Anion Gap.

Anion Gap.

Gap = Na+ + K+ - Cl- - HCO3-

Some causes of metabolic acidosis, e.g., lactic acidosis, release anions into the extracellular fluid which are not normally measured. When this occurs there will be an unexpected discrepancy between the sums of the principal cations and anions. The usual sum is:

Gap   =   Na⁺   +   K⁺   -   Cl^-   -   HCO₃^-

  15    =   140   +   5     -  105  -     25     mMol/L

In addition to Cl^- + HCO₃^- there are extra unmeasured anions, e.g., lactate, phosphate, sulphate, which increase the "gap". A gap greater than 30 indicates a significant concentration of unmeasured anions. 

Because this calculation relies upon the accuracy of the other measurements, small errors in these large numbers cause a disproportionately great error in the "gap". If information is required about the unmeasured anions, it is probably more appropriate to measure their concentration, i.e., lactate in tissue hypoxia, 3-hydroxybutyrate in diabetic ketosis, and phosphate or sulfate in renal failure

Anaesthetic perioperative management of patients with pancreatic cancer 2

EVALUATION AND OPTIMISATION OF PREOPERATIVE PHYSICAL

CONDITIONS AND MEDICATIONS

A growing number of old patients benefits from a surgical procedure[19]. Age is an independent risk factor of postoperative mortality and postoperative complications and can cause a gradual progressive loss in the biological reserves for maintaining physiological homeostasis under stress.

In addition, an increasing number of patients present with one or more age-related chronic conditions, which further decrease their ability to respond to stress.

Cardiac and pulmonary diseases are the most frequently observed co-morbidities that anaesthetists and surgeons must manage during this complex surgery.

A complete history of prior medical and surgical conditions and a full medication list are particularly important[20,21].

CARDIOVASCULAR RISK EVALUATION:

Cardiovascular complications are among the most common and significant postoperative problems in elderly patients.

A practical guideline for perioperative cardiovascular evaluation for non-cardiac surgery has been proposed by the American College of Cardiology and American Heart Association Task Force[22].

Patients should be assessed using an approach that considers clinical predictors, the risk of the proposed operation and the functional capacity.

Ageing is accompanied by increased vascular and ventricular stiffness, diastolic dysfunction and an increased risk of heart failure[23].

Diastolic dysfunction even with a normal or supranormal ejection fraction might elicit a significant effect on the perioperative outcome and management of elderly patients[12].

Diastolic dysfunction might significantly affect perioperative haemodynamics, response to fluid shifts, anaesthetic drugs and other Perioperative medications.

Patients with cardiovascular diseases are sensitive to haemodynamic instability and often require increased filling pressures to generate an adequate cardiac output.

The anaesthetist must carefully manage fluids during the operation to avoid overload or rapid volume administration.

Moreover, the anaesthetist must maintain a normal haemoglobin value (Nair et al[24] demonstrated that anaemia was strongly associated with diastolic dysfunction in patients with coronary artery disease)

if possible, must choose volatile anaesthetics that appear to improve diastolic parameters (in contrast to propofol, which elicits the opposite effect) as measured by echocardiography[25].

Thoracic epidural analgesia should be strongly suggested, not only for pain management and for decreasing respiratory complications but also because its use appears to improve cardiac function by improving the diastolic characteristics of the left ventricle[26,27].

PROPHYLACTIC PERIOPERATIVE Β-BLOCKADE:

In general, cardiovascular medication should not be discontinued prior to surgery.

In the perioperative setting, β-blockers are not contraindicated in patients with diastolic heart failure and should be continued in patients with systolic heart failure.

However, caution is warranted with the acute administration of β-blockers in situations of decompensating systolic heart failure.

Nonetheless, given the risk of acute withdrawal, β-blockade in patients with coronary artery diseases or coronary artery disease risk factors should not be discontinued preoperatively.

Rather, perioperatively increasing the dosage of the patient’s β-blockade regimen would most likely be beneficial[28-30].

If a patient who is scheduled for elective pancreatic surgery requires a new prescription, it should be started at least 1 mo prior to the procedure to allow for dose adjustment[ 31,32].

PULMONARY RISK EVALUATION:

Pulmonary complications such as pneumonia, failure to wean, and postextubation respiratory failure represent the second most frequent types of postoperative complication following wound infection, with an estimated incidence rate ranging from 2.0% to 5.6% following surgery[33,34].

Pulmonary disease increases the risk of postoperative complications, accounting for 40% of postoperative complications and 20% of deaths[35].

Age-related changes, such as increased closing volumes and decreased expiratory flow rates can predispose older patients to pulmonary complications.

Some postoperative pulmonary complication (PPC) predictors after pancreatic surgery are summarised in Table 2 (modified from Canet et al[36]).

Identifying the patients who are at high risk for PPCs, can help the anaesthetist to design individually tailored management approaches[37-39].

Pharmacologic measures for managing these complications are either unavailable or limited, and as a result, treatments must be based on physical therapy and respiratory support ventilation.

Finally, the ability to predict PPCs would enable clinicians to give patients more precise risk assessments, thereby facilitating their decision making.

NUTRITIONAL STATUS AND MECHANICAL BOWEL PREPARATION

The prevalence of malnutrition is high in patients who are submitted for surgery and ranges from 35% to almost 60%[40].

Malnutrition has been consistently associated with impaired immunity[41] and can lead to increased complications, such as pressure ulcers, delayed wound healing, increased risk of infections, impaired muscular and respiratory functions[42], as well as increased mortality and poor clinical outcomes.

Nutritional status should be determined because nutritional deficiencies are common in patients who have undergone pancreatic resection for malignant tumours.

Because malnutrition is potentially reversible with appropriate nutritional support, the early identification of high-risk patients is crucial, and preoperative malnutrition screening is required to identify and to treat the malnutrition[ 43].

Recently, the routine screening of patients to iden-tify risk of malnutrition has been recommended by many national, international, and specialist organisations[44,45].

THE MALNUTRITION UNIVERSAL SCREENING TOOL (MUST) for adults was recently validated by several studies, which have demonstrated that as a screening procedure, MUST is rapid and easy to use[46,47].

The MUST appears to be a valid and easy screening tool for pancreatic surgery[20], which can identify patients at high risk for major complications and death.

Furthermore, the MUST can prompt the implementation of effective nutritional interventions to reduce poor outcomes and thereby optimise the use of postoperative critical care beds and hospital resources.

As soon as malnutrition is recognised, preoperative nutritional supplements should be provided when possible.

This supplementation can include high-energy foods, vitamins, enteral feedings, or, if necessary, total parenteral nutrition.

MECHANICAL BOWEL PREPARATION

“Enhanced recovery” or “FAST-TRACK” (FT) PROGRAMMES, which were first developed by Kehlet[48], are structured interdisciplinary strategies that have been introduced to optimise peri-operative care and to accelerate postoperative recovery[49].

A major intervention principle of this approach is the avoidance of preoperative MECHANICAL BOWEL PREPARATION (MBP), which has been employed as a preventative measure in gastrointestinal surgery for more than a century as an essential factor for avoiding infectious complications and anastomotic dehiscence.

FT programmes, which exclude MBP, have been proposed more often in other surgical fields (elective colorectal, gastro-oesophageal and aortic surgery) and rarely have been applied to liver and pancreatic surgery[50].

The application of MBP in this type of surgery has been evaluated by limited studies (a retrospective case-control study by the Jefferson University[51] and a review by Salvia et al[52]), which have shown that it did not improve Perioperative outcomes.

At our institution, MBP has been excluded from clinical practice in pancreatic surgery.

A recen review examined and compared the application of FT protocols with standard care in elective liver and pancreatic surgeries, showing that FT programmes can enhance post-operative recovery and reduce the length of hospital stays with no increase in adverse events, such as re-admissions, morbidity or mortality[53,54].

The avoidance of MBP, together with other measures including the application of epidural analgesia, the prevention of intra-operative hypothermia, fluid restriction, post-operative nutritional care and early mobilisation, collectively represent essential elements of a FT programme that is warranted for complex surgical operations such as pancreatic resection[55,56].

In our experience FT programmes for hepatopancreatic resections appear to be safe and associated with a reduction in the length of hospital stays.

RISK STRATIFICATION, RATIONALE FOR THROMBOPROPHYLAXIS,

AND RECOMMENDATIONS

In patients undergoing general and abdominal-pelvic surgery, the risk of venous thromboembolism (VTE) varies depending on both patient- and procedure-specific factors[57].

Pancreatic cancer is among the most common malignancies associated with thrombosis, as it occurs in 50% of total patients.

Prophylaxis against postoperative venous thromboembolism should be tailored to the patient’s level of risk.

A model (THE CAPRINI SCORE) that can potentially be used for such purposes estimates VTE risk by adding points for various VTE risk factors[59].

Pharmacological prophylaxis reduces the risk of pulmonary embolism by 75% in general surgical patients and by 57% in medical patients[60].

The use of low-molecularweight heparins (LMWHs) to prevent thrombotic events in these patients is a common and well-documented practice.

Current recommendations strongly advise effective and preventive strategies for all hospitalised patients who are defined as moderate to high risk for VTE and are awaiting pancreatic surgery.

LMWHs appear to be effective and are potentially associated with a lower risk of bleeding when the first dose is administered 12 h preoperatively

We recommend the administration of LMWH from the day prior to surgery to all patients scheduled for pancreatic cancer surgery.

In the case of patients who are receiving anticoagulants or antiplatelet therapy and require an elective surgery or procedure, the actual guidelines addressing their management are underlined in Table 3 and are modified from Douketis et al[62].

GUIDELINES ON THE PROPHYLAXIS OF VENOUS THROMBOEMBOLISM AND ANTIPLATELET AND ANTICOAGULANT MANAGEMENT ADJUSTED ACCORDING TO RECENT GUIDELINES

In patients receiving bridging anticoagulation with a therapeutic-dose Ⅳ of unfractionated heparin, treatment is recommended to be stopped no later than at 4 to 6 h prior to surgery

In patients receiving bridging anticoagulation with a therapeutic-dose of LMWH, the last preoperative dose of LMWH is recommended to be administered at approximately 24 h prior to surgery instead of at 12 h prior to surgery

In patients receiving bridging anticoagulation with a therapeutic-dose of LMWH and are undergoing high-bleeding-risk surgery, resumption of the therapeutic dose of LMWH is recommended at 48 to 72 h after surgery instead of within 24 h following surgery

In moderate-to-high-risk patients receiving acetylsalicylic acid who require non-cardiac surgery, treatment with acetylsalicylic acid is recommended to be continued around the time of surgery instead of discontinued at 7 to 10 d prior to surgery

In patients with a coronary stent who require surgery, deferment of surgery is recommended at 6 wk or 6 mo after the placement of a bare metal or drugeluting stent, respectively, instead of initiating surgery during these time periods

In patients requiring surgery within 6 wk or 6 mo of the placement of a bare-metal or drug-eluting stent, respectively, continuing perioperative

antiplatelet therapy is recommended instead of stopping therapy at 7 to 10 d prior to surgery

Anaesthetic perioperative management of patients with pancreatic cancer

·      Postoperative complications such as primarily pancreatic fistula, haemorrhage, abscess, and delayed gastric emptying still occur at a frequency of 30% to 60%, resulting in a mortality rate of 1% to 5%[11].

·      For this reason and due to the lethality of the pancreatic cancer despite surgical treatment, the patient should be informed about the therapeutic procedure and any potential complications or disabilities to facilitate a conscious involvement in the decision-making process.

·      In the case of patients of advanced age who require pancreatic surgery, formal mental status testing can help determine whether a patient can be considered capable of making this type of decision.

Dementia is an extreme predictor of poor outcome, exhibiting surgical mortality rates that are increased by 52%[12]. The decision to classify an elderly patient eligible for surgery cannot exclude preoperative mental status.

Preoperative risk assessment

A complete history, physical, laboratory examinations, and an assessment of the surgical risks should be included in the preoperative evaluation of an elective surgery.

Recently, a variety of scoring systems has been developed, and the Physiologic and Operative Severity Score for the Enumeration of Mortality and morbidity (POSSUM) model by Copeland et al[14] was recognized as the most effective for general surgery[15].

This model, which uses scores relating to 12 physiological and 6 operative variables, was developed to postoperatively predict 30-d mortality and morbidity.

The application of the predictive POSSUM and P-POSSUM (Portsmouth modification of POSSUM)[16] models to cases of pancreatic surgery has generated conflicting results.

The implementation of this scoring system in the routine practice has proven to be difficult, and a recent review by Wang et al[17] has found POSSUM to overpredict postoperative mortality. Despite these limitations, there is still a role for POSSUM as a useful tool in pancreatic surgery. Individual POSSUM scores should not preclude pancreatic resec- tion in clinical practice but might help surgeons modify expectations of postoperative outcomes[18]. Due to the limitations of the POSSUM model, more trials are needed to adequately evaluate this scoring system in predicting postoperative mortality for pancreatic surgery.

Pulse Pressure Variation

Pulse pressure 

is the difference between the systolic and diastolic pressure readings.

It is msured in millimeters of mercury (mmHg). 

It represents the force that the heart generates each time it contracts. 

If resting blood pressure is 120/80 mmHg, pulse pressure is 40

Pulse Pressure Variation

One of the most important concepts in resuscitation is volume responsiveness, or the ability of the cardiac output to increase in response to a fluid challenge. 

As compared to prior static measures of venous filling, such as central venous pressure (CVP) or pulmonary artery occlusion pressure (PAOP or wedge pressure), clinicians are increasingly turning to dynamic means of assessing patients’ volume responsiveness.1,2 One such measure is the pulse pressure variation (PPV).1,2



PPV is a reflection of cardiopulmonary interactions. 

As a patient breathes, both spontaneously and with mechanical ventilation, cardiac output varies. 

The more the cardiac output varies with respirations, the more likely that patient is to respond to a fluid bolus with an increase in cardiac output. 

Using this simple principle, clinicians can take advantage the common arterial line tracing to assess a patient’s volume responsiveness.



To measure the PPV in a given patient, that patient must have consistent and demonstrable cardiopulmonary interactions. This means that the patient must:

1. Be in normal sinus rhythm
2. Be intubated and be mechanically ventilated, making no spontaneous respiratory efforts
3. Be ventilated with at least 8mL/kg of tidal volume
4. Have no significant alternations to chest wall compliance, such as an open chest

If all these conditions are met, the arterial waveform can be assessed for variation in a few simple steps.

1. To make measurements easier, condense the waveform to 6.25 mm/sec from the baseline of 25 mm/s. (This is easy to do, and is located under “speed” on most monitor settings.)

2. Identify the maximum (during inspiration) and the minimum (during exhalation) waveforms on the undulating pattern.

3. Using the cursor, find the systolic pressure value for the largest, or maximum, inspiratory waveform, then find the diastolic pressure. Take care to use the systolic and diastolic values on the same form. The difference in these two values is your pulse pressure. The following four figures have been captured from a monitor and enlarged for detail. The first figure below shows the value for the maximum systolic of 109 mmHg.



This figure shows the measurement of the diastolic of the maximum waveform, or 60 mmHg.

4. Repeat step 3 for the minimum form. Here, the systolic is 91 mmHg.

And the diastolic is 54 mmHg.


5. These steps will give you two pulse pressures. The pulse pressure variation is calculated as:

Therefore, in this case, the PPmax is 49, the PPmin is 37, for a mean PP of 43. This leads to a calculated PPV of 27.9%. (12/43 X 100)

Numerous studies have found that a PPV of > 12% is associated with volume responsiveness in the operating room and intensive care unit (ICU) alike. 

A meta-analysis of PPV to predict fluid responsiveness found a sensitivity and specificity of 0.89 and 0.88 respectively.3 

A meta-analysis of 22 studies focused on ICU patients had similar findings, with a pooled sensitivity of 0.88 and pooled specificity was 0.89.2 

To date, no study has specifically looked at PPV in the ED, and this is an area for future study.



The most common pitfalls in using PPV for clinical decision-making involve failing to recognize that the patient does not meet one or more of the required criteria listed above.

 If patients are breathing spontaneously, even while on assist control ventilation, they can vary their intrathoracic pressure and thereby cause exaggerated or blunted responses to the positive pressure breaths.2 

Similarly, clinicians should note that the tidal volumes used in this assessment, of > 8mL/kg,2 are larger than are otherwise used (ideally, around 6mL/kg). 

In our practice, we temporarily increase the tidal volume just for the purposes of this assessment, to ensure that the patient is seeing sufficient positive pressure ventilation to impact hemodynamics. 

However, if these criteria are met, PPV is an easy, rapid means to assess a critically ill patient’s likelihood of volume responsiveness.

A 74-year-old lady with a history of ischaemic heart disease and severe congestive cardiac failure is admitted to the ICU with hypotension and presumed sepsis

A17

A 74-year-old lady with a history of ischaemic heart disease

and severe congestive cardiac failure is admitted to the

ICU with hypotension and presumed sepsis. She is sedated

and ventilated in pressure support mode. On examination

she is confused, BP is 85/35mmHg, HR is 115bpm (sinus

tachycardia), SpO2 is 95% on 60% oxygen. Arterial blood

gas analysis shows a lactate of 4.3mmol/L (39mg/dL).

Which is the BEST guide to the need for further intravenous

fluid replacement?

a. Response of oesophageal Doppler to passive leg raising.
b. Insertion of a pulmonary artery catheter and pulmonary artery
occlusion pressure measurement.
c. Titrate fluid resuscitation against repeated blood lactate
measurements.
d. Assess pulse pressure variation.
e. Urine output measurement.


A

This patient may have hypotension secondary to a variety of causes,

including sepsis and decompensated cardiac failure. 

Pulmonary artery 

occlusion pressure has been shown to be a poor predictor of whether a

fluid bolus will cause an increase in cardiac output (i.e. whether the patient

is volume-responsive). 

Blood lactate is a sensitive marker of shock and

falling levels correlate with improved survival.

 However, this will not

distinguish between cardiogenic shock (where inotropes may be required)

and septic shock (requiring fluids and/or vasopressors). 

Pulse pressure

variation of >13% has been shown to accurately predict response to fluid,

but is only reliable in mechanically ventilated patients without spontaneous

respiratory effort. 

Urine output measurement will again not distinguish

between different causes of shock. 

Passive leg raising autotransfuses

about 300ml of blood into the central circulation. 

If stroke volume

increases significantly (>10% as measured by oesophageal Doppler), this

indicates preload-responsiveness. 

An advantage of this technique is that it

is reversible if no improvement is seen, avoiding worsening pulmonary

oedema in the case of cardiogenic shock.

A15 All the following increase the likelihood of a patient
acquiring an antimicrobial-resistant infection EXCEPT:
a. Use of cefotaxime.
b. High nursing workload.
c. Prolonged mechanical ventilation.
d. Brief hospital admission.
e. Understaffing in the ICU.


D
Multi-drug resistant infections are increasing in incidence in the intensivecare unit. Use of broad spectrum antibiotics such as third generationcephalosporins, carbapenems and quinolones is an independent riskfactor for ventilator-acquired pneumonia with a drug-resistant organism, asis prolonged mechanical ventilation.

Methycillin-resistant Staphylococcus aureus rates have been shown to increase with bothovercrowding andunderstaffing of the ICU.


Prolonged hospital admission, the presence ofindwelling devices and poor hand hygiene also contribute to thedevelopment of antibiotic-resistant infections.

A14 A 48-year-old woman is rescued from a house fire during
which she was trapped in a smoke-filled bedroom for 30
minutes. On arrival in the Emergency Room, she has
marked facial burns and a hoarse voice but no stridor. She
is expectorating carbonaceous sputum, appears confused
and has a cherry-red visage. Which statement is FALSE?
a. Early intubation is advisable.
b. A significant thermal injury to the trachea is likely.
c. Lavage with sodium bicarbonate 1.4% has a role in the
management of this patient.
d. Lung function is likely to worsen over the next 12 hours.
e. A cherry red visage has several causes other than carbon monoxide
poisoning.



A14 B
Inhalational injury has three mechanisms: thermal injury to the upper
airway, smoke inhalation causing chemical pneumonitis to the lower
airways, and systemic absorption of toxins (principally carbon monoxide
and cyanide).

Hoarseness and facial burns are signs of upper airway
thermal injury to the pharynx and glottic area.

Pharyngeal oedema is likely
to increase rapidly especially once fluid resuscitation is commenced, and
early intubation is advised.

Most of the heat from hot gas inhalation is
dissipated in the upper airways, so thermal injury below the glottis is
unusual. 

Sodium bicarbonate lavage of the bronchial tree may be
performed following intubation to neutralise acidic deposits and remove
soot contamination, although evidence for the effectiveness of this therapy
is lacking.

Lung function usually worsens over a period of hours following
inhalational injury.

A cherry red visage is a non-specific sign of carbon
monoxide poisoning, and there are many other causes of facial flushing
including alcohol, emotion and heat, all of which are associated with burns
injuries.

A13 Which of the following statements regarding the use of
antifibrinolytic agents is FALSE?
a. Tranexamic acid is a competitive inhibitor of plasminogen and
plasmin.
b. Aprotinin significantly reduces blood loss and transfusion
requirements in cardiac surgery.
c. Use of antifibrinolytics in trauma is supported by several high quality
randomised controlled trials.
d. Arterial and venous thrombosis are uncommon complications of
tranexamic acid use.
e. The risk of anaphylaxis with aprotinin is 0.5%.


A13 C

• Tranexamic acid (TXA) is a competitive inhibitor of plasmin and plasminogen, while aprotinin forms irreversible complexes with a variety of proteases including plasmin. 
• While TXA is synthetic, aprotinin is isolated from bovine lungs and has a high incidence of anaphylaxis (0.5%). 
• A strong evidence base supports the use of antifibrinolytic agents in cardiac surgery, where they have been shown to reduce the requirement for blood transfusion by about 30%. A Cochrane review 1 failed to demonstrate any increased incidence of thrombosis with antifibrinolytics, although a recent study 2 suggested that aprotinin is associated with an increased incidence of myocardial infarction, stroke and renal failure, leading to its withdrawal in the USA. 
• The rationale for use in trauma is extrapolated from evidence in elective cardiac surgery; the ongoing CRASH (Clinical Randomisation of an Antifibrinolytic in Significant Haemorrhage) II trial should help determine whether they are effective in this context.

The following are prerequisites for the use of recombinant factor VIIa in bleeding trauma patients EXCEPT:

The following are prerequisites for the use of recombinant factor VIIa in bleeding trauma patients EXCEPT:

a. Platelet count >50x109/L.

b. Temperature >36°C.

c. Fibrinogen >0.5g/L.

d. pH >7.20.

e. Ionised Ca2+ >0.8mmol/L (3.2mg/dL).

B

• ·        Recombinant factor VIIa (rFVIIa) may be helpful to induce coagulation in areas of diffuse small vessel coagulopathic bleeding.

·        It follows that all surgical avenues to control bleeding must first be explored.

·        Adequate platelet numbers are required to generate the ‘thrombin burst’ which the rFVIIa provokes, and adequate fibrinogen must be present to translate this thrombin generation into clot formation.

·        Correction of severe acidosis, hypothermia and hypocalcaemia are also warranted to maximise the chances of success for this expensive and unlicensed therapy.

·        However, a temperature of 36°C is difficult to achieve in such patients; 32°C is considered an appropriate minimum threshold.

FEATURES INTENDED TO PREVENT HUMAN ERROR

FEATURES INTENDED TO PREVENT HUMAN ERROR

·        The medical gas pin index and diameter index safety systems ensure that medical gas connections are made correctly.

·        Therefore, one cannot hang a nitrous oxide cylinder in an oxygen hanger yoke or connect a nitrous oxide hose to the oxygen pipeline inlet of the machine.

·        The “fail-safe” valve is a pressure-sensitive device that interrupts flow of all hypoxic gases on the machine to their flow control valves if the supply pressure of oxygen in the intermediate-pressure system (i.e., components downstream of the first stage oxygen regulator that reduces the high pressure in the tank to 45 pounds per square inch gauge [psig] and upstream of the oxygen flow control needle valve) falls below a threshold (˜20 psig in GE machines; 12 psig in Dräger machines).

·        When the pressure of oxygen in the intermediate-pressure system falls below 30 psig, an oxygen supply pressure failure alarm is annunciated.

·        The oxygen flow control knob is “touch coded”; that is, it is fluted and larger in diameter than the other gas flow control knobs and is normally located on the right of all other gas flow control valves.

·        In the Dräger Fabius GS workstation, the gas flow controls are arranged vertically, with the oxygen flow control knob as the lowest.

·        Key-fill systems for anesthesia vaporizers are safety features that also decrease the likelihood of atmospheric contamination during vaporizer filling.

·        The most important of such systems is the Safe-T-Fill system used on desflurane bottles and desflurane vaporizers because filling a vaporizer specific for another agent with desflurane could result in a lethal overdose of desflurane.

·        A pressure relief valve built into the machine common gas outlet, breathing system, or ventilator provides some protection against positive pressure barotrauma.

FEATURES TO CORRECT FOR USE ERROR

·        Gas flow proportioning systems ensure a minimum oxygen concentration of 25% when nitrous oxide and oxygen are being used.

·        Therefore, if the anesthesiologist were to accidentally attempt to increase the flow of nitrous oxide, either the oxygen flow would be increased automatically or the flow of nitrous oxide would be limited according to the flow of oxygen that was set.

·        On older machines, during use of the anesthesia ventilator, changes in fresh gas flow, inspiratory-expiratory (I:E) ratio, or respiratory rate cause changes in delivered tidal volume that might result in overventilation, underventilation, or even barotrauma.

·        On modern systems (e.g., GE workstations that incorporate a ventilator with the Smart Vent feature, Dräger Fabius GS, and Apollo workstations), once the ventilation parameters have been set, they are maintained because the ventilator or circuit automatically compensates for changes in gas flow settings.

·        In the GE workstations with Smart Vent, tidal volume is monitored continually by a computer.

·        If measured tidal volume changes from that set to be delivered, the computer adjusts the volume delivered by the ventilator bellows.

·        Dräger and Datascope Anestar workstations use fresh gas decoupling to maintain a constant tidal volume.

·        In this system, during the inspiratory phase, a decoupling valve closes so that fresh gas entering the breathing system is directed into the reservoir bag and only gas from the ventilator is delivered to the patient.

·        During exhalation, the ventilator chamber refills from the fresh gas flow and the fresh gas that was collected in the reservoir bag during the previous inspiration.

·        A vaporizer interlock system prevents the unintentional simultaneous use of more than one vaporizer.

MONITORING SYSTEMS

·        A monitor of oxygen in the gas delivered to the patient is mandatorily enabled, and the low oxygen concentration alarm is

activated whenever the anesthesia workstation is capable of delivering an anesthetic gas mixture from the common gas

outlet.

Other monitors in the anesthesia workstation include pressure, volume, flow, and gas composition. Some also incorporate

airway gas flow monitoring. The breathing system lowpressure monitor alarm is automatically enabled when the ventilator

is turned on.

ALARM SYSTEMS

Contemporary workstations incorporate an integrated prioritized alarm system with visible and audible alerts when set

parameter limits are exceeded.

An important safety feature of all modern machines/workstations is the preuse checkout. In 1993, the U.S. Food and Drug

Administration (FDA) published anesthesia apparatus checkout recommendations. The machine should be checked by

an educated user. Item no. 1 on the FDA checklist is that an alternative means to ventilate the patient's lungs should be

present and functioning. Therefore, if a problem arises with the machine, the patient's lungs can be ventilated using a selfinflating

resuscitation bag (e.g., Ambu bag). If a machine problem arises and the cause/remedy is not immediately obvious, one should instinctively reach for the resuscitation bag and call for help. A recent analysis of the American

Society of Anesthesiologists Closed Claims Project data found that 35% of adverse outcomes were likely preventable if a

proper preuse checkout had been performed.

Recognizing that not all of the FDA 1993 checkout recommendations can be applied to many of the contemporary

workstations, in 2008, the American Society of Anesthesiologists published guidelines applicable to all anesthesia

delivery systems so that individual departments can develop their own workstation-specific preuse checkout that can be

performed consistently and expeditiously. The 2008 guidelines are intended to provide a template for developing checkout

procedures that are appropriate for each individual anesthesia machine design and practice setting. They discuss which

systems and components should be checked, the checkout interval (e.g., before first case vs. before every case), and who

may be responsible for performing each checkout procedure—the anesthesiologist or technician (Table 59.1). Examples

of user-developed workstation-specific checkouts are available on the American Society of Anesthesiologists'