The anaesthetic management of patients with thoracic ascending aortic aneurysms: A review

Introduction

Two of the greatest scientific minds of the 20th century were affected by thoracic aortic aneurysms, and this condition remains prevalent today. In December 1948, Albert Einstein, the renowned scientist, underwent surgery for a large aortic aneurysm by Rudolph Nissen MD, in New York. Nissen wrapped the anterior portion of the aneurysm with cellophane, but he did not try anything further, as aortic mobilisation was considered forbidden at the time. Einstein eventually died of a ruptured aneurysm six years later at the age of 76. Then, in December 2005, 97-year-old Michael DeBakey MD, a pioneering cardiac surgeon, was alone at home preparing a lecture when he experienced symptoms that he himself recognised as those of an acute aortic aneurysm dissection. He initially refused anaesthesia and surgery. His situation deteriorated, and his wife demanded that her husband have surgery. The Ethics Committee at the Methodist Hospital in Houston, Texas approved the planned surgery, and it indeed took place. Dr DeBakey’s postoperative recovery was prolonged and complex, requiring a tracheostomy, prolonged mechanical ventilation, haemodialysis, parenteral nutrition, and extended rehabilitation. In retrospect, Dr DeBakey publicly acknowledged that he was happy that he underwent the surgical procedure. He died of natural causes in July 2008 (Silvay & Castillo 2015). The repair of aortic aneurysm is now common practice, but the management of such cases poses challenges for anaesthesia, surgery, perfusion, and monitoring (Silvay 2009, Tolis & Sundt 2016).

As stated by Drs Tolis and Sundt in 2016, the ultimate medical goal in the management of Type A aortic dissection (TAAD) ‘is an alive patient’ (Kuzmik et al 2012). Thoracic aortic aneurysms are frequently silent until catastrophe strikes, and over 95% of patients with thoracic aortic aneurysms have no idea of their condition and are undiagnosed (Kuzmik et al 2012, Silvay 2009). The management of thoracic aortic aneurysms remains challenging in both the elective and emergent setting (Olsson et al 2006). The mortality of ruptured thoracic aortic aneurysms approaches 100% (Johansson et al 1995), and it can be a difficult and complicated decision whether or not to operate on a patient of whom thoracic aortic dissection has been diagnosed. Clinician opinions differ on when to initiate aggressive surgical procedures, and these decisions hold tremendous consequences for patients. The incidence of thoracic aortic aneurysms has been estimated to be somewhere around 10–30 in 1000, although studies reporting this figure define thoracic aortic aneurysms as greater than 5cm, which is already a large aneurysm (Itani et al 2002, Kälsch et al 2013). It is highly probable that the reported incidences of thoracic aortic aneurysms are underestimates, as fatal thoracic aortic aneurysm ruptures are sometimes misdiagnosed as myocardial infarctions (Kuzmik et al 2012). No two aortic aneurysms are the same, and it is critical to understand the aetiology and management of this phenomenon, especially as the management of thoracic aortic aneurysms changes over time (Stehbens 1958). Currently, efforts to stent the aortic arch are being trialled in numerous international healthcare facilities around the world. Patients selected for this procedure are considered high risk for conventional open arch replacement (Zoller et al 2019).

Major milestones in the development of the contemporary clinical model for the management of thoracic aortic aneurysms include one-lung ventilation (OLV), cardiopulmonary bypass, haemodilution, hypothermic circulatory arrest, cerebral perfusion, as well as advances in anaesthesia, pharmacology, and monitoring, including echocardiography (Gutsche et al 2014). These multiple clinical advances in surgery and anaesthesia facilitated the development of techniques for the reconstruction of the ascending aorta and aortic arch following dissection (Cooley & De Bakey 1956, Dubost et al 1952, Gibbon 1953).

Historical notes

The dilatation of arteries was first recognised as a disease of the cardiovascular system in Egypt in the 1550s BC, and the first to describe an abdominal aortic aneurysm was the Flemish physician Vesalius about a millennium later in 1555. Afterward, the English surgeon John Hunter began to treat aneurysms of the peripheral vessels by ligation (Livesay et al 2005). In 1948, accompanied by the advancement of other therapeutic modalities Rudolph Nissen MD (the aforementioned physician that treated Einstein), used a wrap with cellophane to treat the easily accessible portions of the aortic aneurysm with minimal mobilisation (Cervantes 2003). In 1951, Charles Dubost in Paris performed the first successful resection of abdominal aortic aneurysm, utilising a 15cm long cadaver aortic homograft (Dubost et al 1952). A new era of surgical treatment for aortic aneurysms was launched with the introduction and clinical application of cardiopulmonary bypass by Gibbon (1953) just two years later (Appoo et al 2006, Augoustides et al 2013, Belway et al 2011). The importance of the development of the cardiopulmonary bypass technique is hard to overstate; it held tremendous ramifications for the treatment of aortic aneurysms, and this technique is still widely utilised today. Another notable development included the use of aortic allografts as aortic replacements, which began in 1950. Dacron was introduced by Dr DeBakey (De Bakey et al 1964), and later, a combined operation to replace the ascending aortic aneurysm as well as the aortic valves, which was accompanied by the reimplantation of the coronary arteries, was performed by Drs Bentall and De Bono in 1968 (Cooley 2012, Stehbens 1958). In 1990, the first successful endovascular aneurysm repair was completed, and in 1992, the first successful thoracic endovascular aortic repair was achieved. A timeline with some of the most significant events pertaining to thoracic aortic aneurysms can be seen in Figure 1.

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Figure 1

Timeline of accomplishments in the treatment of thoracic aortic aneurysms. EVAR: endovascular aneurysm repair; TEVAR: thoracic endovascular aortic repair

Understanding thoracic aortic aneurysms

The incidence of thoracic aortic aneurysms in the United Kingdom is hard to estimate, and the prevalence of asymptomatic thoracic aortic aneurysms has been estimated to be anywhere from 0.16 to 0.34% (Itani et al 2002, Kälsch et al 2013). TAAD is specified in the Stanford classification as a dissection of the ascending aorta, regardless of the distal extent. Around 60% of all aortic aneurysms are located in the ascending aorta. The principal causes of death due to TAADs are dissection and rupture. Overall, pooled hospital mortality from a recent systematic review and meta-analysis demonstrated that hospital mortality for all surgical repairs of TAADs was 11.9% (Melissano & Chiesa 2018, Smith et al 2017). Patients with thoracic aortic aneurysms pose clinical challenges that require cooperation between cardiologists, surgeons and anaesthesiologists. Aneurysms of the ascending aorta, in an acute setting, may necessitate prompt surgical repair in order to prevent rupture with catastrophic complications (Silvay & Flynn 2009). Diagnosis and the early treatment of TAADs are of great importance, although most cases are diagnosed by routine chest X-ray or scanning for some other medical problem (Benumof 1994, Elefteriades 2008, Silvay 2009). An artistic illustration of a thoracic aortic aneurysm is presented in Figure 2.

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Figure 2

An artistic rendering of an aortic aneurysm located superior to the left mainstem bronchus as well as a computed tomography scan of the same (Goodwin et al 2013)

Aetiologies of TAADs include hypertension, atherosclerosis, connective tissue disorders, trauma, infection, and previous cardiac or vascular surgery. The inherited disorders associated with TAADs include aortopathies associated with Marfan syndrome, Ehlers–Danlos syndrome, Loeys-Dietz syndrome, and the bicuspid aortic valve (Czerny et al 2018).

In the acute setting, management of a TAAD may be urgent and require immediate surgery, with or without optimal medical stabilisation. Symptomatic patients with leaking aneurysms, for example, require urgent treatment, and there is generally little time to perform more than the most basic preoperative assessment. In less urgent situations, however, preoperative evaluation prior to elective open thoracic aortic surgery is of paramount importance, and one must take the time to consider nearly every organ system. In both emergent and elective surgical procedures, expert anaesthetic management is required for the prevention of intraoperative hypertension and aortic rupture. The anaesthetic approach to the patient with a TAAD depends on the urgency of surgical intervention. One of the most difficult decisions facing patients and physicians is whether or not to perform surgery when a thoracic aortic aneurysm is diagnosed, and in many cases, expert consensus rather than evidence-based recommendations is followed (Elefteriades 2008). Elective surgery for non-urgent cases may provide the best results. In cases in which surgery is not recommended, patients are followed over time and evaluated periodically until the potential benefits of surgery outweigh risks (Flynn et al 2009, Silvay & Stone 2006).

Close communication between the surgeon, anaesthesiologist and perfusion team is required before and during surgery for a TAAD, integrating strategies relating to patient positioning, OLV, cannulation, monitoring, and anticoagulation management. The preoperative care team also consists of an emergency medicine physician in emergent cases, a radiologist who can assist in diagnostic confirmation of aortic aneurysms, other surgical consultants, and cardiologists. The perioperative team includes the head cardiac surgeon, one or two junior surgeons or physician assistants, an attending cardiac anaesthetist as well as a cardiac anaesthesia fellow or resident, a surgical technician, a circulating nurse, and a perfusionist. Postoperatively, the patient should be followed by cardiology and be seen by the in-hospital physical therapist and occupational therapist. Depending upon a patient’s functional status following the procedure, they can either be discharged home or be sent to subacute rehabilitation or long-term rehabilitation.

The anaesthetic approach to the open repair of a TAAD must account for the aneurysm’s severity, including its size and location, aetiology, as well as general condition of the patient (Chiesa et al 2009, Grande & Ganter 2018, Guo et al 2018). Detailed information pertaining to the extent and location of aneurysm, as well as the functional status of the heart and coronary anatomy, is critical. Most patients undergo coronary angiography before the surgery. Preoperative pulmonary work-up, neurological examination, and coagulation studies may also provide important information (Castillo et al 2011, Silvay & Stone 2006). The type of repair is dictated not only by location and extent of the disease, but also the general condition of the patient and the underlying aetiology of the TAAD (Yan et al 2014).

TAAD and anaesthetic management: An overview

TAAD is a severe, life-treating disease with a high overall mortality when untreated. Mortality rate is up to 50% if surgery is delayed beyond 48h with the average mortality rate of 1–2% per hour in this critical period (Criado 2011). The anaesthetic management and surgical treatment of TAAD is challenging, especially for elderly patients. Early diagnosis and stabilisation, prompt referral to an aortic centre, and early operation significantly reduce mortality (Mathur et al 2016, Guo et al 2018). The preoperative phase should be abbreviated.

The anaesthetic management in these complex procedures is typically a regimen titrated to adequate analgesia (with a synthetic opioid), amnesia (with a volatile inhalational agent or benzodiazepine), and neuromuscular blockade (with a vecuronium, pancuronium, or rocuronium). An appropriate titration of these anaesthetics is critical in all cardiovascular procedures, but it deserves particular attention in a surgery as delicate and complex as TAAD repair. Prevention of hypertension during surgery is the main role of anaesthetic management.

Standard ASA monitoring is required for this procedure, as well as both arterial and venous access, bladder catheterisation, transoesophageal echocardiography, multiple available locations for temperature measurements, jugular vein access, and cerebral function monitoring with oximetry (Campos & Feider 2018, Silvay 2009, Silvay & Castillo 2015, Silvay & Stone 2006). Open surgical repair for a TAAD may require interruption of cerebral perfusion. Deliberate hypothermia is induced with the aid of cardiopulmonary bypass to protect viability of the brain during period of elective circulatory arrest. Periods of deep hypothermic circulatory arrest permit surgical reconstruction of the aortic arch (Augoustides et al 2012, 2013, Griepp et al 1975, Guo et al 2018, Yan et al 2013). The issues of bleeding, haemostasis, and blood transfusion are very important in aortic arch surgery, and a recent comprehensive update on this topic is covered in a paper by Raphael et al (2019).

A cost analysis between endovascular versus open repair in the treatment of thoracic aortic aneurysms found that in-hospital costs were significantly greater in the endovascular group, at $52,008 versus $37,172. Interestingly, however, when modelling the costs using reported complication and intervention rates extracted from the literature, a higher cost for the open group was seen in-hospital ($55,109 versus $48,006) and also after a period of three years ($58,426 versus $52,825) (Gillen et al 2015). The open-heart procedure usually takes around 4–6h and sometimes up to 8h. Patients are generally maintained under anaesthesia for an additional 4–6h. Endovascular repair generally takes around the same amount of time.

Ventilatory modalities

OLV is the preferred modality for the open repair of a thoracic aortic aneurysm. OLV provides numerous benefits and serves as the ideal choice for the repair of a thoracic aortic aneurysm, including providing optimal space in the left chest cavity for surgical manipulation (Campos & Feider 2018, Carlens 1949, Hickey 2017). OLV can help protect the dependent lung from secretions, such as blood, pus, or lavage fluids. The use of fibreoptic bronchoscopy is essential. Lung protective ventilation including low tidal volume, low peak airway pressures, high positive end-expiratory pressure (PEEP), and a lower fraction of inspired oxygen (FiO₂) is an important aspect of anaesthetic management (Campos 2007, Cohen 2001, Goodwin et al 2013, Weigel & Hoaglan 2013).

OLV can be performed with either the intubation of the left or right mainstem bronchus (Slinger 2019). A diagram of right-sided versus left-sided double-lumen tubes can be seen in Figure 3. In general, the use of right-sided double-lumen tubes is more complicated because of the anatomic variability of the right mainstream bronchus, as well as the possibility of right upper lobe-obstruction, which can lead to poor ventilation of this lung segment (Campos et al 2000). The right main bronchus is shorter than the left main bronchus, and theoretically there is an increased risk of upper-lobe obstruction with a right-sided double-lumen tube (Yu et al 2018). Thus, left-sided double-lumen tubes grant anaesthesiologists a wider safety range in placement. In addition, in as many as 3% of the population, the right upper-lobe originates at the carina or even the trachea, meaning right-sided double-lumen tubes may be particularly difficult to place in some patients (Ikeno et al 1996). In general, for most cases, the left-sided double-lumen tube is utilised. This leads to most anaesthesiologists preferring a left-sided double-lumen tube during thoracic surgery. It is generally not as important what type of tube will be used as it is that an anaesthesiologist is experienced with utilising the selected tube. The use of double-lumen tubes is superior to the use of bronchial blockers, since double-lumen tubes offer the possibility of more effective suctioning. Bronchial blockers also achieve lung collapse more slowly than double-lumen tubes, and they have a tendency to dislodge more easily. The only possible disadvantage of using a left-sided double-lumen tube is rupture of an aortic aneurysm during manipulations and positioning of the tube (Alliaume et al 1992, Fitzmaurice & Brodsky 1999).

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Figure 3

The correct positioning of a double-lumen tube. Source: Modified from Butterworth et al (2018)

Intraoperative physiological concerns during OLV

During OLV in lateral decubitus position, a redistribution of pulmonary perfusion occurs in which the dependent lung receives significantly more perfusion. This positioning leads to shunt, which is about 20–25% of cardiac output (Dunn 2000). Gravity and surgical manipulation contribute to decreased perfusion in the collapsed lung, as well as the extent of pulmonary disease in the non-dependent lung. In addition, over one half of the tidal volume is delivered to the non-dependent lung while an anaesthetised and paralysed patient is in the lateral decubitus position (Lohser & Ishikawa 2011b). While under anaesthesia, there is a reduction in the functional residual capacity of the dependent lung as well as a decrease in its compliance (Carramiñana et al 2019). V/Q mismatch occurs in this setting because the non-dependent lung is actually better ventilated and its functional residual capacity is greater than the dependent lung. Separation of the lungs exacerbates the V/Q mismatch that is inherent to anaesthesia, due to a large right-to-left intrapulmonary shunt. The dependent lung becomes better perfused, while the non-dependent lung becomes better ventilated (Dunn 2000, Grichnik & Clark 2005). This scenario leads to hypoxemia, which can be further worsened by lung disease or vasculopathy (Pereira et al 2018). The fractional blood flow to the non-dependent lung in OLV drops in one-lung anaesthesia, and the dependent lung sees an increase in fractional blood flow. Oxygenation during OLV must be optimised (Hickey 2017).

While lower tidal volumes are linked to a decreased incidence of volume trauma, there is a concern about adequate oxygenation in this setting (Karzai & Schwarzkopf 2009, Lohser & Ishikawa 2011a). Without PEEP, OLV with low tidal volumes has been shown to provide inadequate oxygenation. During OLV, the dependent lung is at particular risk of alveolar collapse during expiration, and thus PEEP is especially recommended for OLV (Neto & Schultz 2017). In addition, the initiation of high-pressure inspiration for a short period of time accompanied by PEEP has been demonstrated to prevent alveolar collapse and improves arterial oxygenation during OLV (Karzai & Schwarzkopf 2009). Some cases of hypoxemia that are not severe can be managed with continuous positive airway pressure (CPAP) with high oxygen delivered to the operated lung. This can help maintain the patency of alveoli and enhance oxygenation. CPAP to the non-dependent lung will increase pulmonary vascular resistance and redirect shunted blood to the ventilated lung (Roze et al 2011).

Minute ventilation is an important issue in OLV, and permissive hypercapnia is essential in the management of acute lung injury. Tidal volumes and ventilatory pressures can be lowered by decreasing minute ventilation, and this can prevent volume trauma and barometric trauma in OLV (Slinger 2017). In addition, a similar protective strategy employing decreased minute ventilation has been found to reduce the incidence of acute lung injury (Lohser & Slinger 2015).

Immediate postoperative care

A chief concern in the immediate postoperative period in patients having undergone OLV for the repair of a thoracic aortic aneurysm is adequate hemodynamic and respiratory control. The choice between a single or double-lumen tube in the immediate postoperative setting should not have a major impact on mechanical ventilation. Double-lumen tubes, used in the intraoperative setting, have been suggested to provide greater resistance to mechanical ventilation due to their two-lumen nature and therefore confer a more prolonged weaning period (Campos 2007, Telias & Ferguson 2019). Taking into account a patient’s overall clinical stability and respiratory parameters, the double-lumen tube can be exchanged for a single-lumen tube in the operating room, but in other situations, a patient can be transferred to the cardiac intensive care unit with the double-lumen tube still in place.

Summary for anaesthesiologists: OLV for surgical repair of thoracic aortic aneurysm

OLV provides optimal visualisation and reduces retractions related to mechanical trauma that may have occurred to the left lung during surgery.

Conclusions

The management of a patient with a thoracic aortic aneurysm poses unique clinical challenges, and we describe the separation of the lungs as well as anaesthetic management. The implementation of modern anaesthetic management will improve efficiency and safety throughout the intraoperative and postoperative period.

No competing interests declared