Multimodal Monitoring in the Perioperative Period

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David Green

King’s College Hospital and King’s Health Partners, Denmark Hill, London SE5 9RS, UK (t: +44 203 299 3154/3919 e:

What’s wrong with current monitoring strategies?
A recent UK survey reported: “The low mortality rate of general anaesthesia (GA) (~0.06%, or 1 in 1,718) during surgery is notable. Many patients are ‘scared of anaesthesia’ and this figure can only be reassuring for them”. With the mortality rate so low during anaesthesia, what can be gained from adding additional monitors into a multimodal monitoring (MMM) strategy? [1]

However, the survey authors then added: “This low mortality rate is in marked contrast to the report by the European Surgical Outcomes Study (EuSOS) [2] of an overall 4% (1 in 25) mortality rate [in Europe] for inpatient elective major surgery”.

Referring to the 80-fold difference between immediate and intermediate mortality, they went on to comment: “These differences highlight the potential impact advances in perioperative care – by anaesthetists, surgeons and intensivists – might have on overall mortality rates after surgery”. There is an urgent need to close the gap between the very low immediate perioperative mortality (as highlighted above) and the mortality occurring during the whole perioperative period and accept that poor management of the intraoperative course may be the reason for the initiation of the ‘ticking time bomb’ that leads to complications and mortality further down the line.

The increasingly high-risk surgical population
It has been estimated that 1 million of the estimated 3 million operations that were carried out in the UK NHS in 2013, and included in part of the above survey, were on patients over 65 years of age [1]. A recent Australian and New Zealand study (REASON) reported that 5% of patients over 70 had died within 30 days of surgery [3]. The study investigators commented that “strategies are needed to reduce complications and mortality in older surgical patients”. Strategies and protocols need to be guided by monitors, not by ‘recipe books’, and we need to individualise patient care [4]. Anaesthetists must recognise that deficiencies in intraoperative management may not result in immediate mortality but are deleterious for long-term patient outcomes. MMM, as described in the article, is an attempt to reduce those complications and the overall mortality after high-risk surgery, especially in the elderly.

New monitoring technology and its use in the operating room
There have been very few advances in routine intraoperative monitoring in anaesthesia in the past 30 years since the clinical introduction of the pulse oximeter in 1984. Even in the latest trials, the addition of flow monitoring to routine monitoring has not reduced either mortality or complications [5, 6]. Although there is now evidence to suggest that the use of individual new monitors can influence outcome, it will only be their combination that will radically improve the impact of perioperative management on outcomes in high-risk surgical patients [7, 8].

Blood pressure and flow monitoring as part of a multimodal monitoring strategy
Most anaesthetists will try to avoid hypotension, but what is the evidence that low mean arterial pressure (MAP) is harmful? A recent study in the US showed that a systolic, mean or diastolic pressure less than 70, 50 or 30 mmHg, respectively for more than 5 minutes was associated with a roughly threefold increase in 30-day mortality [9].

Using fluids to maintain MAP
Intraoperative fluid management is still influenced by the concept of the loss of functional extracellular fluid (FECF) during major surgery – the so-called ‘third space loss’ – which leads to large quantities of fluid and Na+ being administered during surgery [10]. However, we now know that: “there is no convincing evidence supporting the existence of the non-anatomical third space loss, neither in haemorrhagic shock nor in surgery of any kind” [11−14]. This highlights another problem. Third space loss does exist in conditions such as septic shock, anaphylaxis and burn injury, and it is important to distinguish fluid and flow requirements in elective surgery in contradistinction to haemodynamic changes in the Intensive Care Unit (ICU), where sepsis may well be a serious problem. Reviews of fluid management often mix up these two completely different scenarios and thus cause unnecessary confusion. The dangers of excessive fluids and volume overload have recently been reiterated by Marik – for example, pulmonary oedema and increased extravascular lung water; impaired oxygenation; altered pulmonary and chest wall mechanics; increased work of breathing; myocardial oedema and so on. The list is long [15].

Using vasoactive agents to maintain MAP
It is very easy to restore MAP to ‘normal’ using vasoactive agents such as metaraminol, phenylephrine or noradrenaline [16]. However, MAP may then be maintained by increasing systemic vascular resistance (SVR), rather than stroke volume (SV), cardiac output (CO) and oxygen delivery (DO2).

Cardiac output changes following induction and maintenance of anaesthesia
A fall in MAP following induction of anaesthesia is often ascribed to peripheral vasodilation with a fall in SVR. However, using the LiDCOrapid monitor (LiDCO, UK) to measure CO, following BISTM (Medtronic, USA), the decrease in MAP using either propofol or etomidate was mainly due to a fall in CO and not to a decline in SVR [17].

Why do MAP and CO fall during anaesthesia?
Experiments using propofol in dogs showed that it produced an increase in venous capacitance due to venodilation (not peripheral arteriolar vasodilation) [18] – a key feature of anaesthesia that has received little recognition [19]. Following venodilation, the decrease in MAP is mainly due to declines in preload, SV, CO and DO2, which mimics hypovolaemia [19]. Should venous capacitance be maintained with liberal fluids or by administering a venoconstrictor? Phenylephrine, in a low-dose infusion and commenced pre-induction, and with the effect monitored by the LiDCOrapid (see below), can maintain venous tone without increasing SVR, as well as maintaining venous capacitance SV, CO and
MAP, without the need for liberal fluid administration [20]. It seems that liberal fluid replacement in surgery (see above) has been administered to overcome the effects of increases in venous capacitance rather than a true loss of FECF. Post-induction measurement of CO greatly underestimates the true resting CO/DO2, so any intervention to ‘optimise stroke volume’ (whatever that means) rather than maintain pre-induction CO/DO2 will be flawed and may result (as described above), in unnecessary excess fluid and Na+ administration without improving outcome in high-risk patients [6, 15].

Recommendation to add flow monitoring
In 2011, the UK National Institute for Health and Care Excellence (NICE) recommended the addition of flow monitoring using Doppler (Oesophageal Doppler, Deltex UK) technology for a distinct group of high-risk surgical patients undergoing particular surgical operations [21]. Even so, this guidance was questioned and argued over in editorials [22, 23].

Update on circulatory physiology during anaesthesia
Distinguishing between venodilation and vasodilation, and the role of the intact endothelial glycocalyx layer (EGL) in controlling fluid exchange in the tissues, has important implications for surgical fluid management [19]. Excess fluids and Na+, used to replace non-existent ‘third space loss’, damage the EGL directly by compression and distortion, and also lead to an increase in atrial natriuretic peptide (ANP), which itself damages the layer. The implication is that the ‘third space loss’ may actually be precipitated by excess fluids.

Does the use of flow monitoring on its own, when added to conventional monitoring, improve outcome and reduce mortality?
As reported in a recent Cochrane collaboration review entitled ‘Perioperative increase in global blood flow to explicit defined goals and outcomes after surgery’, when fluids were used, with or without additional drugs, there was no decrease in mortality and length of stay was decreased on average by only one day [24]. The review authors concluded: “The balance of current evidence does not support widespread implementation of this approach to reduce mortality but does suggest that complications and duration of hospital stay are reduced”. The OPTIMISE trial also did not reduce a composite outcome of complications or 30-day mortality [5]. A recent editorial concluded that SV optimisation/maximisation provides “no marginal benefit in
aerobically-fit patients having elective surgery within a contemporary enhanced recovery pathway” [25].

The role of individualised haemodynamic therapy
In their review of perioperative haemodynamic therapy, the authors stated that “once an individualised approach will be identified, the terms of liberal, restrictive and supranormal values could eventually be replaced by adequate haemodynamic support that fits every patient’s own needs” [4]. Might a better ‘individualised approach’ using MMM improve outcomes and reduce mortality? [26]

Avoiding the build-up of oxygen debt contributes to good outcomes [27]. MMM can be used to assess and then maintain pre-induction DO2, thereby avoiding the build-up of oxygen debt. Lack of oxygen debt at the end of the procedure means that goal-directed therapies (GDTs) intended to increase DO2 by fluids and inotropes in the High Dependency Unit (HDU) or ICU – and thus repay the debt [5] or bring DO2 values up to pre-induction levels – are no longer necessary and may not in any case benefit the patient [28].

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