7.5.3 Anaesthetic monitoring during ECC

Although management of the bypass is the perfusionist ‘s responsibility, the anaesthetist has many monitoring and collaborative tasks to perform during on pump time (Table 7.6) [1].

• Anaesthesia management; drugs are injected directly into the ECC circuit. The infusions are connected to the central line or on the atrial line of the Swan-Ganz catheter, because they can only be connected to the circuit via a valve mounted in advance on the venous tubing or reservoir. The connection to the venous reservoir is technically simple, but it does not ensure a continuous flow because the reservoir level constantly oscillates. The best solution is therefore to add a 3 way cock to the venous line at the reservoir outlet. Depending on the position of the central line in relation to the venous cannulae, the delay in action may take up to several minutes.
• Maintaining sleep; this is a critical and delicate issue during bypass surgery. The dilution of circulating substances by the perfusate of the bypass machine and the sympathetic stimulation of cooling can easily wake up the patient at the beginning of the pump. The clinical criteria for anaesthesia no longer apply during this phase of the procedure: the haemodynamics are regulated by the pump, the heart and lungs are out of circuit, sweating is related to rewarming, the pupils are miosis due to fentanil or mydriasis due to cold, and if deeply curarised it is impossible to notice patient’s awakening [4]. It is therefore important to be able to observe the small movements that precede awakening and to administer curare only in situations where it is necessary (see below), or to use sleep monitoring such as the BIS™ (value 40-60); unfortunately, the latter is unreliable and fails in hypothermia (see Chapter 6 Bispectral Index). Brain activity monitoring indicates that 9% to 20% of patients, depending on the investigative technique used, have a value consistent with inadequate sleep [8]. Careful observation of venous oxygen saturation (drop in SvO₂ ) also provides information about possible awakening. For these reasons, the rate of intraoperative awakening and recollection is highly variable in cardiac surgery (0.1-2.3% of cases), but much higher than in general surgery [7]. Halogen vaporiser (isoflurane or sevoflurane) provides sleep by continuously delivering 1.0-1.5 MAC of the substance to the oxygenator gas circuit; this is the best routine for coronary revascularisation surgery (benefit of preconditioning), but there are some problems with its management (see Ventilation on bypass and Specific agents). Propofol infusion (5mg/kg/hr) and midazolam iterative bolus (5mg) or infusion (2mcg/kg/hr) can replace halogen in other types of surgery. Below 30°C, hypothermia alone ensures sleep since 30-32°C is the brain temperature at which consciousness is lost.

• Ventilation; there are three points to consider [6].
  • When the aorta is clamped, the absence of pulmonary circulation warrants a complete shutdown of the ventilator; maintain a passive flow of O₂ /air mixture (1 L/min, FiO₂3-0.5) at a pressure < 5 cm H₂ O (the bag in the breathing circuit must remain deflated).
  • When aorta is declamped, heart beats and maintains some pulmonary circulation, so it is wise to maintain reduced ventilation by adjusting, for example, the ventilator parameters to half of what would be the beating heart values (FiO₂5, VC 3-4 mL/kg, rate 5-8 cycles/min).
  • Vital capacity manoeuvres (30-40 cm H₂ O for 20-30 seconds) are essential at the time of resumption of ventilation; the surgical field is carefully monitored during these manoeuvres to avoid obstructing the operator or exerting deleterious traction on the mammery graft(s) [3].
• Curarisation; preferably reserved for the following situations: left cardiotomy, spontaneous diaphragmatic movements, deep hypothermia with circulatory arrest, low flow accompanied by venous desaturation, shivering on warming. A relaxograph can be used to control the intensity of these. Sub-curarisation improves the possibility of to diagnose momentary arousal.
• Analgesia; pain stimulation is very low during ECC as long as the rib cage is not manipulated. Opiates are useful to inhibit sympathetic and parasympathetic hemodynamic reactivity. Their injection is mandatory as soon as the retractor is released and chest/pericardial drains are inserted. In addition, opiates are only weakly hypnogenic.
• Maintain MAP at 50-70 mmHg, in principle within the range of cerebral selfregulation, but this varies between individuals (see Haemodynamics). Pressure regulation should not be at the expense of flow [6]. MAP can be momentarily lowered to 40 mmHg in hypothermia (≤ 28°). Vasodilation allows for better heat exchange on rewarming. However, some degree of vasoplegia is common on pump, and the hypotension it causes can become dangerous if prolonged because the perfusionist cannot compensate by increasing flow. If 100 mcg boluses of neosynephrine are insufficient to correct the situation, a norepinephrine infusion is started on the central line or on the right atrial line of the Swan-Ganz. A hypertensive attack is controlled by boluses of phentolamine (1 mg), nicardipine (0.3-1 mg) or nitroglycerin (50-100 mcg); isoflurane (2-4%) helps to regulate hypertension. Nitroprusside is reserved for refractory cases, after ascertaining that systemic arterial resistance is indeed excessive using the formula: SAR = 80 - (MAP/DP) where DP is the pump rate; in hypothermia, the risk of thiocyanate intoxication is high because the activity of the hepatic rhodanases responsible for the biotransformation of nitroprusside is inhibited [5].
• Flow monitoring: 2.4 L/min/m² in normothermia, lowered to 1.8 L/min/m² at 28° and 1.5 L/min/m² at 25°C (see Haemodynamics). The frequency of oscillations due to the pump rollers on the arterial curve provides information about the pump output. The SvO₂ measured in the venous circuit of the ECC provides information on the adequacy of the flow to body needs (SvO₂ ≥ 70%). Maintaining flow rate has priority over maintaining pressure.
  • SaO₂ (on ECC): monitor the oxygenator, not the patient.
  • SpO₂ : usually unreadable due to depulsation and vasoconstriction, frequent artefacts.
  • SvO₂ on venous return from the bypass: measures the adequacy of O supply and consumption₂ ; it represents tissue perfusion.
  • ScO₂ (brain saturation): assesses O₂ supply to the brain and reflects the body's tissue DO₂ ; drop of > 20% of baseline value: risk of neurological and polyorganic damage
  • SvO₂ of the Swan-Ganz catheter: irrelevant when pulmonary flow is minimal or zero, but correlates well with the DO₂ /VO₂ adequacy of the body in case of partial bypass (declamped aorta).
• Monitoring of the face and pupils.
• ECG monitoring; resumption of electrical activity, often disordered, requires administration of
• Transoesophageal echocardiography (TEE) monitoring (see Place of TEE).
• Transfusions; the increase in viscosity when cold determines the desired Ht, which should ideally have the same value in % as the temperature in °C. It is preferable to avoid an Ht < 25% (see Priming). Thresholds for transfusion on bypass: Ht ≤ 21% in simple cases, ≤ 25% in high risk cases.
• Blood glucose monitoring; insulin infusion if blood glucose persists > 10 mmol/L.
• Kalemia; hyperkalemia is common because of the potassium in the cardioplegia.
• Urine output is highly variable in bypass surgery and does not correlate with postoperative renal function; the most predictive factors are bypass duration and preoperative renal function.

Temperature management by the perfusionist during ECC  follows a number of guidelines that  the anaesthetist must know [2].

• During cooling, the temperature gradient between the inlet and outlet of the heat exchanger should never exceed 10°C.
• During rewarming when the temperature is < 30°C, the temperature gradient between the inlet and outlet of the heat exchanger must never exceed 10°C.
• During warming up when the temperature is > 30°C:
  • The temperature gradient between the inlet and outlet of the heat exchanger must remain ≤ 4°C;
  • The heating rate should remain ≤ 0.5°C/min.
• The temperature of the blood leaving the heat exchanger should never exceed 37°C to avoid cerebral hyperthermia.
• The temperature gradient between the water in the heat exchanger and the blood should never exceed 10°C, and the water temperature should not fall below 12°C when cooling and not exceed 38°C when heating.
• The gradient between rectal/vesical and oesophageal temperature should remain below 10°C; rectal or bladder T° is 2-4°C below brain temperature during rewarming.

  © CHASSOT PG, GRONCHI F, April 2008, last update, December 2019

References

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2. ENGELMAN R, BAKER RA, LIKOSKY DS, et al. The Society of Thoracic Surgeons, the Society of Cardiovascilar Anesthesiologists, and the American Society of Extracorporeal Technology: clinical practice guidelines for cardiopulmonary bypass - Temperature management during cardioplmonary bypass. J Cardiothorac Vasc Anesth 2015; 29:1104-13
3. GARCIA-DELGADO M, NAVARETTE-SÀNCHEZ I, COLMENERO M. Preventing and managing perioperative pulmonary complications following cardiac surgery. Curr Opin Anesthesiol 2014; 27:146-52
4. GHONEIM  MM, BLOCK RI, HAFFARNAN M, et al. Awareness during anesthesia; risk factors, causes and sequelae: a review of reported cases in the literature. Anesth Analg 2009; 108:527-35
5. MOORE RA, GELLER EA, GALLAGHER JD, et al. Effect of hypothermic cardiopulmonary bypass on nitroprusside metabolism. Clin Pharmacol Ther 1985; 37:680-3
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7. RANTA SO, HERRANEN P, HYNYNEN M. Patients conscious recollections from cardiac anesthesia. J Cardiothorac Vasc Anesth 2002; 16:426-30
8. TIREN C, ANDERSON RE, BARR G, et al. Clinical comparison of three different anaesthetic depth monitors during cardiopulmonary bypass. Anaesthesia 2005; 60:189-93