Impact of positive end-expiratory pressure and recruitment maneuver on healthy lungs in dogs assessed by functional and anatomical monitoring methods

Martina Mosing ^1,2 Andreas Waldmann ³ Thom C. Gent ² Giselle Hosgood ⁴ Nadja Sieber-Ruckstuhl ⁵ Matthias Dennler ⁶ Peter Herrmann ⁷ Karin Unger ^1*

• ¹ Department of Small Animals and Horses, University of Veterinary Medicine, Vienna, Austria
• ² Section of Anesthesiology, Vetsuisse Faculty, University of Zurich, Switzerland, Zurich, Switzerland
• ³ Department of Anaesthesiology and Intensive Care Medicine, Rostock University Medical Centre, Rostock, Germany, Rostock, Germany
• ⁴ College of Veterinary Medicine, School of Veterinary and Life Sciences, Murdoch University, Perth, Western Australia, Australia
• ⁵ Clinic for Small Animal Internal Medicine, Vetsuisse Faculty, University of Zurich, Zurich, Zurich, Switzerland
• ⁶ Clinic of Diagnostic Imaging, Vetsuisse Faculty, University of Zurich, Switzerland, Zurich, Switzerland
• ⁷ Department of Anesthesiology, University Medical Center, Georg-August-University, Goettingen, Germany., Goettingen, Germany

Atelectasis is a common occurrence during anesthesia, and positive end-expiratory pressure (PEEP) ventilation and recruitment maneuvers (RM) can be used to mitigate this. However, both techniques may be associated with side effects in healthy lungs, and close monitoring is indicated. This study aimed to evaluate the effects of PEEP and RM in healthy dogs and compare functional lung monitoring methods by electrical impedance tomography (EIT), volumetric capnography (VCap), and blood gas analysis with the gold-standard anatomical monitoring provided by computed tomography (CT). Nine healthy Beagle dogs underwent anesthesia and mechanical ventilation three times. After 35 minutes using zero end-expiratory pressure (ZEEP), CT images, VCap, EIT measurements, and arterial blood gas samples were taken. Thereafter, either (1) ZEEP was continued, (2) PEEP initiated or (3) an RM was performed followed by PEEP. Ten minutes after changing the ventilation mode measurements were repeated. Only one ventilation mode was employed during each anesthesia. During RM, we found a significant increase in the percentage of overaerated lung (Vhyper) (p< 0.001), while the amount of normally aerated lung (Vnormal), poorly aerated lung and non-aerated lung decreased (p≤0.001). VCap showed an increase in airway dead space (VDaw/VT) (p=0.002), and a decrease in alveolar dead space (VDalv/VTalv). For PEEP, an increase in airway dead space (p=0.003) was found. For both groups, the amount of carbon dioxide exhaled per breath (VTCO2,br) decreased (p<0.001), and EIT showed a shift of the center of ventilation to the dependent lung areas (p=0.021 and p=0.046, respectively). Oxygenation was superior in RM compared to ZEEP (p=0.033). The arterial partial pressure of carbon dioxide decreased in RM (p=0.012). Positive associations were found between Vhyper and VDaw/VT (p=0.004), Vhyper and Vnormal with VTCO2,br (p=0.002 for both). Negative associations were found between Vhyper and VDalv/VTalv (p=0.004) and non-dependent silent spaces (p=0.050), and Vnormal with oxygenation (p=0.030). While RM may be effective in improving gas exchange, it appears to be not benign in healthy lungs, and PEEP might be the preferable strategy to avoid lung collapse during anesthesia. Functional monitoring – EIT, VCap, blood gas analysis – does not detect changes corresponding to anatomical findings on CT.