Respiratory function

• Lung function tests should be able to predict the type and severity of lung disease
• Can predict risk of complications and postoperative mortality
• Tests fall in to three categories
  • Lung mechanics
  • Gas exchange
  • Control of breathing
• Useful radiological investigations include chest x-ray and high-resolution thoracic CT
• Arterial blood gases may be invaluable

Lung function tests

• Allows assessment of :
  • Lung volumes
  • Airway calibre
  • Gas transfer

Spirometry

• Lung volumes are assessed with spirometry
• Volumes measured include:
  • IC = Inspiratory capacity
  • IRV = Inspiratory reserve volume
  • TV = Tidal volume
  • VC = Vital capacity
  • FRC = Functional residual capacity
  • RV = Residual volume
  • ERV = Expiratory reserve volume
  • TLC = Total lung capacity

Peak flow rates

• Airway calibre can be assessed with peak flow measurements
• Requires co-operation and maximum voluntary effort of the patient
• Flow rates measured include
  • FVC = Forced vital capacity
  • FEV1 = Forced expiratory volume in one second
• Absolute values depend on height, weight, age, sex and race
• FEV1 / FVC ratio is important
• Lung function can be classified as:
  • Normal
  • Restrictive
  • Obstructive
• In restrictive lung disease FVC is reduced but FEV1/FVC is normal
• In obstructive lung disease FVC is normal or reduced and FEV1/FVC is reduced

Gas transfer

• Arterial blood gases are best measure available of gas transfer
• Also allow assessment of ventilation / perfusion mismatch
• Important parameters to measure are:
  • pH
  • Partial pressure of oxygen
  • Partial pressure of carbon dioxide
• Pulse oximetry gives an indirect estimate of gas transfer
• Technique is unreliable in the presence of other medical problems (e.g. anaemia)

Cardiac function

• Simple non-invasive and more complicated invasive tests of cardiac function exist
• Non-invasive
  • Chest x-ray
  • ECG
  • Echocardiography
  • Exercise test
• Invasive
  • Coronary angiography
  • Thallium scanning

Chest x-ray

• Routine chest x-ray is not recommended
• It is indicated in the presence of cardiorespiratory symptoms or signs
• Important signs associated with increased cardiac morbidity are:
  • Cardiomegaly
  • Pulmonary oedema 
  • Change in the cardiac outline characteristic of specific diseases

ECG

• Resting ECG is normal in 25-50% of patients with ischaemic heart disease
• Characteristic features of ischaemia or previous infarction may be present
• Exercise ECG provides a good indication of the degree of cardiac reserve
• 24-hour monitoring is useful in the detection and assessment of arrhythmias

Echocardiography

• Can be performed percutaneously or transoesophageal
• Two-dimensional echocardiography allows assessment of
  • Muscle mass
  • Ventricular function / ejection fraction
  • End-diastolic and end-systolic volumes
  • Valvular function
  • Segmental defects
• Doppler ultrasound allows assessment of valvular flow and pressure gradients

Nuclear medicine

• Myocardial scintigraphy allows assessment of myocardial perfusion
• Radiolabelled thallium is commonest isotope used
• Areas of ischaemia or infarction appear as 'cold' spots
• Vasodilators can be used to evaluate reversibility of ischaemia
• Radiolabelled albumin or red cells can be used to assess ejection fraction
• Such dynamic studies are performed 'gated' to the ECG

Renal function

• Glomerular filtration rate is the gold standard test of renal function
• Can be calculated by measuring creatinine clearance rate
• Requires 24-hour urine collection
• Serum creatinine allows a good estimate of renal function
• Use of serum creatinine may be inaccurate in patients with:
  • Obesity
  • Oedema
  • Pregnancy
  • Ascites

Bibliography

Cohn S L,  Goldman L.  Preoperative risk evaluation and perioperative management of patients with coronary artery disease. Med Clin North Am 2003;  87:  111-136

Doyle R.  Assessing and modifying the risk of postoperative pulmonary complications.  Chest 1999;  115 (Suppl 5);  S77-81.

Hollenberg S M.  Preoperative cardiac risk assessment.  Chest 1999;  115 (Suppl 5):  S51-57

Powell C A,  Caplan C E.  Pulmonary function tests in the preoperative pulmonary evaluation.  Clin Chest Med 2001;  22:  703-714.

Smetana G W.  Preoperative pulmonary assessment of the older patient.  Clin Geriatr Med 2003;  19:  35-55.