1

What are the important aspects of thoracic anatomy for an anaesthetist (Figure 1)?

Endotracheal or endobronchial intubation will be facilitated by anatomical knowledge of the tracheobronchial tree, including the:

a)

trachea – which is usually 10-12cm in length and 1.5-2.5cm in diameter, with 15-20 ‘C-shaped’ cartilaginous rings. It divides at the carina into the left and right main bronchi. usually at the level of the 5th thoracic vertebra;

b)

main bronchi – where the right is shorter (approximately 2.5cm vs. 5cm) and more vertical (20-30° vs. 35-45°) than the left;

c)

right upper lobe (RUL) bronchus – which is usually found approximately 2.5cm from the carina. This may be occluded by a right-sided double-lumen tube (DLT). An unusually high take-off of the right main bronchus further increases this risk.

An understanding of the alterations in cardiorespiratory physiology with one-lung ventilation (OLV) is facilitated by an awareness of pulmonary anatomy, including the:

a)

right lung – which is larger and heavier with three lobes (upper, middle and lower), and receives approximately 55% of the cardiac output;

b)

left lung – which has two major lobes (upper and lower), with a tongue-like projection to below the cardiac notch (lingula), and receives approximately 45% of the cardiac output.

2

Which routine investigations are important to an anaesthetist prior to thoracic surgery?

Full blood count (FBC) – which may demonstrate anaemia and post-chemotherapy neutropaenia.

Urea and electrolytes (U&E) – which may provide a useful indicator of renal impairment or the need for electrolyte supplementation.

Chest radiograph (CXR) – which provides information regarding the anatomy of the tracheobronchial tree, as well as the presence of any collapse or consolidation (Figure 2).

Electrocardiogram (ECG) – which may demonstrate arrhythmias or evidence of ischaemic heart disease (common in smokers).

3

Which lung function tests are important to an anaesthetist prior to thoracic surgery?

Spirometry (Figure 3) – which measures the volume of air expelled during a single forced expiration. Spirometry measurements aid the decision-making process regarding fitness for thoracic surgery.

Static lung volume determination – which measures lung volumes and capacities (see Chapter 5 for further details).

Diffusing capacity of the lungs for carbon monoxide (DLCO) – which measures the ability of oxygen to diffuse across the alveolar-capillary membrane. It is particularly useful to measure if diffuse parenchymal lung disease is evident radiologically or if exertional dyspnoea cannot be adequately explained by spirometry results.

Arterial blood gases (ABG) – which demonstrate the baseline partial pressure of oxygen (pO₂), and carbon dioxide (pCO₂), as well as carboxy-haemoglobin levels (which may be increased in smokers) that may impair the ability to transfer oxygen.

Cardiopulmonary exercise testing (CPEX) – which provides an objective measure of the functional ability of a patient to respond to the increased metabolic demands of major surgery. The maximum capacity of an individual’s ability to transport and use oxygen during incremental exercise (VO_{2 max}) is measured. This can be used to estimate the peri-operative risk of surgery:

a)

VO_{2 max} <15mL/kg/min indicates an increased risk of peri-operative complications;

b)

VO_{2 max} <10mL/kg/min suggests a very high risk and usually precludes the patient from surgery.

If CPEX is unavailable, stair climbing, shuttle walk or 6-minute walk tests (6MWT) should be considered. Shuttle walk and 6MWT distances correlate well with peak VO₂ and maximum work capacity as measured by CPEX.

4

What are the important aspects of spirometry for an anaesthetist prior to thoracic surgery?

A single forced expiration is simple to perform and informative in predicting suitability for thoracic surgery.

Spirometric values following optimal bronchodilator therapy are helpful as they may indicate the need (or otherwise) for peri-operative treatment. Both the insertion of airway devices and administration of anaesthetic and analgesic drugs are associated with bronchoconstriction. If bronchoconstriction is present prior to anaesthesia, optimal therapy (with or without steroids) may be able to reduce its impact.

Previous British Thoracic Society (BTS) guidelines suggest that a patient with an FEV₁ >1.5L in the absence of symptoms could safely undergo lobectomy. Spirometry alone, however, is now known to be a poor predictor of outcome following thoracic surgery and it is recommended that the diffusing capacity (DLCO) should also be assessed. Nevertheless, if the FEV₁, FVC and FEV₁/FVC are within normal limits and the patient has a good exercise tolerance, problems are unlikely.

5

What are the important aspects of lung volume assessment for an anaesthetist prior to thoracic surgery?

Lung volume assessment is important for an anaesthetist for:

a)

peri-operative issues – where:

i)

any gas in the lung at the end of expiration (functional residual capacity [FRC]) provides a reservoir of oxygen for gas exchange during periods of apnoea. If periods of apnoea are going to be necessary for surgical reasons, then filling the FRC with oxygen prior to this will extend the duration of apnoea;

ii)

closing capacity (CC) represents the volume in the lungs at which its smallest airways (the alveoli) collapse. It will be important to avoid collapse of these airways in the ventilated lung so as to minimise the proportion of the alveoli suffering shear stress on re-opening and the risk of respiratory distress syndrome;

b)

postoperative planning – where:

i)

predicted postoperative (ppo) FEV₁ should be calculated taking into account the number of lung segments that will remain following surgery. The ppo FEV₁, however, correlates less well with outcome than the ppo DLCO;

ii)

both FEV₁ and FVC are reduced by as much as 65% on the first postoperative day, mandating the early institution of effective physiotherapy. Anaesthetists will need to ensure appropriate analgesia supports this;

iii)

patients with a ppo FEV₁ <40% have an increased risk of developing respiratory complications;

iv)

patients with a ppo FEV₁ <30% have an increased risk of requiring postoperative mechanical ventilation, and an intensive care bed should be made available.

No single test shows validity in assessing likely postoperative outcome pre-operatively. Thus, the tripartite system should be used, incorporating:

a)

assessment of respiratory mechanics (FEV₁);

b)

parenchymal function (DLCO and arterial blood gases);

c)

cardiopulmonary reserve (VO_{2 max}, SaO₂ alteration with exercise).

6

How does functional residual capacity (FRC) change with position and anaesthesia?

Functional residual capacity (FRC) decreases with:

a)

placing a patient in a supine, lateral or prone position – which reduces chest compliance, caused by the abdominal contents pushing up against the diaphragm;

b)

anaesthesia – which decreases the tone of the diaphragm and intercostal muscles, thereby pushing the diaphragm up into thorax.

During normal expiration, FRC usually exceeds the CC and therefore prevents collapse of the alveoli.

During anaesthesia, however, CC may equal or exceed FRC, which results in small airway collapse during normal tidal expiration producing atelectasis and air trapping.

7

How does anaesthesia alter the matching of ventilation and perfusion?

During OLV, ventilation (V) varies across the different areas of the lungs, including:

a)

non-dependent (operated) lung – which is not ventilated, whilst perfusion continues;

b)

dependent (ventilated) lung – where ventilation is relatively more effective than perfusion in the upper segments, because of the relatively reduced pressure from the abdominal contents as compared to the lower segments. Similar physiological effects occur in the non-dependent (operated) lungs during periods of normal two-lung ventilation. The airways in the dependent ventilated lung may collapse during surgery, which usually occurs secondary to either FRC impinging on CC or obstruction by secretions.

Perfusion (Q) of the lungs also varies according to area:

a)

non-dependent (operated) lung – where perfusion continues but is reduced in comparison to normal because of the effects of hypoxic pulmonary vasoconstriction and surgical manipulation;

b)

dependent (non-operated and ventilated) lung – where perfusion is greater due to the effects of gravity. Certain areas of the dependent (ventilated) lung may, however, have reduced perfusion due to gravity (relative), regional collapse with subsequent hypoxic pulmonary vasoconstriction, and systemic hypotension.

Ventilation-perfusion (V/Q) matching varies with position, anaesthesia and type of ventilation. In a spontaneously ventilated patient, the changes with position include:

a)

upright position – where ventilation and perfusion are matched adequately throughout the lung with the bases receiving substantially more of both than the apices;

b)

supine position – where the posterior more dependent regions receive more ventilation and perfusion thereby providing matching.

Any improvement in V/Q matching augments gas exchange, thereby increasing oxygenation and facilitating CO₂ removal.

With sedation or anaesthesia in the absence of muscle paralysis, V/Q matching in the ventilated lungs changes, as anaesthesia decreases FRC by 15-30% (greater in the morbidly obese). If this causes FRC to encroach upon CC, hypoxaemia may result.

With anaesthesia utilising muscle paralysis and intermittent positive pressure ventilation (IPPV) of two lungs, the passive diaphragm is displaced preferentially in the non-dependent (upper) areas, where the resistance to movement by the abdominal contents is least. This is the area with the least effective perfusion. The diaphragm, however, is minimally displaced in the dependent (best perfused) portion, where the resistance to movement by the abdominal contents is greatest and V/Q mismatch is therefore increased, along with the potential for hypoxaemia.

8

What are the principles of pre-operative anaesthetic assessment of a thoracic surgical patient?

The pre-operative assessment aims to identify patients at increased risk of peri-operative complications and long-term disability.

It should include a thorough history and examination, in combination with the least invasive tests possible, to assess:

a)

airway – including obstruction of the upper or lower airways, which may be static (due to a mass) or dynamic (consequent upon bronchial reactivity). Symptoms, including stridor, wheeze, hoarseness and dyspnoea, indicate the potential of an increased risk of problems managing the airway;

b)

breathing – where a ppo FEV₁ or ppo DLCO <40% is associated with an increased risk of postoperative dyspnoea and mortality;

c)

circulation – where symptoms, including angina, exercise intolerance and syncope, may indicate the presence of ischaemic heart disease, valvular heart disease or arrhythmias. Patients with underlying cardiac disease have an increased peri-operative risk.

Cardiac complications, including myocardial ischaemia, occur in 3.4% of patients, with supraventricular arrhythmias being the most common postoperative complication.

Mortality for thoracic surgery is relatively high (3.1%) after lung cancer resection in Europe, with an increased rate in patients aged >70 years (lobectomy 4-7% and pneumonectomy 14%).

9

What are the principles of anaesthetic optimisation for a thoracic surgical patient?

Although any modifiable risk factors should be addressed pre-operatively, surgery should not be unnecessarily delayed if the primary disease is likely to progress in the interim (making surgery ineffective).

Particular attention should be paid to:

a)

smoking – which is a risk factor for postoperative pulmonary complications. Importantly, the duration of abstention from smoking does not affect complication rates. In particular, acute removal of the irritant effect of smoke (with inhibition of cough) does not increase complications, as has previously been described;

b)

reversible airways disease (including COPD or asthma) – where steroids may be required to reduce airway inflammation and narrowing;

c)

exercise – where an increased pre-operative exercise capacity is associated with improved pulmonary function. Any such improvement may also help decrease postoperative respiratory complications.

10

What are the principles of anaesthetic monitoring during thoracic surgery (Figure 4)?

An experienced anaesthetist must be present throughout general anaesthesia for thoracic surgery.

Monitoring during induction and maintenance of anaesthesia must always include:

a)

airway – end-tidal carbon dioxide (EtCO₂) and airway pressure;

b)

breathing – airway gas (including oxygen and anaesthetic vapours) and pulse oximetry;

c)

circulation – ECG, blood pressure (invasive or non-invasive) and pulse oximetry;

d)

nerve stimulator – whenever a muscle relaxant (paralysing agent) has been used;

e)

body temperature – as peri-operative hypothermia is associated with increased morbidity.

Additional monitoring that may be useful includes:

a)

arterial line – which can be used to assess blood pressure invasively and efficacy of gas exchange, particularly during OLV;

b)

central venous line (which should be inserted on the side of the surgery) – which can assess volume status and provide access to administer cardiovascular agents, if required;

c)

bispectral index (BIS) – which provides an assessment of the depth of anaesthesia. It is particularly helpful as thoracic surgery has a relatively high risk of awareness under anaesthesia, especially during rigid bronchoscopy;

d)

transoesophageal echocardiography (TOE) – which can be used to assess cardiac function.

11

What is one-lung ventilation (OLV)?

One-lung ventilation (OLV) by definition represents ventilation of only one lung and thereby allows the other lung to collapse. It can be achieved by a:

a)

double-lumen tube (DLT);

b)

bronchial blocker;

c)

single-lumen endotracheal tube.

The use of OLV facilitates pulmonary and other thoracic surgery but risks inadequate ventilation.

In patients with impaired underlying lung function, where OLV is poorly tolerated, it may be possible to selectively collapse one lobe of a lung with bronchial blockers, whilst allowing ventilation of the other lobes of the same lung.

12

What are the indications for OLV?

Absolute indications:

a)

to prevent contamination of an uncontaminated lung – including from blood or purulent secretions/infection;

b)

to control the distribution of ventilation – where one lung is severely compromised, including in patients with a bronchopleural fistula or bronchial disruption;

c)

to facilitate prolonged bronchopulmonary lavage;

d)

to allow video-assisted thoracoscopic surgery (VATS) procedures.

Relative indications – to facilitate surgical exposure:

a)

high priority – such as in patients undergoing a pneumonectomy, upper lobectomy or lung volume reduction surgery;

b)

lower priority – such as in patients undergoing a middle or lower lobectomy, sympathectomy, mediastinal mass resection or oesophageal surgery.

13

What are the principles of a double-lumen tube?

A double-lumen tube (DLT) represents two tubes bonded together side by side, with each lumen intended to ventilate one lung.

The DLTs may be right- or left-sided and include:

a)

Mallinckrodt DLTs (Figure 5);