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GENERAL ARTICLES

The Effects of Tidal Volume and Respiratory Rate on Oxygenation and Respiratory Mechanics During Laparoscopy in Morbidly Obese Patients

Juraj Sprung, MD PhD^*, David G. Whalley, MB ChB, Tommaso Falcone, MD^,, William Wilks, RT, James E. Navratil, MD, and Denis L. Bourke, MD^||

*Department of Anesthesiology, Mayo Clinic, Rochester, Minnesota; Department of Anesthesiology, The Cleveland Clinic Foundation, Naples, Florida; Department of Obstetrics and Gynecology, Minimally Invasive Surgery, The Cleveland Clinic Foundation, Cleveland, Ohio; and ||Department of Anesthesiology, University of Maryland, and Veterans Administration Medical Center, Baltimore, Maryland

Address correspondence to Denis L. Bourke, MD, University of Maryland, Anesthesiology Service, Baltimore VA Medical Center, 10 North Greene St., Baltimore, MD 21201. Address e-mail to bourkedenisl{at}aol.com Address reprint requests to Juraj Sprung, MD, PhD, Mayo Medical School, Department of Anesthesiology, Saint Mary’s Hospital, MB 2-752, 200 First St. SW, Rochester, MN 55905. Address e-mail to sprung.juraj@mayo.edu.

Morbidly obese (MO) patients undergoing laparoscopy have lower PaO₂ compared with normal-weight (NW) patients. We hypothesized that increases in tidal volume (V_T) or respiratory rate (RR) would improve oxygenation. All measurements were performed at: 1) baseline: V_T 600–700 mL and 10 breaths/min, 2) double V_T: V_T 1200–1400 mL and 10 breaths/min, and 3) double rate: V_T 600–700 mL and 20 breaths/min. We calculated static respiratory system compliance (Cst,rs) and inspiratory resistance (RI,rs). End-tidal CO₂ was measured with a mass spectrometer, and PaO₂ and PaCO₂ with a continuous blood gas monitor. Supine anesthetized MO patients had 29% lower Cst,rs than the NW patients (P < 0.05). Positioning patients head-up or head-down before pneumoperitoneum did not significantly affect Cst,rs in either group (P = 0.8). Doubling the V_T, but not RR, increased Cst,rs in both groups. Pneumoperitoneum caused large decreases in Cst,rs in both groups (both P < 0.001). During pneumoperitoneum, changing the body position, V_T, or RR did not further affect Cst,rs in either group (P > 0.7). Before pneumoperitoneum, RI,rs was higher in the MO patients compared with the NW patients regardless of body position (P = 0.01). Doubling either RR or V_T before pneumoperitoneum did not change RI,rs in either group. After pneumoperitoneum, RI,rs increased in both the head-down and head-up positions (P < 0.05), but not in the supine position. Regardless of the conditions studied, alveolar-arterial difference in oxygen tension was always significantly higher in MO patients (P < 0.05). The alveolar-arterial difference in oxygen tension was not affected by body position, pneumoperitoneum, or the mode of ventilation. Arterial oxygenation during laparoscopy was affected only by body weight and could not be improved by increasing either the V_T or RR.

IMPLICATIONS: Morbid obesity decreases arterial oxygenation and respiratory system compliance. During laparoscopy, arterial oxygenation is affected only by the patient’s body weight. Increases in tidal volume or respiratory rate do not improve arterial oxygenation.

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