3. One lung ventilation

Pulmonary, oesophageal and spinal surgery. It may be necessary during surgery on the thoracic aorta, and it is also used for relatively minor procedures such as transthoracic cervical sympathectomy and pleurodesis. VATS It may be indicated to prevent contamination of the contralateral lung by empyema or significant endobronchial haemorrhage. Lung isolation or protection is also necessary in cases of bronchopleural fistula.

For the duration of anaesthesia the surgical side is uppermost, and the non-ventilated upper lung is usually described as the non-dependent lung. When ventilation is interrupted, the remaining blood flow takes no part in gas exchange, creating ventilation–perfusion mismatch and a shunt, which contributes to hypoxia. The shunt will further reduce if non-dependent blood vessels are ligated surgically, and will largely disappear if, for example, the pulmonary artery is clamped prior to pneumonectomy. Hypoxic pulmonary vasoconstriction (HPV) decreases the flow to the nondependent lung by around 50%, and may reduce the shunt from 50% down to 30% (which is nonetheless still significant). Secretions may pool in the dependent lung, but suction removal via a double-lumen tube may be very difficult.

The ventilator settings are similar to those used for double-lung ventilation with tidal volumes of around 10–12 ml kg Higher volumes increase both mean airway (Paw) and vascular resistance, with the result that more blood may flow to the non-ventilated lung and increase shunt. Lower tidal volumes are likely to lead to pulmonary atelectasis.
Although shunt is not substantially improved by supplemental oxygen, many anaesthetists routinely increase the FiO2 to 0.8–1.0.
The respiratory rate is adjusted to keep the end-tidal carbon dioxide (ETCO2) at around 5–6% or 40 mmHg.

• Pre-existing disease, either pulmonary or cardiac, may be an important contributory factor. • Check the FiO2 and increase it if necessary. This may not help if significant shunt is the problem, but it is probably the swiftest intervention available. • Check the tidal volume and other ventilator indices. Again, these are interventions that can be made rapidly. The ETCO2 should be maintained at 5–6% because hypocapnia may decrease hypoxic pulmonary vasoconstriction, although small increases in tidal volume can help oxygenation. • The double-lumen tube position should then be checked with a fibreoptic bronchoscope. Displacement to a suboptimal position is very common, particularly if the patient has been moved.

• If oxygenation still does not improve, then CPAP of around 5 cmH2O can be added to the upper lung, but the surgeon will have to be warned that the lung may partially re-expand. • Alternatively, oxygen can be insufflated in the upper lung, but many anaesthetists do this routinely from the start of surgery. • PEEP (~5 cmH2O) can be added to the lower lung, which may increase volume in potentially atelectatic areas. This manoeuvre may, however, increase vascular resistance and divert blood to the non-ventilated upper lung. • Both CPAP and PEEP can be increased in small increments. 9.. If none of these interventions is successful, intermittent inflation can be tried, or it may finally be necessary to revert to full double-lung ventilation (with lung retraction which will allow surgery to continue).

Difficulties with double-lumen tubes are probably the most important cause of mortality and morbidity associated with one-lung anaesthesia. In the 1998 National Confidential Enquiry into Peri-Operative Deaths (NCEPOD), which looked at oesophagectomy, problems with double-lumen tubes were implicated in 30% of perioperative deaths. Studies have confirmed that critical malpositioning occurs in more than 25% of cases, and general misplacements complicate more than 80% of uses. This is not surprising. The anatomy may be distorted by tumour or effusion, and the tubes are bulky and more complex to insert than single-lumen tubes, requiring rotation within the airway of between 90 and 180. Complications include failure to achieve adequate lung separation and one-lung ventilation, prolonged surgical retraction and associated pulmonary trauma, occlusion of a major bronchus with lobar collapse and secondary infection, contamination of the dependent lung by infected secretions from the upper lung and trauma during insertion.

A double-lumen tube is positioned correctly when the upper surface of the bronchial cuff lies immediately distal to the bifurcation of the carina. This tube position can be assessed clinically, but this may be unreliable. The average depth of insertion for a patient of height 170 cm is 29 cm, and the distance alters by 1 cm for every 10 cm change in height. This distance from the incisors can be used as an approximate guide. Auscultation of the lung fields during clamping and release can be performed, although findings may be equivocal if access to the chest wall is limited because surgery has begun. Oximetry and capnography will not give specific enough information about where the tube is sited. The tube position should therefore be checked using a fibreoptic bronchoscope