Abstract
Cardiac surgery induces profound sympathetic nervous system and inflammatory responses. This so-called ‘stress response’ to surgery causes a multitude of adverse haemodynamic, metabolic, haematological, endocrine and immunological effects. In the setting of cardiac surgery, the attenuation of pain and sympathetic autonomic activity has many theoretical attractions. The introduction of high-dose opioid techniques into cardiac anaesthesia was based, in part, on the belief that they would inhibit the stress response. Failure to block the stress response completely, combined with equivocal evidence of clinical benefit and the need for prolonged postoperative mechanical ventilation, made the technique unpopular. The demonstration that thoracic sympathetic blockade improves the blood flow in severely diseased coronary arteries, the emergence of less invasive cardiac surgical techniques and economic pressures have prompted renewed interest in regional anaesthetic techniques.
Cardiac surgery induces profound sympathetic nervous system and inflammatory responses. This so-called ‘stress response’ to surgery causes a multitude of adverse haemodynamic, metabolic, haematological, endocrine and immunological effects. In the setting of cardiac surgery, the attenuation of pain and sympathetic autonomic activity has many theoretical attractions. The introduction of high-dose opioid techniques into cardiac anaesthesia was based, in part, on the belief that they would inhibit the stress response. Failure to block the stress response completely, combined with equivocal evidence of clinical benefit and the need for prolonged postoperative mechanical ventilation, made the technique unpopular. The demonstration that thoracic sympathetic blockade improves the blood flow in severely diseased coronary arteries, the emergence of less invasive cardiac surgical techniques and economic pressures have prompted renewed interest in regional anaesthetic techniques.
Thoracic Epidural Anaesthesia
The first report of thoracic epidural anaesthesia (TEA) for analgesia after cardiac surgery appeared in 1976. It was not until 1987, however, that the first report of TEA before cardiac surgery was published. In most published series the interspaces between C7 and T1 and T3 and T4 have been used for TEA. The higher approaches are technically easier, although it should be borne in mind that the ligamentum flavum in the thoracic region is thinner and more delicate than in the lumbar region. Most practitioners use a midline approach, with the conscious patient sitting or lying. The method used to identify the epidural space varies. The avoidance of the ‘asleep’ epidural in this setting is more a function of retaining the ability to assess the efficacy of the block before induction of anaesthesia than a response to concerns about medicolegal implications of neurological injury.
Typical initial doses include 3 ml lidocaine 2% and 5 ml levobupivacaine 0.5% with or without fentanyl ~25 μg. repeated after 10 minutes, as necessary. Regardless of the epidural infusion regimen used, it is imperative that a bilateral T1–T5 dermatome block is achieved before proceeding. A continuous epidural infusion (e.g. levobupivacaine ~0.125% + fentanyl 1–5 μg ml–1 ± clonidine 0.5 μg ml–1 at a rate of 4–10 ml h–1) is then started during surgery and usually continued for up to 3 days. The synergy between agents of different classes (i.e. opioids and local anaesthetics) permits the total dose and side effects of each to be reduced. Regular input from an acute pain management team maximizes epidural efficacy and allows early detection of adverse events. The ‘pros’ and ‘cons’ of TEA in cardiac surgery are shown in Box 38.1.
Cardiac sympathetic blockade
- Pro
Unmyelinated sympathetic neurones very sensitive to local anaesthetics
Blockade of sympathetic neurones from T1 to T5
Dilatation of severely diseased coronary arteries
↓ Incidence of postoperative arrhythmias
↑ Myocardial contractility – remains unproven
- Con
Risk of hypotension
May inhibit sympathetic vasodilatation in normal coronary arteries
Attenuation of stress response
- Pro
Local anaesthetics superior to epidural opioids alone
Attenuation of ↑ circulating catecholamine levels
BP and HR response to surgery blunted
Less effect on secondary metabolic, immune and haematological responses
- Con
Unequivocal evidence of stress response attenuation difficult to obtain
Analgesia
- Pro
Intense intraoperative and postoperative analgesia
Avoids adverse effects of parenteral narcotic analgesics
Early tracheal extubation and mobilization
Improved postoperative pulmonary function
Possible ↓ incidence of chronic pain syndromes
- Con
Unilateral block or missed segments render technique ineffective
Motor and proprioception block may limit mobilization
TEA is almost invariably used as an adjunct to general anaesthetic techniques, tailored to permit early recovery and neurological assessment. Recently some centres have been assessing the feasibility of using TEA alone in beating heart surgery. Apart from showing that it is possible to rise to this fresh challenge and the associated publicity, it is difficult to think of any other reason why anyone would want to routinely perform cardiac surgery on awake patients. The additional anxiety for both patients and staff is unnecessary, spontaneous respiration with open pleurae is physiologically undesirable, respiratory depression due to paralysis of the diaphragm or thoracic musculature may occur and TOE is impossible. Common sense suggests that the goals of anaesthesia for cardiac surgery should include the prevention of unanticipated patient movement during surgery.
The insertion of an epidural catheter prior to full anticoagulation is considered less taboo than in the past. Emerging evidence suggests that those most likely to benefit are patients with borderline pulmonary function, opioid addicts and patients likely to be incompletely revascularized by surgery. Recently published prospective studies suggest that TEA is associated with a lower incidence of postoperative respiratory tract infection, supraventricular dysrhythmias and renal dysfunction. No study yet published has had sufficient power to demonstrate any statistically significant reduction in perioperative mortality. Double-blind studies of TEA, with placement of a ‘non-therapeutic’ epidural catheter in control group patients, have not been undertaken.
Spinal Anaesthesia
The first report of spinal anaesthesia in cardiac surgery was published in 1980. In this report, as in the vast majority of subsequent publications, the agent used was preservative-free morphine sulphate. The potential benefits of spinal anaesthesia in cardiac surgery are the same as those for TEA, although the risk of epidural haematoma is probably less. Unlike TEA, however, spinal anaesthetic techniques in this setting have tended to be ‘single shot’ and opioid-based. The limited duration of drug action dictates that lumbar puncture has to be performed shortly before heparinization. The major safety concerns, therefore, are respiratory depression and neuraxial bleeding, although pruritus, nausea, vomiting and urinary retention are usually more troublesome. The low lipid solubility of morphine results in the delayed onset of analgesia and unpredictable effects. Although some investigators have demonstrated superior postoperative analgesia, others have reported either no benefit or delayed recovery. This may explain the failure of intrathecal morphine to attenuate the stress response to surgery.
In contrast, published investigations of intrathecal local anaesthesia in cardiac surgery are scarce. In a retrospective study published in 1994, Kowalewski et al. reported that the combination of hyperbaric bupivacaine (30 mg) and morphine (0.5–1.0 mg) produced excellent postoperative analgesia compatible with early (same-day) tracheal extubation. Lee et al. demonstrated that general anaesthesia combined with intrathecal bupivacaine (37.5 mg) resulted in the significant attenuation of the stress response and improved the LV segmental wall motion (Box 38.2). When compared to patients who had a ‘sham spinal’, study patients had significantly lower serum levels of epinephrine, norepinephrine and cortisol, and a significantly enhanced preservation of atrial β-adrenoceptor function. Unfamiliarity with the technique and the perception of haemodynamic instability may limit the widespread adoption of high-dose intrathecal bupivacaine and intrathecal morphine by cardiac anaesthetists. Although haemodynamic instability is cited as one of the major risks of high spinal anaesthesia, the physiological responses to total sympathectomy can be managed with small, titrated doses of IV vasoconstrictors.
Box 38.2 Technique used for delivering a high or total-spinal anaesthesia (adapted from Lee et al, 2008)
Patient pre-medicated with oral diazepam (0.1 mg kg–1) or oral gabapentin (1.5 mg kg–1)
IV volume repletion and loading accomplished with crystalloid 500 ml
Lumbar spine is prepared and draped with the patient in the lateral decubitus position
25-gauge pencil point spinal needle is used to administer the intrathecal blockade
Single-shot dose of 37.5–45 mg of hyperbaric bupivacaine 0.75%, in combination with 2–3 μg kg–1 (maximum total dose 300 μg) of preservative free spinal morphine injected into intrathecal space. Bevel of the spinal needle facing cephalad during injection may maximize local anaesthetic spread
Operating table placed in <5° Trendelenburg, with the patient in supine position. A C8 or higher sensory block can take ~10 minutes to develop
Small doses of IV phenylephrine and ephedrine are used to maintain a MAP of >65 mmHg
General anaesthesia induced, after complete cardiac sympathectomy achieved, using IV propofol 0.5–1.0 mg kg–1 and rocuronium 0.6–1.0 mg kg–1. IV narcotics may not be required for maintenance
To ensure amnesia and hypnosis, general anaesthesia must be maintained at a minimum of 0.5–1.0 MAC
Prior to chest closure, adjunctive analgesia can be provided using bilateral parasternal blocks
Prior to emergence, the patient is given IV morphine or IV hydromorphone to achieve an RR between 15 and 20 bpm
Trachea extubated in operating room immediately after procedure
Lee et al. examined the impact of high spinal anaesthesia on the perioperative inflammatory response in 2016 (Figure 38.1). Patients receiving high spinal anaesthesia in addition to general anaesthesia, compared to general anaesthesia alone, had significantly increased anti-inflammatory biomarker serum concentrations (interleukin-10). This small study suggests that patients exposed to high spinal anaesthesia during cardiac surgery may obtain an incremental benefit up to 28 days after surgery.
Figure 38.1 Interleukin-10 (IL-10) responses of patients receiving high spinal anaesthesia with general anaesthesia. Median values and P values are shown. High spinal plus general anaesthesia patients (open bars) versus general anaesthesia alone (solid bars) are shown.
Other clinical impacts of high regional anaesthesia for cardiac surgery may also include a reduction in postoperative delirium. In a retrospective, propensity-matched study, Petropolis et al. found that the incidence of postoperative delirium was significantly decreased in patients receiving high spinal anaesthesia when compared to controls.
High Spinal Anaesthesia and Organ Donor Harvest
In an animal model of donor organ harvest, Almoustadi et al. examined the effects of regional anaesthesia for cardiac surgery after brainstem death. Total spinal anaesthesia as a supplement to general anaesthesia in a large-animal model of heart donation after brainstem death resulted in reduced catecholamine release and improved cardiac function of the donor animal. Since haemodynamic collapse is often observed after brainstem death in humans, it is intriguing to consider that perhaps the administration of high spinal anaesthesia prior to organ harvest may be of some future benefit to donor organ functional preservation.
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