Care for the patient with aortic dissection

Though aortic dissection is most commonly the domain of vascular surgeons and interventional radiologists, it does occasionally involve the arch of the aorta, and the management for both conditions involves the tight control of haemodynamic variables. In short, it seemed reasonable to put this chapter into the cardiothoracic section. The most useful parts offered below are the management of the Type A dissection back from a recent aortic root repair,  the conservative medical management of an uncontrolled Type B dissection, and the possible reasons one might resort to surgery in the latter.

In short:

Up until Question 13 from the second paper of 2021, this has not come up in the fellowship exam, except as a question regarding diagnostic modalities: Question 13 from the second paper of 2010. LITFL have an excellent summary of this topic, in succinct point form and complete with recent references. As the diagnostic steps have already been covered elsewhere, this section will focus on the potential complications and management strategies. The canonical source for this information would have to be the most recent iteration of the ACCF/AHA Guidelines for Diagnosis and Management of Patients With Thoracic Aortic Disease.

Classification of aortic dissection

The first classification of aortic dissections was fashioned by Michael DeBakey and his colleagues. They had the distinction of being the first to successfully perform a surgical repair in 1955, using a lateral thoracotomy approach. Some excellent pencil art accompanied this paper:

After ten years of experience, this group (DeBakey et al, 1965) classified "dissecting aneurysms of the aorta" into three major subtypes:

• Type 1: Entire aorta affected
• Type 2: Confined to the ascending aorta
• Type 3: Descending aorta affected distal to left subclavian artery

This classification was mainly directed by the difference of the surgical approach used by these pioneers. Type 1 was recognised as having "a tendency ... to progress rapidly toward a fatal termination", and urgent surgery was recommended. The authors also described their (at that time, novel) technique of transecting the aorta on bypass and repairing the dissection by approximating the inner and outer walls. Type 2 required the replacement of the entire ascending aorta with a graft, and Type 3 required the replacement of only the descending segment. 

The main criticism of this classification was that the distal extent of the dissection (i.e. whether it extends more distally than the left subclavian) appeared to have little effect on the mortality, whereas the proximal extent was very important. People with dissections of the ascending aorta were sicker, and tended to die. People with predominantly descending aortic involvement tended to have less invasive surgery, and largely survived (as both DeBakey Type 1 and Type 2 responded to basically the same sort of surgical approach, and required a median sternotomy to repair, whereas Type 3 could be done with a thoracotomy). On the basis of this, Daily et al (1970) came up with the Stanford classification (Stanford not because of a co-author named Stanford but named after Stanford University Hospital where the co-authors worked). They simplified the dissections into either Type A (involving the ascending aorta) and Type B (the rest). The point of separation was designated as the left subclavian artery: any dissection distal to this point is Type B. On this, there is a difference of opinion between radiologists and surgeons; according to Lempel et al (2014), 

"Many radiologists classify any dissection involving or extending proximal to the left subclavian artery as a type A dissection whether or not it involves an arch vessel. ...However, cardiovascular surgeons believe that only dissections originating proximal to the innominate artery (ie, only those involving the ascending aorta) are appropriately classified as type A dissections"

Other classification systems have been attempted.  Miller et al (1979) further subdivided Stanford A and B into "acute" and "chronic", an acute dissection being one that has been around for less than 2 weeks. Aortic dissection, intramural haematoma and penetrating atherosclerotic ulcer all form a spectrum of related disorders grouped as "acute aortic syndromes", and Svennson et al (1999) tried to represent these in a separate classification system. 

Stanford Type B dissection can be further subclassified as "complicated" or "uncomplicated". This is something of a subjective assessment, as the thing that separates one from the other is the presence of end-organ complications. Moulakakis et al (2014) describes a "malperfusion syndrome involving visceral, renal, or extremity ischemia, rupture or impending rupture, uncontrolled hypertension, persistent abdominal or chest pain, or findings of rapid expansion on computed tomography (CT) imaging". The main reason for this additional classification system is to define the role of surgical management. Uncomplicated dissections can be managed with good blood pressure control, whereas complicated dissections usually require some sort of repair, usually endovascular.  

Risk factors:

To paraphrase Table 9 from the ACCF/AHA guidelines:

• Conditions associated with increased aortic wall stress:
  • Hypertension
  • Pheochromocytoma
  • Cocaine
  • Weight lifting
  • Deceleration or torsional injury
  • Coarctation of the aorta
• Conditions associated with some sort of structural weakness of the aortic wall:
  • Marfan syndrome
  • Ehlers-Danlos syndrome
  • Bicuspid aortic valve (including prior aortic valve replacement)
  • Chronic corticosteroids
  • Infections involving the aortic wall
• Inflammatory vasculitis ("Aortitis")
  • Takayasu
  • Giant cell arteritis
  • Behçet arteritis 
• Iatrogenic
  • IABP counterpulsation
  • Coronary angiography
• Random and misc
  • Pregnancy
  • Polycystic kidney disease

Clinical features

Chest pain is basically standard. The classical textbook description of the abrupt onset chest pain which is described as tearing and which radiates to the back and shoulder blades is actually strongly represented among emergency presentations with aortic dissections, and only about 10% are painless (Ayrik et al, 2006). From an excellent overview by Bossone et al (2018), the following list of features was assembled:

• History:
  • A risk factor or two might be present (see above)
  • They will often be elderly, usually in their seventies
  • They will often be male
  • Their chest pain will be "classical"- tearing and radiating to the back
  • The pain does not have to be in the chest: it could be in the abdomen or a leg
  • End-organ dysfunction may be the dominant presenting feature, for example something like paraplegia from spinal cord infarction, or 
• Examination findings:
  • Hypertension is not mandatory; hypotension and shock may actually be the main haemodynamic feature, as a Type A dissection could extend into the coronaries or create a tamponading pericardial effusion 
  • There may be clinical features of associated LV hypertrophy
  • A diastolic murmur of aortic regurgitation may be present
  • The diastolic pressure may be dismally low, if the Type A 
  • There may be a distinct difference in blood pressures taken from each limb, and some of the limbs may in fact be pulseless
  • The patient may have presented with features of stroke 
  • Venous engorgement may be observed, which could be due to obstructive shock, heart failure, or due to the enlarged aorta compressing the SVC.
• Investigations and imaging
  • Bloods may reveal anaemia, a raised CK, or evidence of end-organ damage like deranged LFTs or a raised creatine and urea.
  • D-dimer is apparently highly sensitive: in that a low D-dimer virtually rules out the possibility of an acute aortic dissection.
  • ABG could have something nonspecific like a raised lactate
  • ECG may demonstrate ischaemia or LVH
  • CXR may demonstrate mediastinal widening
  • TTE may demonstrate an effusion or tamponade
  • TOE should be able to image the false lumen
  • CT angiography is the gold standard, as it will comprehensively assess the aorta and all of its branches, identify a pericardial effusion, and define the threatened organs.

Complications of aortic dissection

It would surprise nobody to learn that the sudden structural failure of the main blood supply channel in your body could have some major consequences. A few:

• Death

  • About 40% die immediately. They are underidentified, as few are subjected to autopsy.
  • About 1% die per hour thereafter.
  • About 5-20% will die shortly after their definitive surgery.
• Organ failure
  • Cardiac failure due to aortic regurgitation
  • Cardiac failure due to cardiac tamponade
  • Myocardial ischemia or infarction
  • Ischemic stroke
  • Paraplegia due to spinal ischemia
  • Aortopulmonary fistula with haemorrhage
  • Mesenteric ischemia
  • Renal ischemia
  • Limb ischemia

Medical management of aortic dissection

The medical therapy for aortic dissection is probably of greatest interest to the intensivist, as we tend to be caring for the patient before they go for surgery.  And when surgery is delayed, deferred, or where a conservative  course of management is embarked upon, the medical therapy becomes all-important.

The most critical issue is to prevent the propagation of the dissection. The intima is hydrodissected into by two main properties of the flowing blood: the force imparted on the wall of the aorta by each ventricular contraction, and by the pressure of the blood, which is actually exerted mainly in diastole. Of these, the former seems to be more important for dissections, at least on the basis of mathematical modelling (Attinger, 1964). This is the force exerted by the left ventricular pulse, which is a product of mass and acceleration. Specifically, the term impulse is usually seen in the literature, which is a term borrowed from physics by Rushmer (1964), and which describes the product of force and time. The mass of the blood is not something you can do much about, but acceleration and time are definitely amenable to medical control. These are defined by the contractility of the left ventricle, often expressed in the literature as dP/dT. 

From this, it follows that, to decrease the rate of propagation of a dissection flap, we should aim to decrease dP/dT. This was recognised very early by Wheat et al (1965), who described the rationale and practical use of contractility-targeting antihypertensive therapy in acute aortic dissection. Apart from purely theoretical physiological observations, the support for their thesis came from a comically random source. Apparently, some weird selective breeding practices in in 1950s America applied to commercially farmed turkeys ultimately selected a population of incredibly hypertensive birds, to the point where a dominant mode of death for them was aortic dissection, often before the poor gobbler reached a marketable weight. As soon as reserpine became available, farmers began to administer it to these flocks in subclinical doses (parts per million), with excellent effect. From this, Wheat and colleagues derived their rationale for using reserpine and trimethapham on humans.

Control of blood pressure in aortic dissection

Now, reserpine is an indole alkaloid which acts as a monoamine reuptake inhibitor by irreversibly blocking the VMAT-2 transport protein, leading to depletion of all monoamines (yes catecholamines, but also dopamine and serotonin, to the point where it could have made a decent antipsychotic). Trimethapham is a ganglionic blocker. Thankfully, these days we do not need to resort to these toxic substances. 

What endpoints do you aim for? We have no satisfactory method of continuously monitoring dP/dT, but we do have invasive arterial pressure monitors, which make reasonable surrogates. Oh's Manual recommends a systolic BP target of 100-110 mmHg, or a MAP of 55-65 mmHg (pp. 1033), which fits well with other recommendations in the literature (100-120 mmHg seems to be the range most review articles tend to fall into). However, from end-stage CICM trainees, some additional level of sophistication is probably expected, and the monodimensional pursuit of blood pressure targets is probably not going to be enough for exam success. Thus:

• Control contractility: Beta-blockade is the standard of care, as beta blockers decrease the force of cardiac contraction and therefore reduce the velocity of the aortic jet. Labetalol or esmolol are described by the AHA document as "excellent" choice.
• Control rate. With higher heart rates, contractility increases because the myocardium does not have time to pump the calcium out of the cytoplasm (this is usually referred to as the Bowditch effect or the Treppe phenomenon). Fortunately, beta blockers will also help with this.
• Maintain preload. Yes, increased preload increases contractility. With slower heart rates and longer diastolic filling time, the effect of preload on contractility is accentuated. However, one needs to take into account the fact that these people usually come from a background of poorly controlled hypertension, and their ventricles will be thick and stiff. The poor LV compliance in this group makes them more sensitive to reductions in preload. Plus, they will have recently had a CT aortogram, and they would probably be headed for a repeat CT aortogram in the near future, which is a nontrivial contrast dose. In short, it would probably be counterproductive to dry them out too much. 
• Control afterload.  Increased afterload increases the end-systolic volume, which increases contractility (this is usually referred to as the Anrep effect).  Various arterial vasodilators (eg. sodium nitroprusside) can also be used, provided beta-blockade is firmly in place (one does not wish to see any reflex tachycardia).

The choice of drugs to achieve these goals is relatively broad. Wealthy American guidelines recommend esmolol, which is basically liquid money. A cheaper alternative may be IV labetalol. The added α-antagonist effect from this drug seems to play some beneficialrole. The excellent management guidelines table from Tran & Khoynezhad (2009) recommends a 15mg bolus, followed by a 5mg/hr infusion. Esmolol or diltiazem can then be used to control any additional tachycardia; sodium nitroprusside or clevidipine can help with any refractory hypertension once the rate is well-controlled.

Supportive care

Some additional elements of care can be suggested, which all serve the main purpose, that being improvement of blood pressure control:

• Intubation is not essential, as many of these patients are perfectly alert and cooperative with no respiratory compromise. However, intubation is usually required for interventions. In these scenarios, a "smooth and deep" induction is recommended. Practically, that means huge sympatholytic amounts of opiates.
• Mechanical ventilation,  where possible, should be mandatory, particularly where there has been no repair. If the dissection flap is especially fragile, one would want to avoid any sort of Valsalva-like scenario, where the patient coughs, and then turns pale and dies instantly. If not neuromuscular blockade and total controlled ventilation, one may instead opt for sedation with plenty of opiates and minimal suctioning.
• Sedation and analgesia are important because pain and distress are potent triggers for hypertensive episodes, and we wouldn't want any of those. Analgesia therefore becomes very important. Clonidine and dexmedetomidine are effective co-analgesics which could serve a dual purpose here.

Surgical management of aortic dissection

Type A  aortic dissection

Most authors (eg. Goldfinger et al, 2014), when writing a review of aortic dissection, will mention the figure of 1% per hour. That is the rate at which the risk of death increases for a Type A aortic dissection waiting for surgery, and is supposed to illustrate the dangers of delaying definitive repair. Everybody quotes those numbers but nobody seems to ever mention where they came from. It appears that this 1%/hr figure comes from the IRAD database, which is an international registry of acute aortic dissection. On average, it appears that these patients only wait about 4 hours before getting into theatre. The audit of IRAD data by  Evangelista et al (2018) actually contains a lot of very valuable insights; for example, only 86% of patients are managed surgically, and the mortality for nonoperative management, though high (56%), is not 100%.

In short, conservative management is a valid option, but it's not a very good option. It's really reserved for the situation where surgery is absolutely contraindicated. For example, the patient has had a large ischaemic stroke, and will not tolerate the 30,000 units of heparin required for bypass. Or they are frail and elderly, and it seems a shame to subject them to sternotomy and bypass. Whatever the reason, the mortality from this approach seem to double whatever the mortality would have been from a surgical approach.

Surgical management of Type A dissection is a variation on the theme of aortic root replacement and repair. To discuss it in detail would extend the author into an area where he has neither expertise more interest, and for the CICM trainee, it will suffice to say that the surgical approach is individualised, and some people will end up with more Dacron inside them than others. Tirone et al (1999) is dated, but an otherwise solid overview of the techniques. The outcomes are spectacular. According to Yang et al (2018), in-hospital mortality can be as low as 8%.

Type B aortic dissection

Uncomplicated Type B: the vast majority of these can be managed non-operatively.  Estrera et al (2006) reported a mortality of 10% for this approach, which the authors had described as an "acceptable" outcome. However, a fair proportion of these "uncomplicated " Type B patients go on to develop late complications, like aneurysmal degeneration. These are entirely "uncomplicated", i.e they have no organ damage, but they do have a large false lumen, or a large aorta in general (more than 4cm). They are at greater risk of complications in the long term, and might benefit from endovascular repair, albeit with a delay.  The optimal window seems to be around 2 weeks to 3 months from the original dissection - too early, and the risk of complications from the TEVAR is too great (Tadros et al, 2019).

Complicated Type B usually need early TEVAR. The intervention is usually endovascular. Open repair is hideous, and most vascular surgeons would do whatever they can to avoid these. For Harky et al (2019), TEVAR had a 1-year mortality of around 7.4% as compared to  18.6% for open repair. 

In order for a Type B dissection to be described as "complicated", the following criteria need to be present:

• Features suggesting instability:
  • Aortic rupture
  • Refractory hypertension
  • Aortic total diameter of > 4.5 cm
  • Extension of dissection on repeat imaging
• Ischaemia:
  • Visceral and renal ischaemia
  • Lower extremities ischaemia
  • Spinal cord ischaemia
• Malperfusion:
  • Paraparesis or paraplegia
  • Abdominal pain, nausea, or diarrhea
  • Renal failure or LFT derangement

This list is from Moulakakis et al (2014), but it resembles what is seen in UpToDate. One could also add patients who are entirely "uncomplicated", but who have a large false lumen, or a large aorta in general (more than 4cm). They are at greater risk of complications in the long term, and might also benefoit