Objective—To evaluate the effectiveness of reduction of inspired oxygen fraction (Fio2) or application of positive end-expiratory pressure (PEEP) after an alveolar recruitment maneuver (ARM) in minimizing anesthesia-induced atelectasis in dogs.
Animals—30 healthy female dogs.
Procedures—During anesthesia and neuromuscular blockade, dogs were mechanically ventilated under baseline conditions (tidal volume, 12 mL/kg; inspiratory-to-expiratory ratio, 1:2; Fio2, 1; and zero end-expiratory pressure [ZEEP]). After 40 minutes, lungs were inflated (airway pressure, 40 cm H2O) for 20 seconds. Dogs were then exposed to baseline conditions (ZEEP100 group), baseline conditions with Fio2 reduced to 0.4 (ZEEP40 group), or baseline conditions with PEEP at 5 cm H2O (PEEP100 group; 10 dogs/group). For each dog, arterial blood gas variables and respiratory system mechanics were evaluated and CT scans of the thorax were obtained before and at 5 (T5) and 30 (T30) minutes after the ARM.
Results—Compared with pre-ARM findings, atelectasis decreased and Pao2:Fio2 ratio increased at T5 in all groups. At T30, atelectasis and oxygenation returned to pre-ARM findings in the ZEEP100 group but remained similar to T5 findings in the other groups. At T5 and T30, lung static compliance in the PEEP100 group was higher than values in the other groups.
Conclusions and Clinical Relevance—Application of airway pressure of 40 cm H2O for 20 seconds followed by Fio2 reduction to 0.4 or ventilation with PEEP (5 cm H2O) was effective in diminishing anesthesia-induced atelectasis and maintaining lung function in dogs, compared with the effects of mechanical ventilation providing an Fio2 of 1.
Objective—To evaluate the effects of 10 cm H2O of positive end-expiratory pressure (PEEP) on lung aeration and gas exchange in mechanically ventilated sheep during general anesthesia induced and maintained with propofol.
Animals—10 healthy adult Bergamasca sheep.
Procedures—Sheep were sedated with diazepam (0.4 mg/kg, IV). Anesthesia was induced with propofol (5 mg/kg, IV) and maintained with propofol via constant rate infusion (0.4 mg/kg/min). Muscular paralysis was induced by administration of vecuronium (25 μg/kg, bolus IV) to facilitate mechanical ventilation. After intubation, sheep were positioned in right lateral recumbency and mechanically ventilated with pure oxygen and zero end-expiratory pressure (ZEEP). After 60 minutes, 10 cm H2O of PEEP was applied for 20 minutes. Spiral computed tomography of the thorax was performed, and data were recorded for hemodynamic and gas exchange variables and indicators of respiratory mechanics after 15 (T15), 30 (T30), and 60 (T60) minutes of ZEEP and after 20 minutes of PEEP (TPEEP). Computed tomography images were analyzed to determine the extent of atelectasis before and after PEEP application.
Results—At TPEEP, the volume of poorly aerated and atelectatic compartments was significantly smaller than at T15, T30, and T60, which indicated that there was PEEP-induced alveolar recruitment and clearance of anesthesia-induced atelectasis. Arterial oxygenation and static respiratory system compliance were significantly improved by use of PEEP.
Conclusions and Clinical Relevance—Pulmonary atelectasis can develop in anesthetized and mechanically ventilated sheep breathing pure oxygen; application of 10 cm H2O of PEEP significantly improved lung aeration and gas exchange.
OBJECTIVE To evaluate the impact of 2 tidal volumes (TVs) with or without positive end-expiratory pressure (PEEP) on lung mechanics, aeration, and gas exchange in healthy anesthetized dogs.
ANIMALS 40 mixed-breed dogs with healthy lungs.
PROCEDURES Anesthetized dogs were randomly assigned to 4 groups (n = 10/group) with different ventilatory settings: TV of 8 mL/kg and PEEP of 0 cm H2O (low TV group), TV of 8 mL/kg and PEEP of 5 cm H2O (low TV plus PEEP group), TV of 15 mL/kg and PEEP of 0 cm H2O (high TV group), or TV of 15 mL/kg and PEEP of 5 cm H2O (high TV plus PEEP group). Expired CO2 and respiratory rate were titrated on the basis of a predetermined stepwise protocol. Gas exchange, respiratory mechanics, and pulmonary aeration were evaluated by means of CT 30 minutes after starting mechanical ventilation at the assigned setting.
RESULTS Partial pressures of arterial and expired CO2 were higher in the low TV and low TV plus PEEP groups than in the high TV and high TV plus PEEP groups. Peak and plateau airway pressures were higher in the PEEP group than in the other groups. Static lung compliance was higher in the high TV plus PEEP group than in the low TV group. Relative percentages of atelectatic and poorly aerated lung were lower in the high TV plus PEEP group than in the other groups. Oxygenation was similar among groups.
CONCLUSIONS AND CLINICAL RELEVANCE Differences in TV and PEEP application during mechanical ventilation may affect respiratory function in anesthetized dogs with healthy lungs. Ventilation with a TV of 15 mL/kg and PEEP of 5 cm H2O significantly improved lung compliance and reduced the amount of atelectatic and poorly aerated lung.
Objective—To compare the effect of 2 concentrations of oxygen in inspired gas (fraction of inspired oxygen [FIO2] 1.0 or 0.4) on pulmonary aeration and gas exchange in dogs during inhalation anesthesia.
Animals—20 healthy dogs.
Procedures—Following administration of acepromazine and morphine, anesthesia was induced in each dog with thiopental and maintained with isoflurane in 100% oxygen (100% group; n = 10) or a mixture of 40% oxygen and air (40% group; 10). Dogs were placed in dorsal recumbency and were mechanically ventilated. After surgery, spiral computed tomography (CT) of the thorax was performed and PaO2, PaCO2, and the alveolar-arterial oxygen tension difference (P[A–a]O2) were assessed. The lung CT images were analyzed, and the extent of hyperinflated (−1,000 to −901 Hounsfield units [HUs]), normally aerated (−900 to −501 HUs), poorly aerated (−500 to −101 HUs), or nonaerated (−100 to +100 HUs) areas was determined.
Results—Compared with the 100% oxygen group, the normally aerated lung area was significantly greater and the poorly aerated and nonaerated areas were significantly smaller in the 40% oxygen group. The time to CT (duration of surgery) was similar in both groups. Although PaCO2 was similar in both groups, PaO2 and P(A–a)O2 were significantly higher in the 100% oxygen group. In both groups, pulmonary atelectasis developed preferentially in caudal lung fields.
Conclusion and Clinical Relevance—In isoflurane-anesthetized dogs, mechanical ventilation with 40% oxygen appeared to maintain significantly better lung aeration and gas exchange than ventilation with 100% oxygen.
Objective—To evaluate the use of the oxygen content–based index, Fshunt, as an indicator of venous admixture (s/t) at various fractions of inspired oxygen (Fio2s) in anesthetized sheep undergoing Flung or 2-lung ventilation.
Animals—6 healthy adult female sheep.
Procedures—Sheep were anesthetized and administered 5 different Fio2s (0.21, 0.40, 0.60, 0.80, and 1.00) in random order during 2-lung mechanical ventilation. Arterial and mixed venous blood samples were obtained at each Fio2 after a 15-minute stabilization period. Vital capacity alveolar recruitment maneuvers were performed after blood collection. The previously used Fio2 sequence was reversed for sample collection during Flung ventilation. Blood samples were analyzed for arterial, pulmonary end-capillary, and mixed venous oxygen content and partial pressure and for hemoglobin concentration. Oxygen hemoglobin saturation, s/t, Fshunt, and oxygen tension–based indices (OTIs; including Pao2:Fio2, alveolar-arterial difference in partial pressure of oxygen [Pao2 – Pao2], [Pao2 – Pao2]:Fio2, [Pao2 – Pao2]:Pao2, and Pao2:Pao2) were calculated at each Fio2; associations were evaluated with linear regression analysis, concordance, and correlation tests. Intermethod agreement between s/t and Fshunt was tested via Bland-Altman analysis.
Results—Strong and significant associations and substantial agreement were detected between Fshunt and s/t. Relationships between OTIs and s/t varied, but overall correlations were weak.
Conclusions and Clinical Relevance—Whereas OTIs were generally poor indicators of s/t, Fshunt was a good indicator of s/t at various Fio2s, regardless of the magnitude of s/t, and could be potentially used as a surrogate for s/t measurements in healthy sheep.