# 17.4 Assessing and preparing patients with medical

# 17.4 Assessing and preparing patients with medical conditions for major surgery 3860 Tom Abbott and Rupert Pearse

ESSENTIALS
The assessment of patients before surgery is complex. However, 
since surgery is offered to increasing numbers of patients with mul-
tiple comorbidities, the demand for comprehensive preoperative 
assessment is expected to increase. Perioperative medicine provides 
a patient-​centred approach from preoperative assessment through 
to hospital discharge and beyond.
Preoperative assessment serves to identify comorbidity that may 
require optimization before surgery, plan perioperative care, identify 
a need for a non​standard anaesthetic technique, assess functional 
reserve, brief patients on the perioperative care pathway, and provide 
patients with an opportunity to have questions answered.
Patients with active cardiac or respiratory conditions are among  
those most likely to benefit from preoperative optimization. 
Smoking cessation reduces the incidence of postoperative pul-
monary complications. During surgery, goal-​directed haemo-
dynamic therapy is used to optimize cardiovascular performance. 
It is unclear what practical steps can be taken to prevent illness 
in the immediate postoperative period, over and above increased 
vigilance.
There are a variety of tools for preoperative assessment and rec-
ognized approaches to managing patients with existing chronic 
disease during the perioperative period, but the absence of robust 
evidence to favour any particular clinical approach is striking.
Introduction
Surgical treatments are now offered to more and more patients. 
This due to growing global population, increased healthcare cap-
acity, and because improvements in surgical and anaesthetic care 
mean that patients considered too high-​risk only a few years ago 
are now routinely offered surgery. The result is a shifting demo-
graphic of the surgical population toward older patients with 
multiple comorbidities. Modern perioperative care therefore re-
quires a robust and considered approach to each stage of a patient’s 
journey and a multidisciplinary approach best termed periopera-
tive medicine. Here, we review current approaches for assessing 
and preparing patients for surgery.
Perioperative morbidity and mortality:   
A global perspective
An estimated 300 million surgical procedures are performed world-
wide each year, which is greater than the worldwide prevalence of 
malaria (c.200 million cases). However, little is known about the 
global incidence of postoperative death and disability. Accepted es-
timates of perioperative mortality in developed countries range be-
tween 0.5 to 2.0% of surgical procedures, although one estimate was 
as high as 4% for non​cardiac surgery. In contrast, data from small 
studies suggest the postoperative mortality rate may be as high as 
5–​10% in resource-​poor countries. The most common causes of 
death within two days of surgery are cardiovascular in origin, fol-
lowed by sepsis and multiorgan failure. Death is more common fol-
lowing emergency compared to elective surgery. However, different 
population demographics and public health profiles make accurate 
comparisons between developing and developed countries complex.
Non​fatal complications following surgical procedures are 
common. These range from wound pain and immobility to more 
serious conditions, including pneumonia, surgical site infection, 
myocardial infarction, acute kidney injury, and stroke. The presence 
of any postoperative complication is associated with reduced long-​
term survival. In developed countries, estimates of the proportion of 
patients with complications or adverse events following surgery vary 
between 3 to 20% of procedures (Table 17.4.1) with the incidence 
affected by patient factors, the nature of the surgical procedure, and 
the quality of the care the patient receives.
The ‘high-​risk’ surgical patient
Epidemiological studies have described a high-​risk group of patients 
that account for 80% of postoperative deaths, but represent only 15% 
of patients having major surgery. The characteristics of this high-​risk 
group are not well defined, but the typical patient is older, with sig-
nificant comorbid disease, and often undergoes emergency surgery. 
The risk of postoperative complications is influenced by multiple 
factors, which can be divided into two broad categories: procedure-​
related factors and patient-​related factors. Procedure-​related factors 
include surgical technique (e.g. open or laparoscopic), duration, 
17.4
Assessing and preparing patients 
with medical conditions for major surgery
Tom Abbott and Rupert Pearse


17.4  Assessing and preparing patients 
3861
and type of surgery, intraoperative complications (e.g. blood loss), 
procedure-​specific postoperative complications (e.g. anastomotic 
leak after bowel resection), and—​much less frequently—​surgical 
or anaesthetic error. The influence of these factors is compounded 
by abnormal physiology during emergency procedures, resulting 
in worse outcomes. However, most postoperative complications are 
likely to occur, not as a direct result of the surgical procedure, but 
as a secondary consequence of indirect factors such as immobility, 
tissue inflammation, or hospital-​acquired infection.
Patient-​related factors refer to an individual’s state of general 
physical health or functional capacity. This is usually considered in 
the context of the ability to withstand the physiological stress as-
sociated with surgery, such as elevated cardiac output or minute 
ventilation, fluid shifts between body compartments, the effects of 
mechanical ventilation, or inflammation associated with tissue in-
jury. The resultant increase in cellular respiration increases the total 
body oxygen requirement. Inadequate functional capacity in one or 
more organ systems can manifest as an inability to adequately in-
crease oxygen transport to the body tissues during surgery, which is 
thought to be causally associated with postoperative complications 
and death. Functional capacity is influenced by age, cardiorespira-
tory fitness, and comorbidities, but cannot be reduced to a list of 
chronic diseases or measurement of any one variable. Inter-​relation 
of functional reserve, comorbidity, and procedural factors is com-
plex (Fig. 17.4.1). There is considerable overlap between functional 
reserve and comorbidity because physical illness affects cardio-
respiratory fitness. For example, patients with congestive cardiac 
failure, or those receiving cytotoxic chemotherapy, often experience 
a reduction in exercise performance. Equally comorbid disease can 
directly influence a surgical procedure, mandating a specific surgical 
or anaesthetic technique.
Preoperative assessment and risk stratification
Ideally, the anaesthetic assessment will be performed days or weeks 
in advance of surgery to allow time for planning of perioperative care. 
However, in many healthcare systems, assessment on the morning of 
surgery is the routine. This often leaves clinical staff with the diffi-
cult decision to either proceed with surgery or postpone surgery at 
the last minute to address the management of chronic disease. The 
preoperative assessment clinic, led by nurses and doctors, provides 
an opportunity to evaluate the patient in the days or weeks before a 
planned surgical procedure and formulate a definitive plan for peri-
operative care. Preoperative assessment serves several purposes:
•	 To identify comorbidity that may require optimization before 
surgery
•	 To plan perioperative care
•	 To identify a need for a non​standard anaesthetic technique
•	 To assess functional reserve
•	 To brief patients on the perioperative care pathway and provide an 
opportunity to have questions answered
A  typical appointment involves a medical history and physical 
examination including:  resting observations, height, and weight; 
blood tests such as routine haematology, biochemistry, and coagu-
lation tests; urine analysis and electrocardiography. Further inves-
tigations, for example, chest X-​ray, spirometry, echocardiography, 
or exercise testing, or referral to a specialist, are performed on a 
case-​by-​case basis. Some centres use standard criteria to trigger a 
review by an anaesthetist or referral to other specialists such as a 
cardiologist. It seems intuitive that patients attending a preoperative 
assessment clinic are less likely to have their procedure postponed 
on the day of surgery. However, growing evidence suggests that at-
tending a preoperative assessment clinic could also result in lower 
postoperative mortality, reduced hospital and high-​dependency 
unit (HDU) length of stay, and fewer unplanned critical care unit 
admissions. Unfortunately, the provision of preoperative assessment 
clinics is not universal and 20% of patients undergoing high-​risk 
surgery in the United Kingdom do not attend a preoperative assess-
ment clinic. The global availability and use of preoperative assess-
ment clinics is unknown.
Opinion leaders suggest that all patients undergoing major surgery 
should be reviewed by an experienced anaesthetist in a preoperative 
Table 17.4.1  Approximate incidence of common postoperative 
complications following elective non​cardiac surgery
Complication
Incidence (%)
All complications
15%
Infectious complications
Sepsis
5%
Superficial surgical site
3%
Pneumonia
2%
Urinary tract
2%
Deep surgical site
1%
Body cavity
<1%
Cardiovascular complications
Myocardial injury
8%
Myocardial infarction
3%
Arrhythmia
3%
Heart failure
2%
Pulmonary embolism
<1%
Stroke
<1%
Cardiac arrest
<1%
Other complications
Postoperative bleeding
3%
Acute kidney injury
2%
Acute respiratory distress syndrome
<1%
Gastrointestinal bleed
<1%
Procedure-related factors
Functional reserve
Co-morbidity
Fig. 17.4.1  The three principle components of perioperative risk.


Section 17  Critical care medicine
3862
assessment clinic. This provides an opportunity to evaluate the risk 
of postoperative morbidity or mortality. Risk estimates can be used 
to identify patients who are likely to require enhanced perioperative 
care, to determine whether a patient requires critical care immedi-
ately following surgery, and to provide patients with important de-
tail during the informed consent process. The optimal approach to 
estimating the risk of perioperative morbidity or mortality remains 
uncertain. Potential methods include risk stratification scores or in-
dices, objective assessment of functional capacity through exercise, 
and plasma biomarkers. Similar approaches are used in audit and re-
search to adjust for individual patient risk factors and facilitate com-
parisons between different patients or institutions. However, the 
most commonly used risk-​adjustment models use a combination of 
preoperative and intraoperative factors, which limit their use for risk 
prediction before surgery.
Risk stratification tools
Clinical prediction rules are ubiquitous to medical practice and 
comprise main five types:  scoring systems, prediction models, 
nomograms, decision trees, and neural networks. The underlying 
principle is to identify factors associated with a particular outcome 
and to calculate the probability of that outcome occurring given the 
presence of one or more predictors. Scoring systems tend to feature a 
concise group of predictors, weighted according to their association 
with the outcome measure. The total score indicates a risk category, 
but does not provide an absolute measure of risk. More complex risk 
prediction models feature a larger number of variables and often 
provide a numerical estimate of risk. However, due to the amount of 
information required, the latter can be cumbersome to use.
A variety of surgical scoring systems and risk prediction models 
are available for general and specific surgical populations and out-
come measures. However, most of these have only been validated in 
single-​centre studies or in specific patient groups. Four preoperative 
risk stratification tools have been validated in multiple centres: the 
American Society of Anesthesiologists’ Physical Status Score (ASA-​
PS), the Surgical Risk Score, the Surgical Risk Scale, and the Charlson 
Comorbidity Index. ASA-​PS is a simple and commonly used risk 
assessment tool, whereby a patient is placed into one of five groups 
according the presence and severity of comorbid disease. However, 
it has been criticized due to reportedly low inter-​rater reliability. The 
Surgical Risk Scale is favoured because it is concise, easy to use, and 
comprises only preoperative variables. It is a composite score de-
rived from the ASA-​PS, urgency of surgery and the grade of surgery 
according to the British United Provident Association (BUPA) clas-
sification (Table 17.4.2). However, it has only a moderate predictive 
accuracy. While it is simple is to use and could be easily integrated 
into clinical practice, it has been criticized for including ASA-​PS, 
which requires a subjective assessment by the clinician. The Charlson 
Comorbidity Index, which is more objective, has a poor predictive 
accuracy. The Revised Cardiac Risk Index is one of the best models 
for predicting postoperative cardiac complications. It consists of six 
components: high-​risk type of surgery, history of ischaemic heart 
disease, history of congestive cardiac failure, history of cerebrovas-
cular disease, preoperative insulin usage, and preoperative serum 
creatinine levels of more than 2.0 mg/​dl (>177 μmol/​litre). Other 
risk stratification models used for risk-​adjustment have a higher 
predictive accuracy, for example, the P-​POSSUM. However, these 
models use intraoperative and postoperative variables, so they do 
not have a preoperative application. Despite their utility, preopera-
tive risk stratification tools are not widely used.
Assessing functional capacity
For many years, anaesthetists and surgeons have subjectively as-
sessed their patients’ functional capacity to provide an indication of 
cardiorespiratory reserve. There are also more objective methods to 
evaluate functional capacity. Metabolic equivalents (METs) provide 
a semi-​quantitative measure of exercise tolerance. One MET repre-
sents resting oxygen consumption of c.3.5 ml/​kg/​min when sitting. 
Nomograms and tables provide a list of day-​to-​day activities with 
corresponding average METS, which allow the clinician to estimate 
functional capacity. A more reliable method is to use a patient ques-
tionnaire, for example, the Duke Activity Status Index (DASI), which 
is a set of 12 standardized questions relating to a range of activity 
levels. The total score correlates with maximal oxygen consump-
tion measured during exercise testing in surgical and non​surgical 
cohorts. However, the ability of the index to predict postoperative 
mortality or morbidity is still being evaluated.
Perhaps the most robust method for assessing functional cap-
acity is cardiopulmonary exercise testing (CPET). Most commonly 
a cycle ergometer operating an incremental ramp protocol is used, 
whereby the workload increases along a fixed gradient according to 
predicted exercise tolerance. The aim is for the patient to reach peak 
exertion within 8–​12 minutes. Non​invasive haemodynamic and gas 
exchange measurements are recorded continuously, which allows 
multiple parameters, with variable predictive accuracies, to be de-
rived. The two most widely used cardiopulmonary exercise testing-​
derived variables are peak oxygen consumption (VO2peak) and 
Table 17.4.2  Surgical Risk Scale, incorporating American Society 
of Anesthesiologists’ Physical Status Score (ASA-​PS)
Category
Description/​Example surgery
Score
Urgency of surgery
Elective
Routine booked non​urgent case
1
Scheduled
Booked admission
2
Urgent
Case requiring treatment within 24–​48 hours 
of admission
3
Emergency
Case requiring immediate treatment
4
Grade of surgery
Minor
Removal of sebaceous cyst, skin lesion, 
oesophagogastric duodenoscopy
1
Intermediate
Unilateral varicose vein, unilateral hernia 
repair, colonoscopy
2
Major
Appendicectomy, open cholecystectomy
3
Major plus
Gastrectomy, colectomy
4
Complex major
Carotid endartarectomy, AAA repair, limb 
salvage, anterior resection, oesophagectomy
5
ASA-​PS
I
No systemic disease
1
II
Mild systemic disease
2
III
Systemic disease affecting activity
3
IV
Serious disease but not moribund
4
V
Moribund, unlikely to survive
5
AAA, abdominal aortic aneurysm.


17.4  Assessing and preparing patients 
3863
oxygen consumption at the anaerobic threshold (VO2AT), the point 
where the metabolism switches from predominantly aerobic res-
piration to predominantly anaerobic respiration (Fig. 17.4.2). Most 
evidence supporting preoperative cardiopulmonary exercise testing 
comes from single-​centre studies of restricted cohorts of surgical 
patients. In only a handful of the studies were clinicians blinded 
to the result of the cardiopulmonary exercise testing, adding fur-
ther potential for bias. Growing evidence suggests that VO2peak may 
be the best cardiopulmonary exercise testing-​derived predictor of 
surgical outcome. However, VO2AT is probably the most commonly 
used. Cardiopulmonary exercise testing is becoming increasingly 
popular as an objective method of preoperative assessment in some 
countries. In the United Kingdom, approximately half of hospitals 
have access to it.
Biochemical markers of risk
Biochemical markers are a core feature of modern medical practice. 
Familiar examples include creatinine as a marker of renal function 
and cardiac troponin as a marker of cardiac injury. In the context of 
surgery, biochemical markers are used before, during, and after pro-
cedures to identify and categorize disease, track clinical progress and 
response to treatment, and to aid prognostication. However, there is 
growing interest in their use to predict short-​ and long-​term surgical 
outcomes. Current evidence is mainly restricted to morbidity and 
mortality associated with perioperative cardiac and renal disease 
and features a limited number of candidate molecules.
Brain natriuretic peptide (BNP) is most commonly used to aid 
diagnosis and prognosis in patients with heart failure, but it is be-
coming increasingly clear that increased preoperative BNP is as-
sociated with postoperative mortality and non​fatal myocardial 
infarction. When combined with the Revised Cardiac Risk Index, 
preoperative BNP improves the accuracy of risk prediction com-
pared with the risk index alone. Other candidate biochemical 
markers are cardiac troponin and cystatin-​c. Troponin is elevated 
before 10% of cardiac surgical procedures and may be associated 
with postoperative cardiac complications, although this has not 
been widely studied in patients undergoing non​cardiac surgery. 
Preoperative cystatin-​c may be associated with kidney injury after 
surgery, but needs further investigation.
One in ten patients experience myocardial injury after non-​
cardiac surgery, defined by a transient increase in serum troponin 
concentration. This is associated with mortality, the risk of which 
increases with the magnitude of troponin release. Most cases of 
postoperative myocardial injury are asymptomatic and there is 
increasing awareness that traditional tests of myocardial ischaemia 
and infarction do not identify the bulk of these. Postoperative brain 
natriuretic peptide and C-​reactive protein are both associated with 
mortality and adverse cardiac events.
Gas exchange
AT
0:00
2:00
4:00
6:00
8:00
10:00
12:00
Time (min)
14:00
16:00
Increasing workload
VO2 and VCO2
18:00
20:00
22:00
Slope (ml/min / W) = 10.0
Fig. 17.4.2  Gas exchange during a cardiopulmonary exercise test. Blue line—​VO2, red line—​VCO2. The anaerobic threshold (AT) is indicated.


Section 17  Critical care medicine
3864
Perioperative medicine: managing the high-​risk 
surgical patient
The supposition that most postoperative complications arise either 
during surgery or in the immediate postoperative period seems 
intuitive. Anaesthesia, mechanical ventilation, and surgical ma-
nipulation are physiologically abnormal, hence it is not surprising 
that postoperative complications occur. Patients undergoing 
high-​risk procedures or those with existing medical conditions 
are obvious candidates for perioperative interventions aimed at 
improving outcomes after surgery. The goal of perioperative medi-
cine is to facilitate surgery and minimize associated morbidity and 
mortality—​this has considerations before, during, and after sur-
gery (Fig. 17.4.3).
Before surgery
Patients with active cardiac or respiratory conditions are among 
those most likely to benefit from preoperative optimization 
(Fig. 17.4.4). Suspected cardiac failure or valvular disease can be 
investigated with echocardiography. Hypertension is often treated 
if the systolic or diastolic pressures are greater than 180 mm Hg 
or 110 mm Hg, respectively. New guidelines suggest that patients 
referred for elective surgery should have a blood pressure of less 
than 160/100 mm Hg recorded in primary care. Gradual reduc-
tion in blood pressure before surgery is preferable to rapid control 
using intravenous agents. Some patients with unstable coronary 
artery disease may require procedural intervention before sur-
gery. However, prophylactic preoperative revascularization may 
not reduce postoperative cardiac events but will delay surgery. 
Pharmacotherapy to reduce postoperative cardiac events is also 
controversial. Until recently, perioperative blockers were recom-
mended for patients with cardiovascular risk factors. However, 
while β-blockade reduces rates of postoperative myocardial infarc-
tion, it increases the risk of stroke, hypotension, and death. These 
agents are now only recommended for patients at intermediate or 
high-​risk of myocardial ischaemia and when prescribed should be 
started far enough in advance of planned surgery to allow safety 
and tolerability to be assessed prior to surgery. Other negatively 
chronotropic agents like ivabradine may represent an alternative, 
but this has yet to be investigated. Patients undergoing vascular 
surgery may benefit from risk factor modification with statins. 
Similarly, patients taking aspirin often continue to do so, when 
the risk of cardiovascular complications if aspirin is withheld out-
weighs the risk of bleeding if aspirin is continued. In this respect, 
patients with coronary artery stents require special consideration.
Patients with existing respiratory disease are at increased risk of 
postoperative pulmonary complications and may benefit from en-
hanced perioperative care. This is particularly important for patients 
with chronic obstructive pulmonary disease (COPD), poorly con-
trolled chronic respiratory conditions, or obesity-​related disease such 
as obstructive sleep apnoea. The mainstay of preoperative manage-
ment is optimization of existing treatments and considering a lower 
threshold for postponing surgery in the event of an exacerbation. In 
addition, several preoperative respiratory interventions are gaining 
widespread support. Smoking cessation reduces the incidence of 
postoperative pulmonary complications, although the optimum 
duration of abstinence before surgery is unclear. Respiratory physio-
therapy in the immediate postoperative period is widely adopted 
as prophylaxis against respiratory complications, and physio-
therapy before surgery may provide added benefit. Similarly, there 
is growing evidence that increasing preoperative fitness through ex-
ercise training can improve surgical outcomes, but more research is 
needed to better to define the optimum target population and exer-
cise regimen.
During surgery
It is widely accepted that tissue oxygenation may become im-
paired during surgery due to the effects of tissue injury, inflam-
mation, and the sympathetic response to surgical stimulation. 
Many clinicians use intraoperative goal-​directed haemodynamic 
Before surgery
During surgery
After surgery
Home
Risk assessment
Procedure-related factors
Functional capacity
Co-morbidity
Biomarkers
Optimization
Optimising existing treatments
Starting new treatment where appropriate
Consider goal-directed therapy
Surveillance
Consider screening for asymptomatic
disease
Reduce long-term harm
Routine follow-up
Identify perioperative complications
Fig. 17.4.3  Flow diagram of the perioperative care pathway.


17.4  Assessing and preparing patients 
3865
therapy to optimize cardiovascular performance, thus improving 
cellular respiration and tissue oxygenation with the aim of 
improving perioperative outcome. The most common method 
of achieving this is with intravenous fluid therapy and inotropic 
agents guided by cardiac output monitoring. Meta-​data sug-
gest that goal-​directed therapy might be associated with lower 
rates of postoperative complication, but further evidence from 
multicentre trials is needed.
After surgery
The genesis and mechanism of most postoperative complications 
are poorly understood. Major surgery triggers a systemic inflam-
matory response similar to that which follows sepsis or major 
trauma, although it is usually less severe. New evidence suggests 
that surgery can suppress the immune response for several days, 
indicating a period of potentially increased vulnerability to noso-
comial infection. Furthermore, most postoperative myocardial in-
jury, detected by raised cardiac troponins, occurs in the first 24 
hours after surgery. However, it is unclear what practical steps can 
be taken to prevent illness in the immediate postoperative period, 
over and above increased vigilance. In the case of myocardial in-
jury, routine troponin sampling is a potential surveillance option, 
but little is known about the aetiology of postoperative myocardial 
injury and myocardial infarction, and there are no proven treat-
ment strategies. Further research is needed to define the clinical 
approach to prevention and treatment of postoperative cardiac 
complications.
It is also possible that extending intraoperative treatment strat-
egies, for example, goal-​directed therapy and high intensity nursing, 
into the early postoperative period may improve patients’ outcomes. 
This already occurs in some centres operating postanaesthesia care 
units for patients that are not transferred to the critical care unit 
immediately after surgery. Given the clear association between 
postoperative morbidity and subsequent mortality, enhanced sur-
veillance of patients for postoperative complications after hospital 
discharge may be prudent. However, there is no defined protocol or 
pathway for postoperative surveillance over and above existing sur-
gical outpatient follow-​up.
FURTHER READING
Abbott, et al. (2017). Frequency of surgical treatment and related hos-
pital procedures in the UK: a national ecological study using hos-
pital episode statistics. Br J Anaesth, 119, 249–57.
Fleisher LA, et al. (2014). 2014 ACC/​AHA guideline on perioperative 
cardiovascular evaluation and management of patients undergoing 
General haemodynamic measures
1.
5% dextrose at 1 ml/kg/hr
2.
Transfuse blood to maintain haemoglobin >8 g/dl
3.
Clinician retains discretion to adjust therapy if concerned about risks of 
hypovolaemia or fluid overload
4.
Mean arterial pressure 60–100 mmHg; SpO2 ≥ 94%; core temperature 37°C; 
heart rate <100 bpm
Administering fluid to a stroke volume end point
1.
250 ml colloid boluses to achieve a maximal value of stroke volume
[Note: Start dopexamine after first fluid challenge –see below]
2.
Fluid challenges should not be continued in patients who are not fluid
responsive in terms of a stroke volume increase
3.
Fluid responsiveness is defined as a stroke volume increase ≥10%
4.
If stroke volume decreases further fluid challenge(s) are indicated
5.
Persistent stroke volume responsiveness suggests continued fluid loss
Dopexamine
1. Start dopexamine infusion at fixed rate of 0.5 µg/kg/min after first colloid fluid
challenge
2. Halve dose if heart rate rises to the greater of: (a) >120% of baseline value, or 
(b) >100 bpm for more than 30 minutes. 
3. Stop dopexamine if tachycardia persists
Example intraoperative optimization algorithm (OPTIMISE Trial)
Fig. 17.4.4  Intraoperative goal-​directed therapy algorithm from the OPTIMISE Trial.


Section 17  Critical care medicine
3866
noncardiac surgery: a report of the American College of Cardiology/​
American Heart Association Task Force on Practice Guidelines. 
Circulation, 130, e278–​333.
International Surgical Outcomes Study Group (2016). Global pa-
tient outcomes after elective surgery: prospective cohort study in 
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Khuri SF, et al. (2005). Determinants of long-​term survival after major 
surgery and the adverse effect of postoperative complications. Ann 
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Moonesinghe SR, et al. (2013). Risk stratification tools for predicting 
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National Institute for Health and Care Excellence (NICE) (2003). 
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The Royal College of Anaesthetists (2015). Perioperative Medicine: The 
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