As it sounds, one lung ventilation (OLV) involves deliberately ventilating just one lung, isolating it from the other one.
One lung ventilation may be used for several reasons:
Lavage e.g. cystic fibrosis
Traumatic bronchial disruption
One lung ventilation can be achieved by several means:
Double lumen endotracheal tube
A long normal OETT (advanced endobronchial in an emergency)
This is perhaps the most common method for one lung ventilation. They have two lumens, one lumen for the trachea, and another which sits in the bronchus of the lung which will be ventilated. There are also two cuffs, one in the trachea, and the other in the bronchus. This allows the lungs to be ventilated together, or just the single lung, as well as allowing access to both lungs. Tubes are designed specifically to be a left or right sided tube, as the will require a specific shape.
The tubes are inserted via laryngoscopy in the usual manner. They have two curves that require consideration during insertion. The tube is initially held rotated to about 90 degrees, to allow the curve of the bronchial tube to be in the sagittal plane as it is passed through the vocal cords. If there is a stylet, this is then removed, and the tube is rotated so that the bronchial tube is then on the side of the lung that is being isolated. The tube will then be advanced until it sits appropriately (often a degree of resistance will be felt). The exact final position needs confirmation through inspection with a bronchoscope, or in some cases this may be possible clinically.. Using a fiberoptic bronchoscope to look down the tracheal lumen should show the carina with the blue bronchial cuff just visible down the appropriate bronchus.
The correct position can also be checked clinically be a 3 step approach:
After insertion, inflate tracheal cuff to prevent air leak through the glottis, and confirm ability to ventilate bilaterally (auscultation and capnography) - essentially the steps for normal ETT insertion.
Clamp the tracheal limb of the tube and disconnect at the ETT end (to be able to hear for any air leak). Inflate the bronchial cuff (1-3ml) and ventilate through the bronchial lumen only, confirming unilateral ventilation through auscultation.
Reattach the bronchial lumen and recommence ventilation through both lumens. Again ensure ventilation is occuring bilaterally through auscultation.
This method is not considered suitable for insertion of a right sided tube due to the problem of the early origin of the right upper bronchus which can be occluded. In this case fiberoptic examination is the only appropriate method to ensure the correct placement.
Collapse of the non-intubated lung (if the purpose is surgical access) can be achieved by detaching the tracheal lumen. If the pleura has been breached, this will allow the lung to collapse down. This can be increased by applying suction to the lung.
If there is a choice available (e.g. for surgical access) then a left sided tube is prefered. This is because of the shorter length of the right main bronchus, which means it is more likely for occlusion of the right upper main bronchus. Right sided tubes have an aperture for this (Murphy’s eye), but getting an adequate positioning is still harder.
Types of double lumen tube include:
Tubes come in sizes from 26 to 41 French gauge (French is the external diameter in mm multiplied by 3). Specific guidance on size selection is not really possible, but depends on the patient’s gender, height and size of right main bronchus. The most accurate method probbaly involves CT measurement of the left main bronchus diameter (as this is where the tube will ultimately sit) Usual sizes are: Female - 37 to 39 Male - 39 to 41 The properly sized tube should pass easily through the glottis and the trachea. The common approach will be to select the smaller size for the gender and try this, with adjustment based on height and weight i.e. either a step up or down.
Advantages of DLT over other techniques for OLV include:
Quickest to insert
Fibreoptic scope not absolutely necessary
Ventilation of either lung
Higher risk of trauma
Limited selection of sizes
Harder to use in difficult airway or anatomy
These are devices inserted through a normally placed OETT. It is essentially a hollow bougie with a cuff. It can be inserted into the bronchus of the lung that is not being ventilated, often by hooking it onto a bronchoscope and then being guided into position. The cuff can then be inflated, isolating that lung from ventilation. Examples of this design include the Arndt and Coopdeck design There are some designs where the bronchial blocker is included in the design of the endotracheal tube e.g. Univent torque control blocker.
Bronchial blockers do have a not insignificant risk of dislodgement and the problems that arise from this. However, there are scenarios where the choice of a bronchial blocker may be preferable to a double lumen tube, but they only really apply when the indication does not involve protection from contamination. In this indication, some scenarios where it may be preferable include:
Difficult laryngoscopy - insertion of a DLT is often harder
There is a strong preference/need to really minimise airway trauma - DLTs are generally associated with a greater degree of this e.g. post op hoarseness.
Very tall patients - DLTs often harder to site in this group
Ongoing ventilation - this may be ventilation before the need for OLV (e.g. they are on ICU) or after the procedure. Changing of the OETT will therefore not be needed.
Possible increased difficulty of insertion
Increased risk of displacement
Need for fiberoptic insertion
Suction not possible
Reduced flexibility of ventilation
Physiology of OLV
There are understandably some significant physiological changes that occur with OLV, which need understanding to help manage the complications, the most common of which is hypoxia. In OLV for thoracic surgery, the ventilated lung is often the dependent lung, as the patient is often positioned on their side to allow the lung which the surgeon is operating on to be at the top. As such it is probably easier to discuss the physiology in this scenario.
The goals of the anaesthetist in OLV are to allow the function of the lungs (i.e. gas exchange) to continue to occur despite loss of ventilation of one lung. Whilst there is often adequate amounts of total lung tissue to allow this (and diffusion surface area), there are variations in the ventilation, perfusion and matching of these two variables that can result in suboptimal gas exchange.
If we consider the theoretical scenario where the only difference was that one lung was no longer ventilated, we can see that the oxygen in this lung would be rapidly taken up by the pulmonary blood flow and become depleted. The blood passing through that lung would therefore be unable to be oxygenated on its passage through the lung (or lose CO2) and would essentially leave the lung with the same oxygen content as it entered. This would then join the oxygenated blood of the other lung, and serve to ‘dilute’ the oxygen content of the blood from the ventilated lung, a process that is termed shunt. As the lungs and haemoglobin normally function at a high degree of capacity (i.e. nearly fully saturated with oxygen), and because of the physiology of oxygen transport with haemoglobin, there is no method for compensating for this. The main problem that this causes is a reduced saturation of haemoglobin with oxygen, which impairs oxygen delivery to the body. However, physiological mechanisms exist to combat this problem, primarily hypoxic vasoconstriction. This is covered in more detail elsewhere, but can be summarised as:
Parts of lung that are poorly ventilated have a quick reduction in oxygen due to tissue or blood uptake
This results in a low partial pressure of oxygen in that lung unit and surrounding tissue, including the vasculature.
This low partial pressure of oxygen results in activation of local mechanisms to increase vascular tone, and thus reduce blood flow.
The result is diversion of blood away from hypoxic regions of lung to better ventilated ones, and thus reducing shunt.
This concept is important to consider when commencing one lung ventilation, as manipulation of it will help reduce the desaturation that can occur in one lung ventilation, due to the incomplete nature of hypoxic vasoconstriction.
In the common position for thoracic surgery (lateral with the ventilated lung inferior) there are some fortunate effects. The impact hydrostatic pressure means that lung perfusion is better in the dependent lung, which is the ventilated lung, and so this aids V/Q matching, reducing shunt. This effect will not be as prevalent in other positions for OLV, and so may result in greater degrees of shunt and hypoxia.
The switch to OLV has the potential for causing to the lungs, which may manifest immediately e.g. barotrauma related rupture, or later e.g. ARDS. Strategies to limit this include:
Low tidal volumes - 6ml/kg predicted body weight still suitable for just a single lung
This is common and almost to be expected to some degree. It is commonly related to shunt through the unventilated lung, but other causes should also be kept in mind. In general, an SpO2 of over 90% is considered the recommended range, although it not clear what the lowest ‘safe’ value is.
A step-wise approach can be used to manage hypoxia in these circumstances:
Increase FiO2 to 100%
Check tube position
Position of tube overall
Position of bronchial lumen
Position of bronchial blocker
Ensure satisfactory haemodynamics - important for adequate V/Q matching
Perform recruitment manoeuvre - optimise gas exchange and reduce shunt in the ventilated lung
Apply/adjust PEEP to the ventilated lung - as for 4
Insufflate oxygen to non ventilated lung - will provide oxygenated of shunted blood, reducing its impact.
Apply CPAP to non ventilated lung - as for 6
Ventilate non ventilated lung - may be necessary is severe hypoxia. However, in addition transient ventilation can improve the hypoxic vasoconstrictor response
Clamp the pulmonary artery of non ventilator lung - eliminates the shunt, but is only really plausible in pneumonectomy for any significant length of time.