Tests of pulmonary function before thoracic surgery

THORACIC ANAESTHESIA

Tests of pulmonary function before thoracic surgery

Learning objectives After reading this article, you should be able to: C explain why tests of pulmonary function are necessary C list what tests are commonly performed C describe how common tests are performed C discuss how these tests are used to guide surgery and counsel patients

Neil G Britton Matthew Stagg

Abstract

ventilation and oxygenation whilst reduced dynamic volumes can impair ability to cough, clear secretions and therefore lead to further atelectasis and pneumonia.2 Preoperative pulmonary function testing allows: diagnosis and severity scoring of pulmonary comorbidities; potential optimization; counselling of patients about the risks of short-term pulmonary complications and long-term dyspnoea and thus informed consent. Tests of pulmonary function can be divided into four areas: respiratory mechanics, parenchymal function, cardiorespiratory reserve and anatomical. Together these four areas combine to enable predictions of postoperative function.

Respiratory function declines following surgery due to atelectasis. After thoracic surgery, there is an even greater decline due to resection and lung handling. Patients undergoing thoracic surgery often have concomitant respiratory disease and testing pulmonary function preoperatively allows: diagnosis and optimization of lung disease; counselling of patients accurately to obtain truly informed consent and guide the multidisciplinary team to the best operation. Tests of pulmonary function can be divided into tests of mechanical function, tests of parenchymal function, tests of cardiorespiratory reserve and function, and anatomical tests. When these tests are combined with knowledge of the lobes resected they allow predicted postoperative values to be calculated. Evidence-based guidelines show which investigations should be performed preoperatively and risk-stratify patients for postoperative dyspnoea, morbidity and mortality.

Tests of respiratory mechanics Gas must move in and out of alveoli for gas exchange to take place, with obstructive and restrictive disease processes limiting this function. Spirometry is a common, non-invasive and versatile test of mechanical functions and volumes of the lungs. A patient breathes via a mouth piece, with nose clipped, through the spirometer which records flows and volumes over time. Plotting of volume against time for a maximal forced expiration from total lung capacity to residual volume gives FEV1 (the volume expired in 1 second) and FVC (forced vital capacity) (Figure 1). Plotting of flow against volume for a complete cycle of maximal expiration and inspiration gives the flowevolume loop, which allows VC (vital capacity) and PEF (peak expiratory flow) to be derived as well as evaluating the degree of obstructive and restrictive disease and the presence of significant extra-thoracic obstruction (Figure 2). To obtain TLC (total lung capacity) a helium dilution test is used to interrogate the residual volume left after full expiration. Values obtained are reported as absolute values and as compared to values derived from healthy population studies and vary with age, race, weight and height. FEV1 and FVC should be more than 80% of that predicted, and the ratio of FEV1/FVC greater than 0.8. Arterial partial pressure of carbon dioxide (PaCO2) is inversely related to minute volume, and so may be used as a surrogate marker of adequacy of ventilation preoperatively. An elevated PaCO2 may indicate abnormal ventilation.

Keywords Cardiopulmonary exercise test; lung function tests; preoperative assessment; spirometry; thoracic surgery Royal College of Anaesthetists CPD Matrix: 3G00

Why do pulmonary function tests before thoracic surgery? Thoracic surgery remains high-risk surgery despite advances in anaesthetic and surgical technique (30-day mortality for lobectomy 2.3% and 5.8% for pneumonectomy1). Atelectasis is common after thoracic surgery and is thought to predispose to pneumonia and respiratory failure in the acute phase. Posttreatment dyspnoea can lead to significant morbidity, disability and decreased quality of life. Patients for thoracic surgery will generally have concomitant pulmonary disease or abnormal respiratory function, such that they would be at risk of postoperative complications from any anaesthetic or surgical insult. Thoracic surgery however necessitates the handling, and often resection of, lung tissue and a thoracotomy incision which causes a reduction in static and dynamic lung volumes as well as function. Often these reductions exceed the volume of lung resected. Reduction in static volumes and gas-exchange function may result in inadequate

Neil G Britton FRCA is an ST7 in Anaesthesia in the North West Deanery, UK. No conflicts of interest are declared.

Tests of parenchymal function

Matthew Stagg FRCA FFICM is a Consultant in Cardiothoracic Anaesthesia and Intensive care at the Lancashire Cardiac Centre, Blackpool, UK. No conflicts of interest are declared.

Gases must diffuse across the alveolar capillary membrane for normal function and parenchymal function tests assess and quantify performance of the lung at this process. Oxygen content

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involves breathing 0.3% carbon monoxide (CO) and measuring inspired and expired fraction of gases. In the commonly used single breath technique 0.3% CO with 10% helium is inhaled and held for 10 seconds. Helium does not diffuse and so the ratio of CO to helium allows calculation of DLCO and total lung capacity. DLCO is expressed as the amount of CO diffusing per minute per unit of pressure (mmol/minute/kPa). DLCO is decreased by diseases that impair the alveolar capillary membrane, such as pulmonary fibrosis, but also by diseases or surgeries that decrease total lung area. To compensate for this the transfer coefficient (KCO) is sometimes given, which is corrected for lung volume. KCO is expressed as the amount of CO diffusing per minute per unit of pressure per unit volume (mmol/minute/kPa/ litre). DLCO is given as absolute values and as a percentage predicted, based on age, gender, height, exercise and altitude. For preoperative assessment DLCO is a better tool than KCO.

Volume (litres)

FVC FEV1

1 second

Time

Tests of cardiorespiratory reserve and function

FEV1, forced expiratory volume in 1 second; FVC, forced vital capacity.

Recovery from surgery requires an elevated oxygen delivery from approximately 110 ml/m2 to approximately 170 ml/m2 whilst oxygen extraction falls, thus cardiac output may need to increase 2.5-fold for the postoperative period.3 Tests of cardiorespiratory function attempt to predict a patient’s ability to elevate their metabolism, cardiac output, and VO2 for a prolonged period without requiring anaerobic respiration and thus attempt to predict a patient’s risk of postoperative morbidity and mortality.

Figure 1 Forced vital capacity curve.

Flow rate (litres/minute)

PEF

Cardiorespiratory exercise testing Cardiorespiratory exercise testing (CPET)3 is a non-invasive test whereby a patient uses a bicycle ergometer (arm-ergometers are available for those unable to cycle) to gradually increase work done whilst CO2 and O2 are measured on a breath-by-breath basis. SpO2, non-invasive blood-pressure and ECG are recorded. The test may be terminated by the patient due to exhaustion, breathlessness or chest pain. Or terminated by the operator if ST changes or significant arrhythmias are detected, or all values required are obtained. VO2Peak is the maximal rate of oxygen utilised (VO2) during the test, measured as ml/minute/ kg. Except in trained and motivated individuals VO2Peak is limited by motivation rather than a true physiological limitation of O2 uptake, which is known as the maximal uptake of O2 (VO2Max). Anaerobic threshold (AT), which is the VO2 at a workload where demand for O2 exceeds supply, is not motivation dependent and is easily achieved since it occurs at around 50% of VO2Max. It may, therefore, be a better test value to compare between individuals. However, the majority of studies for thoracic surgery, and thus guidelines, have used VO2Peak. Values of AT less than 11 ml/minute/kg and VO2Peak less than 15 ml/minute/kg are considered high risk for thoracic surgery. Whilst values of VO2Peak greater than 20 ml/minute/kg or over 75% predicted are necessary for pneumonectomy and less than 10 ml/minute/kg or under 35% predicted precludes any resection.

VC

Volume (litres)

VC, vital capacity. Figure 2 Flow-volume loop during inspiration and expiration.

of blood is dependent on this process (to a much greater degree than carbon dioxide) and so low saturation of arterial oxygen (SaO2) or partial pressure of oxygen (PaO2) will reflect poor parenchymal function. However, the relationship is difficult to quantify and there may be significant reduction in parenchymal function before reduction in SaO2 or PaO2. Diffusion (or total) capacity of the lung for carbon monoxide (DLCO/TLCO) is a more invasive but quantifiable test of alveolar capillary membrane function. Carbon monoxide (CO) is used because haemoglobin has over 400 times the affinity for CO as oxygen and so, at low inspired concentrations, CO uptake is diffusion limited. Different methods are possible but each

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Other tests Attempts have been made to use simpler tests to measure fitness. The Six-minute Walk Test (6MWT) involves recording how far a

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THORACIC ANAESTHESIA

patient can walk on flat ground at their own pace in 6 minutes. A saturation probe is worn and the test terminated if desaturation occurs. A 6MWT of greater than 600 m predicts low risk and the test correlates well with CPET. The Shuttle Walk Test (SWT) has a standardized flat 10-m course which the patient repeatedly walks at increasing pace, as set by a protocol. When breathlessness or failure to maintain the required pace occurs the test is terminated. The distance walked or shuttles completed is recorded. The ability to walk more than 400 m correlates with low risk from CPET. Stair climbing is often used as a measure of fitness, but there is no standardization regarding height of steps, number of steps or test protocol and so it is difficult to compare studies. Inability to complete one flight of stairs, however, indicates poor fitness and increased risk. In general these simple tests may be best used to identify patients who are low risk and do not need to complete formal CPET.

Predicting postoperative values Thoracic surgery, unlike other surgeries, often involves a planned reduction in lung volume and therefore lung function. Greater resection will result in greater reduction in postoperative lung function. Predicted postoperative (ppo) values for lung function can be calculated. Normal lungs have 19 segments in total (ten right, nine left). The number of functional lung segments (T) is measured using CT or V/Q scans, and the postoperative residual lung segments (R) calculated as: R ¼ T e functional segments resected ppo value ¼

Preoperative value R T

Using pulmonary function tests in thoracic surgery The National Institute for Health and Clinical Excellence (NICE),4 British Thoracic Society (BTS) and Society of Cardiothoracic Surgery (SCTS)1 have all published evidence-based guidelines on the use of investigations, patient selection and patient counselling for lung cancer surgery. The NICE guidelines focus on which investigations should be performed. If a patient has no shortness of breath on exertion (SOBOE) or interstitial lung disease (ILD) and an FEV1 greater than 80% predicted and 2 litres then no further investigation is recommended. SOBOE or ILD necessitates measurement of DLCO and ppoDLCO. FEV1 less than 80% predicted or 2 litres necessitates calculation of ppoFEV1 and performing CPET to allow prediction of postoperative lung function and risk of long-term morbidity.

Anatomical tests The majority of patients having thoracic surgery for cancer will have had CT scans, and may have had ventilation/perfusion (V/ Q) scans and positron emission tomography CT (PET-CT). This allows identification and quantification of parenchymal disease, emphysematous disease and areas of obstructed and nonfunctional lung. In brief this is important since resection of non-functional or proportionally less-functional areas of lung will not affect lung function as adversely as that of fully functional lung.

SCTS, BTS and NICE guidelines for predicting risk of postoperative dyspnoea SCTS/BTS guidelines

NICE guidelines

Yes DLCO

FEV1 and DLCO SOBOE or ILD? LOW RISK ppoFEV1 and DLCO ≥40%

MODERATE TO HIGH RISK ppoFEV1 and DLCO <40%

No FEV1 <80% or <2 litres

CPET

MODERATE RISK VO2Peak >15 ml/minute/kg

Yes

ppoVal calculated and CPET

No

HIGH RISK VO2Peak <15 ml/minute/kg

No further investigations

SCTS, Society of Cardiothoracic Surgery; BTS, British Thoracic Society; FEV1, forced expiratory volume in 1 second; DLCO, diffusion capacity for carbon monoxide; VO2Peak, peak oxygen consumption; ppo, predicted postoperative; CPET, cardiopulmonary exercise testing; ILD, interstitial lung disease; SOBOE, shortness of breath on exercise.

Figure 3

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FEV1 values required for pneumonectomy are at least 80% predicted or more than 2 litres, ppoFEV1 over 800 ml or at least 40% predicted. For lobectomy FEV1 more than 1 litre or over 40% predicted is sufficient. Patients with a ppoFEV1 less than 0.8 litres or under 40% predicted are generally considered very high risk for resection surgery, thus those with a FEV1 under 1 litre need significant counselling before even minimal resection surgery. Those with a ppoFEV1 lower than 30% should be considered for non-operative and non-standard operative techniques. For pneumonectomy, a DLCO of at least 80% predicted and ppoDLCO of at least 40% predicted is required. Patients with ppoDLCO under 40% have high risk of perioperative death or morbidity and should be thoroughly investigated and counselled. Values for required VO2Peak for pneumonectomy differ between American5 and British1 guidelines: over 20 ml/minute/kg or over 15 ml/minute/kg, respectively. It is generally accepted that a VO2Peak less than 10 ml/minute/kg predicts a very high risk of perioperative mortality after any surgery and therefore these patients are considered unsafe for any resection surgery. Patients with non-malignant, emphysematous disease may have a postoperative improvement of respiratory function after resection of poorly functioning lung segments and so those with ppoFEV1 and ppoDLCO greater than 20% predicted may be considered for surgery. A

The BTS/SCTS guidelines predict the risk of postoperative dyspnoea (POD). Patients with a ppoFEV1 and ppoDLCO of greater than 40% predicted are low risk for developing POD. Patients with a ppoFEV1 and DLCO less than 40% predicted are moderate to high risk of POD and require a CPET. A VO2Peak below 15 ml/minute/kg predicts high risk of POD and long-term ventilator dependency, whilst a patient with VO2Peak over 15 ml/ minute/kg but ppoFEV1 and ppoDLCO less than 40% predicted have moderate risk of POD. These guidelines are summarized in Figure 3. From a composite of NICE guidelines,4 BTS/SCTS guidelines1 and American College of Chest Physicians5 suggested lung function criteria for resection surgery can be synthesized.6 The decision for pneumonectomy, lobectomy or wedge resection must be based upon probability of curative success but also minimizing risk of decreasing postoperative quality of life (Table 1).

Lung function criteria for thoracic surgical procedures Operation/resection

Suggested lung function criteria

For pneumonectomy

FEV1 >2 litres or >80% DLCO >80% VO2Peak >75 % or >20 ml/minute/kg ppoFEV1 >40% or >800 ml ppoDLCO >40% FEV1 1e1.5 litres or >40% predicted FEV1 >0.6 litres FEV1 <1 litre VO2Peak <35% or <10 ml/minute/kg ppoFEV1 <30% or <800 ml ppoDLCO <30% PaCO2 >6 kPa SaO2 <90% on room air

For lobectomy For Wedge resection Unsafe for resection

REFERENCES 1 Lim E, Baldwin D, Beckles M, et al. Guidelines on the radical management of patients with 76 lung cancer. Thorax 2010; 65: iii1e27. 2 Weissman C. Pulmonary function after cardiac and thoracic surgery. Anesth Analg 1999; 88: 1272e9. 3 Agnew N. Preoperative cardiopulmonary exercise testing. Contin Educ Anaesth Crit Care Pain 2010; 10: 33e7. 4 NICE Clinical Guideline. Lung cancer: diagnosis and management, 2011. 5 Brunelli A, Kim AW, Berger KI, Addrizzo-Harris DJ. Physiologic evaluation of the patient with lung cancer being considered for resectional surgery: diagnosis and management of lung cancer, 3rd ed: American college of chest physicians evidence-based clinical practice guidelines. Chest 2013; 143. 6 Davies AN, Saravanan P. Tests of pulmonary function before thoracic surgery. Anaesth Intensiv Care Med 2014; 15: 495e8.

FEV1, forced expiratory volume in 1 second; DLCO, diffusion capacity for carbon monoxide; VO2Peak, peak oxygen consumption; ppo, predicted postoperative; PaCO2, partial pressure of arterial CO2; SaO2, arterial oxygen saturation. Adapted from Ref.6

Table 1

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