Tidal Volme and Respiratory Rate
While every patient has optimal ventilatory settings for OLV, it takes time and effort to discover these, and may not be practical in the context of an anesthetic. Thus, a reliable starting point is desired. 5-6 cc/kg IBW with 5 cm H2O PEEP is recommended by Miller as a good starting point in patients without COPD. According to Barash, however, a TVOLV < 8 mL/kg can result in dependent-lung atelectasis as well as a decrease in FRC, thus the recommended TVOLV is 10-12 mL/kg. By contrast, a TVOLV > 15 mL/kg can increase PVR in the dependent lung, thus shunting blood to the operative lung. Barash also advocates a PaCO2 of 35 mm Hg, as significant hypocapnia may inhibit the hypoxic pulmonary vasoconstriction response
PEEP in the Dependent Lung
PEEP10 has the theoretical advantages of increasing FRC in the dependent lung, thus improving the V/Q ratio and preventing atelectasis, and does not require cessation of surgery. That said, early studies of PEEP during FiO2 of 1.0 failed to find significant improvements in oxygenation [Tarhan et al. Can Anaesth Soc J 17: 4, 1970; Capan LM et al. Anesth Analg 59: 847, 1980]. A subsequent study of PEEP10 in diseased lungs (with PaO2 < 80 mm Hg) found some benefit [Cohen E et al. Anesth Analg 64: 200, 1985]. More recent studies have shown mixed results - Mascotto et al. randomized 50 patients to ZEEP versus PEEP, and found that PEEP lowered the PaO2/FiO2 ratio, with no differences in the requirement for 100% oxygen, reinflation of the operative lung, or PACU time [Mascotto et al. Eur J Anaesthesiol 20: 704, 2003]. Valenza et al. showed that 10 cm of PEEP in OLV was able to improve oxygenation in patients with an FEV1 of > 72%, but not in those with poor pulmonary function (< 72%) [Valenza et al. Eur J Anaesthesiol 21: 938, 2004, 48 patients]. Senturk et al. showed that the addition of 4 cm PEEP to PCVOLV, PEEP lowered plateau pressures, peak pressures, and Qs/Qt, and increased PaO2 in [Senturk et al. J Cardiothorac Vasc Anesth 19: 71, 2005, 25 patients]
Auto-PEEP
Tends to occur in emphysematous or elderly patients, and is worsened by the small DLT lumen. Can be beneficial if it move the patient towards FRC on the compliance curve (rare), but more often than not it moves that patient away from FRC. Most standard anesthesia ventilators cannot detect auto-PEEP (this requires end-expiratory flow interruption). Auto-PEEP is proportional to tidal volume and inversely proportional to expiratory time, thus, if it is suspected, consider lowering TV and increasing the expiratory time
CPAP in the Operative Lung
The MOST effective method by which PaO2 can be increased is application of CPAP to the operative lung [Capan LM et al. Anesth Analg 59: 847, 1980; Cohen E et al. J Cardiothorac Vasc Anesth 2: 34, 1988; Hogue et al. Anesth Analg 79: 364, 1994], but it must be preceded by a recruitment maneuver, as the opening pressure of atelectatic lung regions is > 20 cm H2O [Rothen HU et al. Br J Anaesth 71: 788, 1993]. In fact, if one fully inflates the lungs, only 1-2 cm H2O CPAP may be needed [Hogue CW. Anesth Analg 79: 364, 1994], which will improve intraoperative conditions
By contrast, some authors have suggested simply giving 5-10 cm H2O immediately after an inspiratory breath [Capan LM et al. Anesth Analg 59: 847, 1980], although it has been shown that insufflation only can increase PAO2 eventually (~ 45 minutes [Rees and Wansbrough. Anesth Analg 61: 507, 1982], as does intermittent reinflation of the lung [Malmkvist. Anesth Analg 68: 763, 1989]. CPAP10 has no hemodynamic consequences [Van Keer et al. J Clin Anesth 1: 284, 1989]. According to Barash, anything greater than 10 cm H2O may cause overdistention and/or hemodynamic consequences
