The assessment of the patient’s ability to withstand and respond to surgery is a key determinant in risk stratification for predicting outcome, providing informed consent planning appropriate surgery and the level of anticipated postoperative care. Major elective colorectal surgery carries significant risk (particularly in the elderly or those with comorbidities) and imposes significant perioperative demands on the patients, in particular causing a catabolic response and an increase in oxygen consumption. Patients who are unable to meet this increase in oxygen consumption with a concomitant rise in oxygen delivery are at risk of oxygen debt, anaerobic metabolism and the build up of lactic acid. Moreover, in patients undergoing colorectal surgery the situation is compounded by the fact that haemodynamic upset (e.g. from hypovolaemia) will divert splanchnic blood other organs, which may put anastomotic healing at particular risk.

For many years, intensivists have focussed on the improved outcomes of patients who are able to increase oxygen delivery (DO₂) in the intraoperative and postoperative period to supranormal levels e.g. a DO₂ of >600 ml·min⁻¹·m⁻² [8]. In addition a number of risk scores have also been described to aid in the preoperative assessment. Goldman’s original risk score [9] has been superseded by a number others, the most well known is Lee’s Revised Cardiac Risk Index [10]. There others that are widely used including Physiological and Operative Severity Score for the enUmeration of Mortality and Morbidity (POSSUM) scores and data from large surgical databases such as the American College of Surgeons National Surgical Quality Improvement Program (NSQIP).

More recently, there has been a paradigm shift to dynamic assessment of patients. This may includes relatively simple assessment such as estimating metabolic equivalents (MET) which is the resting oxygen uptake (approx. 3 ml/kg/min), with, for example walk up a flight of steps or a brisk walk on level ground representing 4 METs and strenuous sports representing >10 METs. The gold standard is increasingly being viewed as cardiopulmonary exercise testing (CPET), in which a number of measurements are made including maximal oxygen consumption (VO₂ peak) or more usefully the estimation of anaerobic threshold (AT), which is effort independent. Although the evidence is still growing a commonly used threshold is Older’s study in elderly patients undergoing abdominal surgery, with a mortality in a patients with an AT of >11 ml/kg/min was 1 %, increasing to 18 % with an AT of 8–11 ml/kg and over 50 % with an AT of <8 ml/kg/min [11]. More recently, studies for colorectal surgery patients have broadly agreed with this, such as West’s group who demonstrated the risk threshold to be an AT of <10.1 ml/kg/min and VO₂ peak < 16.7 ml/kg/min [12].

What is the value of this assessment of physiological status? There is compelling evidence that patients with poor reserve have a poorer outcome, both in terms or mortality but also morbidity. The latter, in the form of complications has the potential to cause chronic ill health, and given the volume of colorectal surgery undertaken worldwide annually clearly represents not only huge personal burdens, but also widespread social and economic burdens too. Above all, good preoperative assessment permits risk stratification, guiding consent, appropriate surgery and the postoperative care. For example some patients may not be able to withstand the cardiovascular and respiratory sequelae of prolonged pneumoperitoneum for a laparoscopic resection and may be best served by an open procedure. Other patients may be deemed at such high risk that stenting may be the safest alternative, or require a planned admission to intensive are for targeted organ support.

There is little controversy for patients with an ERAS programme should have their health optimized preoperatively. Much of this involves coordination with primary care physicians and includes optimization of haemoglobin levels (e.g. with iron therapy), and of comorbidities such as diabetes, angina hypertension, atrial fibrillation asthma etc. For some patients, preoperative assessment may unmask other serious comorbidities and these may need surgical intervention prior to surgery, such as coronary stenting.

An exciting area of perioperative medicine emerging is the concept of prehabilitation. Given that there seems to be an improved outcome in with increasing physiological reserve, it is logical to take steps to improve this reserve preoperatively. Indeed, the parallel between some of the physiological changes of major surgery and a major sporting endeavour has been made [13] and there exists a wealth of data in exercise physiology as to how training may improve exercise performance as measured by an increase in VO₂ max or lactate threshold [14]. Therefore if exercise can improve the results of CPET for athletes, it is very tempting to suppose that this might have the same effect in patients and thus be translated into an improved outcome. This is an emerging field with no unified agreement as to an exercise regime. However there seems to be a dose–response effect with increased benefits occurring following increased exercise. Prehabilitation is a wider, multimodal discipline, encompassing not only exercise regimes, but nutritional supplementation (see below) and psychological preparation too. Although in some circumstances effective prehabilitation may be impractical, either because of the relative urgency of surgery or because of other comorbidities, this area is key within ERAS programme, as patients with poor preoperative functional capacity and grip strength have a slower recovery, with increased complications, sleep disturbance and fatigue persisting weeks or even months. Moreover the least fit patients appear to have the most to gain [15].

Nutritional supplementation and advice and has been mentioned above, and is one of the most exciting areas in the preoperative care and indeed we have coined the phrase “nutritional perioperative medicine” [13]. The use of nutritional support and supplements for those with a high degree of nutritional risk may need to escalated so to formal enteral (and in some cases parenteral) for 7–10 days prior to surgery, even if surgery may have to be delayed. More recently the effects of immunonutrients and perhaps even other nutrients that improve oxygen efficiency (such as beetroot and other nitrates) are awaited with interest [13, 15].

In addition, simple advice (but often hard for the patients to implement) such as weight loss and cessation of smoking and management of excessive alcohol intake must nevertheless be enthusiastically given to patients, extolling their virtues. In particular smoking cessation has a greater effect the longer the patient is able to stop, and there is a reduction in postoperative complications after just 3 weeks of abstinence. Furthermore, multidisciplinary support, counselling and nicotine replacement patches may aid in this process.

The fundamentals fluid management are best appreciated by understanding two fundamental concepts. Firstly the importance of adequate cellular oxygen delivery and secondly the consequences of poor fluid management.

Adequate tissue oxygenation is a basic physiological principle and over 40 years ago the link between higher levels of oxygen delivery (DO₂) and consumption (VO₂) with increased survival in septic patients [27]. Many, but not all, later trials have supported confirmed these findings in both high risk and septic patients [28, 29] and the concept of ensuring adequate or indeed “supranormal” DO₂ to patients undergoing major surgery to reduce complications and death is now fundamental modern perioperative practice. DO₂ is usually indexed (i.e. expressed per m²) and expressed as DO2I using the well known equation:
Where SV = stroke volume, HR = heart rate, SaO2 = oxygen saturation (expressed as fraction), [Hb] = haemoglobin concentration and BSA = body surface area.

Commonly used targets are cardiac index (CI) of >4.5 l min^–1 and a DO2I of >600 ml·min^–1·m^–2 [30]. This forms the basis of modern fluid management as there is a move away from pressure guided fluid therapy (such as blood pressure or central venous pressure – CVP) to flow guided fluid therapy. Given that HR, SaO2 and [Hb] are often not markedly deranged, the major focus is directed at stroke volume optimisation (SVO) according to Frank-Starling curve.

Secondly, an appreciation is required of the consequences of poor fluid management, either with inadequate fluid or excess fluid. The former will ultimately result in a reduced SV, CO and DO₂, and morbidity and mortality will be increased, secondary to inadequate tissue perfusion and organ dysfunction. The latter – excess fluid administration – will result in salt and water overload and thus oedema, which will not only effect major organs such as the lung, but critically in colorectal surgery, gut tissues and in particular any anastomosis as well as potentially increasing myocardial workload. Both these extremes of fluid management will therefore ultimately impair tissue oxygenation, leading to anaerobic metabolism, an oxygen debt, lactic acidosis and ultimately organ dysfunction and anastomotic failure. In particular excess fluids increases complications: for every litre of excess increased complications by 32 % [7] and prolonged hospital stay by 1 day [31].

Thus if we accept that excess or inadequate fluid for the patient is harmful and leads to increased morbidity and mortality, the key question is how to find the “sweet spot” of just the right amount of fluid, i.e. achieve SVO. It is just over a 100 years since Starling was credited with the concept that the more a heart muscle is stretched the greater the power of contractility (although the concept was first described 50 years prior to that) [32]. The process of SVO involves measurement and then the administration of an IV fluid bolus of 200–250 ml. The SV is measured again 10–15 min later. If there has been >10 % rise in SV, further boluses are given until SV no longer rises (i.e. optimisation has occurred). Further boluses of fluid are potentially detrimental as SV can reduce with excess of fluid. SVO is sometimes used synonymously with (Individualized) Goal Directed Fluid Therapy (GDFT), but the latter is not restricted to fluids but other methods too (e.g. inotropes) to achieve various targets or goals.

Historically, formulaic methods based on weight, standard 25 years ago (e.g. 5–10 ml/kg/h intraoperatively followed by 3 l in the first 24 h) are generally seen as inferior as they take no account of the patients starting fluid status or cardiac function. Moreover the widely held belief of third space loss and the concomitant requirement to replace these losses is probably flawed [33]. A significant improvement occurred with the adoption of routine CVP monitoring and the use of intermittent estimation of lactate, base excess and central venous oxygen saturation (as a surrogate marker for mixed venous oxygen saturation and hence oxygen extraction) to guide fluid therapy provide a more individualized approach. However, CVP measurement assumes pressure (right atrial pressure) and volume (left ventricular end diastolic volume) measurements are directly correlated which is clearly not always the case e.g. with a poorly compliant myocardium. Arterial and central venous biomarkers are of use but may show a delay of several hours. Thus flow-derived variables such as SVO, whilst not having universal acceptance, are nevertheless regarded as probably the superior method of fluid management.

Nevertheless, how can we make sense of the large amount of studies, some conflicting, in this all-important area? Firstly an appreciation is required in the changes in perioperative care that have occurred which rapidly outdates studies even only a few years old. The ERAS philosophy represents one of the most major changes in modern perioperative care and many of the physiological stresses such as prolonged starvation, the use of bowel preparation have produced a huge change improvement in the fluid status of patients arriving for surgery. Secondly the advent of laparoscopic surgery (vide infra) has dramatically reduced the physiological upset and fluid shifts associated with open surgery. Thirdly, there are various different available methods of measurement of cardiac output (with the majority of good quality data using the oesophageal Doppler monitor (ODM) though some more recent use other method such as arterial waveform analysis e.g. LiDCO rapid). This area has been recently reviewed [34]. Finally, many studies are on various abdominal surgeries and not specific to colorectal.

The key, current area seem are exemplified in the OPTIMISE trial, looking at 734 high-risk patients aged 50 years or older undergoing a variety of major gastrointestinal surgeries [35]. SVO and inotropic support was compared to a control group (CVP guided therapy) during surgery and for 6 h postoperatively. Although complications and 30-day mortality was different to the control group, when included in an updated meta-analysis their intervention group was associated with a reduction in complication rates. Thus we can postulate that SVO probably does no harm, probably reduces complications and probably has no effect on mortality. Two other major reviews would broadly support these findings. The former, most definitive review, studied the effects of increasing perioperative blood flow using fluids (with or without inotropes/vasoactive drugs) and found that mortality was not reduced but three complications – renal failure, respiratory failure, and wound infections – were reduced, with 13 out of every hundred patients treated expected to avoid having complications. The length of stay was also reduced by just over 1 day [36]. The latter systematic review, whilst specifically looing at colorectal surgery had only 691 patients and found no difference in outcome (morbidity and mortality), particularly if GDFT was conducted within modern ‘optimized perioperative care’ [37].

It would seem that the benefits of modern, flow-targeted therapies, using fluids and where necessary inotropes had benefits that have been rather easier to demonstrate in older studies, prior to the adoption of ERAS pathways, and any benefits, are now much harder to prove. Indeed, a recent editorial – Stroke volume Optimization is the fairy tale over? – makes this very point [38]. SVO probably does no harm, and in aerobically fit patients many be unnecessary, but some patients will benefit more than others, especially the very frail and those in whom there is significant blood loss (7 ml/kg), or other large fluid shifts. Overall with the reduction of variability of patient care that comes with protocolised ERAS pathways, preoperative optimization and the reduction of physiological upset have all probably served to negate the earlier benefits of SVO.

This author’s approach to fluid management takes a pragmatic approach of the available evidence. The key areas are [39]

The approach to laparoscopic surgery is dealt with later.

The type of fluid administered is also important and has generated a large number of trials, reviews and debates spanning four decades. A recent area of interest as centred on the endothelial gylcocalyx, where excessive volumes of fluid may disrupt this barrier promoting vascular leakage in to the interstitial space [39].

There are a variety of crystalloids available, but there is little doubt that physiologically balanced crystalloids (e.g. Hartmann’s solution and latterly Plasma-Lyte) are superior to 0.9 % saline, which has consistently been reported to cause hyperchloraemic acidosis, itself predisposing to reduced kidney perfusion, reduced gastric blood flow and motility and reduced myocardial contractility [42]. If 0.9 % saline has a place, it is for the treatment of hypocholoraemic alkalosis, such as prolonged vomiting or large volumes of nasogastric aspirate.

Colloids have been preferred by some as the intravascular half-life is improved, providing that the endothelial glycocalyx is intact. Theoretically, a ratio of crystalloid: colloid might be expected to be about 3:1 to meet the same targets, given the early distribution of the two fluids, but a recent meta analysis demonstrated that in practice although there was much variation amongst the studies, the overall ratio was about 1.5:1 [43]. Of the major synthetic colloids – gelatins and starches – the latter have recently fallen into disuse because of the risk of death and renal failure in critically ill patients, although whether or not this can be extrapolated to elective patients is debatable. Many still use gelatin derivatives, although potential disadvantages include cost, allergy and coagulopathy. Albumin, whole blood and blood products are used to replace pre-existing and/or losses deficiencies.

Thus for critically ill patients who may be septic and/or have renal injury, in whom the endothelial glycocalyx may not be intact, many would avoid colloids to exacerbate any renal dysfunction. For other patients, many still use colloids for SVO to limit excess crystalloid (and saline) load, which is a major predictor for adverse outcomes for ERAS patients [7].

Open colorectal surgery usually necessitates a large incision and requires intensive pain relief, usually with either regional blockade (usually an epidural) or parenteral opioids. For many years a thoracic epidural anaesthesia (TEA) was viewed as the gold standard for these patients rather than opioids as it provided a number of well-documented advantages including not only excellent pain relief, but in addition a swifter return of GI function, both as a result of a reduced in opioid requirements but also due to the sympathectomy and a relative increase in parasympathetic action on the gut. Other advantages from other forms of abdominal and pelvic surgery included a reduced VTE rate, reduced blood loss, and a reduction in some aspects of the stress response (pituitary, adrenocortical and sympathetic responses).

However in recent years enthusiasm for epidurals has declined. There are several reasons for this including a significant failure rate with quoted ranges between 13 and 47 %, with a large study describing an incidence of 32 % for thoracic epidurals and 27 % for lumbar epidural [45]. Although in many cases re-siting or adding adjuvants (such as epidural diamorphine) may rectify the situation, a failed epidural not only leaves the patient in pain, but it may also restrict other alternatives (such as systemic opioids) as these drugs cannot be co-administered if the patient is receiving epidural opioids. In addition, although epidurals are widely perceived as safe, the permanent neurological damage can occur, either due to vertebral canal haematoma, abscess and direct trauma. The NAP3 project highlighted the risks associated with postoperative epidurals having the highest incidence of permanent neurological harm, estimated between 1:5 700 – 1:12 200 [46]. Finally patients with epidurals often receive a greater volume of fluids to combat hypotension, which predisposes to fluid overload and can increase hospital stay [31]. These patients should receive vasoactive drugs rather than fluids to combat the sympathectomy [40, 41], but if so the patient will require nursing in an ICU/HDU environment as these agents cannot safely be administered on a ward and without intrarterial monitoring. Whilst epidurals will nevertheless continue to have a place for open surgery, other techniques have been used successfully used such as bilateral Transversus Abdominis Plane (TAP) catheters [47], rectus sheath catheters, and wound catheters which infuse local anaesthetic into the wound edges.

Whilst the optimum analgesic technique for open colorectal is well studied and has a good evidence base to support the use of epidurals (even if latterly other modalities nerve blocks are gaining popularity), there is a paucity of data for laparoscopic surgery. Generally these patients’ analgesic requirements are much more modest in both intensity and in duration and although epidurals have been used, they are at best probably not necessary and at worst detrimental. A logical alternative to more the modest analgesic requirements is spinal anaesthesia with local anaesthetic and opioid. This has been used with a degree of success by a number of centres, and was used for patients in the first 23-h stay colorectal resections paper [48]. It offers good intraoperative analgesia extending several hours postoperatively with postoperative opioid sparing, following which a regime of regular simple oral analgesics suffices. Compared to epidurals, spinals permit greater mobilization and with no on-going sympathectomy, results in patients receiving less iv fluids, with their attendant complications [31], although others have found its benefits purely limited to opioid sparing [49]. Whilst spinals are approximately 2½ safer than epidurals [46], they are nevertheless not risk free for neurological damage. In addition, the onset of the sympathectomy is much more rapid than an epidural and caution is required, particularly in the elderly. Also the spinal must have “fixed” before adopting any head down position, so it is advisable to leave 10–15 min before adopting this position. Other forms of one-shot local anaesthetics have also been used, such as TAP blocks and wound infiltration, also with success. As with epidurals for open surgery, after initial success with spinals for laparoscopic surgery, many clinicians are opting for a simpler alternative, such as TAP blocks. Spinals are of little use for very prolonged surgery or if conversion to open surgery occurs.