SD), did not differ significantly from that after succinylcholine, at 30 +/- 7 s; however, both were significantly less than that after 0.8 mg/kg rocuronium, 46 +/- 8 s (P < 0.05). The time to 25% recovery from 1.2 micro gram/kg rocuronium, 41 +/- 13 min, was approximately 50% greater than that after 0.8 mg/kg, at 27 +/- 6 min (P < 0.001), and eight times greater than that after succinylcholine, at 5.2 +/- 1.9 min (P < 0.001).     

Effects of Probenecid on Renal Function in Surgical Patients Anesthetized with Low-flow Sevoflurane

Background: Dehydrofluorination of sevoflurane by carbon dioxide absorbents in anesthesia machines produces compound A, which is nephrotoxic in rats. Several clinical studies indicate that prolonged low-flow sevoflurane anesthesia is associated with an increased urinary excretion of biochemical markers, such as protein. Probenecid, a competitive inhibitor of organic anion transport, diminishes compound A nephrotoxicity in rats. The purpose of the present study was to examine the effects of low- and high-flow sevoflurane anesthesia on urinary excretion of biochemical markers in humans and to examine the effects of probenecid on urinary excretion of these markers.

Methods: Elective surgical patients (n = 64) were assigned to four groups (n = 16 each): low-flow sevoflurane plus probenecid (LSP), low-flow sevoflurane (LS), high-flow sevoflurane plus probenecid (HSP), and high-flow sevoflurane (HS). Probenecid (2.0 g) was administered orally 2 h before the induction of anesthesia in both the LSP and HSP groups. Nothing was administered orally 2 h before the induction of anesthesia in either the LS or HS groups. All patients underwent prolonged low-flow (1 l/min) or high-flow (6 l/min) sevoflurane anesthesia. Urinary excretion of protein, albumin, [beta]2-microglobulin, glucose, and N-acetyl-[beta]-d-glucosaminidase was measured for up to 7 days postoperatively.

Results: Sevoflurane doses were similar in all four groups. There were no differences in blood urea nitrogen, creatinine, or creatinine clearance among the four groups after anesthesia. Average values for urinary excretion of protein, [beta]2-microglobulin, and N-acetyl-[beta]-d-glucosaminidase in the LS group were significantly higher than those in the other groups (LSP, HSP, HS;P < 0.05). There was no significant difference between the LS and LSP groups in average values for urinary excretion of albumin and glucose, although there were significant differences between the LS and both high-flow sevoflurane groups (HSP, HS).     

Effects of Muscle Relaxants on Mask Ventilation in Anesthetized Persons with Normal Upper Airway Anatomy

BACKGROUND:: Recent studies suggest advantages of muscle relaxants for facemask ventilation. However, direct effects of muscle relaxants on mask ventilation remain unclear because these studies did not control mechanical factors influencing ventilation. We tested a hypothesis that muscle relaxants, either rocuronium or succinylcholine, improve mask ventilation. METHODS:: In anesthetized adult persons with normal upper airway anatomy, tidal volumes during facemask ventilation were measured while maintaining the neutral head and mandible positions and the airway pressures of a ventilator before and during muscle paralysis induced by either rocuronium (n = 14) or succinylcholine (n = 17). Tidal volumes of oral and nasal airway routes were separately measured with a custom-made oronasal portioning full facemask. Behavior of the oral airway was observed by an endoscope in six additional subjects receiving succinylcholine. RESULTS:: Total, oral, and nasal tidal volumes did not significantly change at complete muscle paralysis with rocuronium. In contrast, succinylcholine significantly increased total tidal volumes at 60 s after its administration (mean ± SD; 4.2 ± 2.1 vs. 5.4 ± 2.6 ml/kg, P = 0.02) because of increases of ventilation through both airway routes. Abrupt tidal volume increase occurred more through oral airway route than nasal route. Dilation of the space at the isthmus of the fauces was endoscopically observed during pharyngeal fasciculation in all six subjects. CONCLUSIONS:: Rocuronium did not deteriorate facemask ventilation, and it was improved after succinylcholine administration in association with airway dilation during pharyngeal fasciculation. This effect continued to a lesser degree after resolution of the fasciculation.     

Hemodynamic and Cardiac Electrophysiologic Effects of Lidocaine-Bupivacaine Mixture in Anesthetized and Ventilated Piglets

Background: The sensory blockade induced by a lidocaine-bupivacaine mixture combines the faster onset of lidocaine and the longer duration of bupivacaine. The current study compared the effects of large doses lidocaine (16 mg/kg), bupivacaine (4 mg/kg), and a mixture of 16 mg/kg lidocaine-4 mg/kg bupivacaine on hemodynamic and cardiac electrophysiologic parameters in anesthetized and ventilated piglets.

Methods: After carotid artery cannulation, a double micromanometer measured mean aortic pressure, left ventricular end diastolic pressure, and the first derivative of left ventricular pressure. Electrocardiogram recording and a bipolar electrode catheter measured RR, PQ, QRS, QTc, JTc, AH, and HV intervals. Lidocaine, bupivacaine, or the mixture was administered intravenously over 30 s, and studied parameters were measured throughout 30 min.

Results: Mean aortic pressure decreased in all groups (P < 0.05). The first derivative of left ventricular pressure was decreased in all groups (P < 0.001) but to a greater extent with the mixture compared with lidocaine (P < 0.04). RR, QTc, and JTc intervals were similarly increased in all groups (P < 0.05). In all groups, PQ, AH, HV, and QRS intervals were widened (P < 0.001). The lengthening of PQ was greater with bupivacaine (P < 0.02). The lengthening of AH was greater and delayed with bupivacaine compared with lidocaine (P < 0.03). The lengthening of HV and the widening of QRS were greater and delayed with bupivacaine (P < 0.01). The widening of QRS was greater with the mixture than with lidocaine (P < 0.01).     

Effects of Vasopressin on Microcirculatory Blood Flow in the Gastrointestinal Tract in Anesthetized Pigs in Septic Shock

Background: Vasopressin increases arterial pressure in septic shock even when [alpha]-adrenergic agonists fail. The authors studied the effects of vasopressin on microcirculatory blood flow in the entire gastrointestinal tract in anesthetized pigs during early septic shock.

Methods: Thirty-two pigs were intravenously anesthetized, mechanically ventilated, and randomly assigned to one of four groups (n = 8 in each; full factorial design). Group S (sepsis) and group SV (sepsis-vasopressin) were made septic by fecal peritonitis. Group C and group V were nonseptic control groups. After 300 min, group V and group SV received intravenous infusion of 0.06 U [middle dot] kg-1 [middle dot] h-1 vasopressin. In all groups, cardiac index and superior mesenteric artery flow were measured. Microcirculatory blood flow was recorded with laser Doppler flowmetry in both mucosa and muscularis of the stomach, jejunum, and colon.

Results: While vasopressin significantly increased arterial pressure in group SV (P < 0.05), superior mesenteric artery flow decreased by 51 +/- 16% (P < 0.05). Systemic and mesenteric oxygen delivery and consumption decreased and oxygen extraction increased in the SV group. Effects on the microcirculation were very heterogeneous; flow decreased in the stomach mucosa (by 23 +/- 10%; P < 0.05), in the stomach muscularis (by 48 +/- 16%; P < 0.05), and in the jejunal mucosa (by 27 +/- 9%; P < 0.05), whereas no significant changes were seen in the colon.     

Decrease of Functional Residual Capacity and Ventilation Homogeneity after Neuromuscular Blockade in Anesthetized Young Infants and Preschool Children

Background: Based on age-dependent differences in pulmonary mechanics, the effect of neuromuscular blockade may differ in infants compared with older children. The aim of this study was to determine the impact of neuromuscular blockade and its reversal by positive end-expiratory pressure (PEEP) on functional residual capacity (FRC) and ventilation distribution in young infants and preschool children.

Methods: The authors studied 14 infants (aged 0-6 months) and 25 preschool children (aged 2-6 yr). FRC and lung clearance index were calculated. Measurements were taken (1) after intubation, (2) during neuromuscular blockade, and (3) during neuromuscular blockade plus application of PEEP (3 cm H2O).

Results: Functional residual capacity (mean +/- SD) decreased from 21.3 +/- 4.7 ml/kg to 12.2 +/- 4.8 ml/kg (P < 0.001) during neuromuscular blockade in infants and from 25.6 +/- 5.9 ml/kg to 23.0 +/- 5.3 ml/kg (P < 0.001) in preschool children. With the application of PEEP, FRC increased to 22.3 +/- 5.9 ml/kg (P = 0.4829, compared with baseline) in infants and 28.2 +/- 5.8 ml/kg (P < 0.001) in children. The lung clearance index increased after neuromuscular blockade, whereas baseline values were regained after the application of PEEP. The changes induced by neuromuscular blockade were significantly greater in infants compared with preschool children (P < 0.001).