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Changes in respiratory physiology during laparoscopy are from the combined effects of pneumoperitoneum, positioning, ongoing CO2 absorption, and patient body weight. The basic principles of respiratory physiology that apply to a routine general anesthetic remain pertinent under laparoscopy (Figure 5.1). In the awake state with spontaneous ventilation, a gravitational gradient promotes greater blood flow, and greater intrapleural pressure surrounding the basilar alveoli. The alveoli at the lung base are more compressed in size because of higher intrapleural pressure. As a result, the dependent (basilar/down) portion of the lungs lies on a steeper part of the pressure volume curve allowing greater expansion during inspiration. Consequently, alveoli in the dependent (basilar/down) part of the lung are better perfused, and better ventilated. Conversely, apical alveoli have less perfusion, are larger in resting size, lie on the plateau of the pressure volume curve, and expand less with inspiration (Figure 5.2). General anesthesia, supine positioning, and muscle relaxation decrease the difference in ventilation between the apical and basilar alveoli. The supine position decreases functional residual capacity (FRC) 10%-15% and the

V/Q ratio

V/Q ratio

Lung Fields

Lung base Lung apex

Rib Number

Figure 5.1. The basic principles of respiratory physiology that apply to a routine general anesthetic remain pertinent under laparoscopy.

Lung base Lung apex

Rib Number

Figure 5.1. The basic principles of respiratory physiology that apply to a routine general anesthetic remain pertinent under laparoscopy.

4000

4000

Apical Alveoli

Transpulmoriary Pressure cm HzO

Figure 5.2. Alveoli in the dependent (basilar/down) part of the lung are better perfused, and better ventilated. Conversely, apical alveoli have less perfusion, are larger in resting size, lie on the plateau of the pressure volume curve (figure at right), and expand less with inspiration. (Reprinted with permission from Johnson ME., Factors Affecting Pulmonary Ventilation and Perfusion In Faust, RJ Anesthesiology Review., 3rd edition; New York, Churchill Livingstone; 2002:9).

Transpulmoriary Pressure cm HzO

Figure 5.2. Alveoli in the dependent (basilar/down) part of the lung are better perfused, and better ventilated. Conversely, apical alveoli have less perfusion, are larger in resting size, lie on the plateau of the pressure volume curve (figure at right), and expand less with inspiration. (Reprinted with permission from Johnson ME., Factors Affecting Pulmonary Ventilation and Perfusion In Faust, RJ Anesthesiology Review., 3rd edition; New York, Churchill Livingstone; 2002:9).

induction of general anesthesia further decreases FRC an additional 20%. Anesthesia and paralysis cause the reduction of lung volume through a continuum related to the body mass index (BMI).5 A reduction in lung compliance with BMI is simply the reduction in FRC, with the intrinsic mechanical characteristics of the lung being approximately normal. Oxygenation expressed as PaO2/PAO2 ratio also decreases with increasing BMI. The major cause of this decrease is likely related to the reduction in FRC. Under anesthesia, the nondependent (apical/ up) lung receives greater ventilation as it moves to a steeper part of the pressure volume curve (Figure 5.2). Supine positioning, muscle relaxation, cephalad displacement of the diaphragm, and compression by the abdominal contents create microatelectatic areas in the dependent part (basilar/down) of the lung and small airways collapse. This phenomenon results in true intrapulmonary shunting and ventilation perfusion mismatch.

Pneumoperitoneum and positioning changes during laparoscopic surgery add to the effects of general anesthesia and muscle paralysis. The pneumoperitoneum shifts the diaphragm cephalad, reduces diaphragmatic excursion, and stiffens the diaphragm/abdomen part of the chest wall.6 The decreased chest wall compliance and increase in intrathoracic pressure, limits lung expansion. The restricted lung expansion elevates peak and plateau airway pressures and decreases oxygenation (PaO2). Controlled ventilation allows the anesthesiologist to increase minute ventilation overcoming the decreased thoracopul-monary compliance and hypoventilation. In normal-weight patients, pneumoperitoneum causes a 47% decrease in lung compliance, a 50%

increase in peak airway pressure, and an 81% increase in airway plateau pressure.7 Morbidly obese anesthetized supine patients have 30% lower static respiratory system compliance and increased inspiratory airway resistance compared with their normal-weight counterparts.8 Laparoscopic surgery causes more severe deterioration in gas exchange in obese patients compared with normal subjects, who show a milder abnormality in alveolar-arterial oxygen difference.

Alterations such as increased tidal volume (TV) or the addition of positive end-expiratory pressure (PEEP) do not reliably improve PaO2.8 Increasing the TV (1000-1200 mL) often fails to improve oxygenation in both normal-weight and morbidly obese patients, suggesting that poorly ventilated, but perfused, areas of the lung are not consistently recruited.8 In morbidly obese patients, ventilation with large TV, especially during pneumoperitoneum, results in high end-inspiratory (plateau) pressures. The end-inspiratory pressure is a measure of paren-chymal stretch during ventilation. The acceptable upper limit is approximately 35 cm H2O.9 Prolonged increases in inspiratory pressures may lead to barotrauma of the lung parenchyma. The addition of PEEP is not a reliable tool for improving gas exchange. The addition of 10 cm of PEEP can reduce or eliminate areas of microatelectasis.10 It may also overstretch alveoli, decrease CO, and worsen V/Q mismatch. The use of PEEP in morbidly obese patients may slightly improved PaO2 (from 110 to 130 mm Hg) compared with normal-weight subjects.11 The decline in pulmonary arterial oxygenation during laparoscopy is primarily the effect of patient weight, which correlates with decreased thoracopul-monary compliance.8 Increasing the inspired oxygen concentration may be the most reliable treatment for hypoxemia in overweight and morbidly obese patients.

Laparoscopic colectomy usually requires the patient to be placed in steep Trendelenburg position. The head down position pushes abdominal contents upward additionally impairing diaphragmatic excursion and lung expansion. Vital capacity (VC) is reduced because of the increased weight of the abdominal viscera against the diaphragm. Prolonged placement in the Trendelenburg position can lead to edema of the airway including the larynx. Despite the appearance of a positive trend, a 30° reverse Trendelenburg position does not have significant beneficial effects on breathing mechanics.12 Inspiratory resistance is increased both in the Trendelenburg and reverse Trendelenburg positions if minute ventilation is manipulated (Figure 5.3). This change in inspiratory airway resistance with position applies to both normal-weight and obese patients. There is also a potential for inadvertent right mainstem bronchial intubation, and hypoxemia with Trendelenburg positioning.13

The CO2 continually insufflated into the abdomen dissolves in the blood elevating arterial CO2, and consequently alveolar CO2. This is reflected as an increase in ETCO2. Spontaneous ventilation, especially in patients with diminished pulmonary reserve, would result in profound respiratory acidosis. Because general anesthesia allows controlled ventilation, it permits the anesthesiologist to increase the minute ventilation either by increasing the TV and/or respiratory rate. Usually, an increase in the respiratory rate is sufficient to overcome hypercarbia.

Breathing Rate During Day

Figure 5.3. Pneumoperitoneum increases inspiratory pressures and resistance in both the Trendelenburg (Trend) and reverse Trendelenburg (rev Trend) positions. (Reprinted with permission from Sprung J, Whalley DG, Falcone T, Wilks W, Navratil JE, Bourke DL: The Effects of Tidal Volume and Respiratory Rate on Oxygenation and Respiratory Mechanics During Laparoscopy in Morbidly Obese Patients Anesthesiology Review; 2003:07:268-274.)

Figure 5.3. Pneumoperitoneum increases inspiratory pressures and resistance in both the Trendelenburg (Trend) and reverse Trendelenburg (rev Trend) positions. (Reprinted with permission from Sprung J, Whalley DG, Falcone T, Wilks W, Navratil JE, Bourke DL: The Effects of Tidal Volume and Respiratory Rate on Oxygenation and Respiratory Mechanics During Laparoscopy in Morbidly Obese Patients Anesthesiology Review; 2003:07:268-274.)

Abdominal distension may not allow an increase in TV without further increase in inspiratory airway pressures. Controlled ventilation throughout laparoscopic surgery helps prevent hypercarbia and respiratory acidosis.

After open abdominal surgery, the VC is reduced by 40%-50% of preoperative values. The VC is gradually restored over the next 5-7 days. FRC is reduced by 70%-80% of preoperative values. Gradual restoration of lung volumes begins on the second to third postoperative day. Full restoration to preoperative status may take as long as 1 week. These postoperative effects in FRC and VC are attributed to pain and reflex diaphragmatic dysfunction.13 Patients undergoing laparoscopic procedures are noted to have better postoperative pulmonary mechanics than those undergoing open procedures.14 A 20%-25% postoperative improvement in forced expiratory volume in 1 second, forced VC, and forced expiratory flow in patients undergoing laparoscopic chole-cystectomy versus an open procedure is likely attributable to minimal abdominal wall disruption, leading to less postoperative pain.14

The maintenance of adequate ventilation and oxygenation during laparoscopy is a challenge for the anesthesiologist. The decrease in pulmonary compliance, lowered lung volumes, and continued absorption of CO2 leads to hypoxia and hypercarbia. Ventilatory adjustments are continued throughout surgery to maintain oxygen and CO2 content near the physiologic norm.

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    DOES LAPAROSCOPIC SURGERY DECREASE LUNG VOLUME?
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