When To Operate

As a general rule of thumb 5cm for ascending aorta, 6cm for descending aorta, and 7cm for the aortic arch. Please note there are a number of exceptions.

Introduction

Aortic root

Ascending Aorta

Threshold for Replacing Ascending Aorta Calculator

Arch Aorta

Descending Aorta

Aortic Aneurysm Expansion Rates

Estimate Rate of Rupture post CT Scan

SUMMARY

Introduction

Emergent operation is indicated in the setting of acute ascending aortic dissection or rupture. Ascending aortic aneurysms rupture into the pericardial space and result in death from acute cardiac tamponade. Aortic dissections may rupture or may compromise coronary or cerebral circulation. Operative mortality is significant in this setting, but death is certain in the case of rupture and probable in the case of acute dissection if not surgically addressed.

Symptomatic aortic insufficiency or stenosis may be the primary indication for operation. When replacing or repairing a diseased valve a decision must be made regarding the moderately dilated aorta. Michel et al  reported that 25% of patients undergoing surgery for aortic insufficiency who had ascending aortic diameters greater than 4 cm required subsequent operation for aortic replacement. Prenger et al  reported a 27% incidence of aortic dissection following aortic valve replacement in patients with aortic diameters greater than 5 cm. Based on these findings it is recommended that aortic diameters of 4 to 5 cm be dealt with at the time of aortic valve surgery. Further incentive for earlier surgery is the improved possibility of native valve preservation.

Size/Growth Rate

Because the diameter of an aneurysm strongly correlates with the risk of rupture or dissection, size has long been used as the criteria for elective surgical intervention. Although size criteria for the abdominal aorta have been well established and generally agreed upon, less of a consensus has emerged for the thoracic aorta. Variable growth rates and propensities for rupture in different regions of the thoracic aorta and with different underlying pathologies are perhaps to blame. The average size of rupture of the thoracic aorta reported in the literature is highly variable. Coady et al report rupture or dissection at a median size of 5.9 cm in the ascending aorta and 7.2 cm in the descending aorta. Because intervention at these diameters would have by definition resulted in rupture or dissection in 50% of patients, preemptive surgical therapy at 5.5 cm and 6.5 cm, respectively, for ascending and descending aneurysms seems appropriate.

 
As opposed to absolute size criteria, some surgeons prefer the use of ratios of measured to expected size. The expected size is based on the body surface area and age of the patient. The ratio indicating intervention is adjusted based on the underlying etiology. Ergin et al advocate a ratio of 1.5 for the average patient with an asymptomatic incidentally discovered ascending aortic aneurysm. This leads to intervention at a size of only 4.8 to 5.0 cm in an adult less than 40 years of age with a body surface area of 2 m². Because the ascending aorta normally increases in size with age, the diameter for intervention would be higher in a patient more than 40 years old.

The rate of expansion is also an important consideration. Reported mean growth rates of thoracic aneurysms vary from 0.10 to 0.42 cm per year. The rate of expansion is usually greater in the descending aorta and in conditions with a weakened aortic wall, such as Marfan syndrome or chronic dissection. Growth at a rate of greater than 1.0 cm per year is certainly an accepted indication for surgical intervention, but more often the rate of dilation is used by the surgeon as supplementary information that helps to guide the timing of surgery rather than serve as an absolute indication.

The Influence of Etiology

Patients with Marfan syndrome or with familial aneurysms, particularly when there is a history of early dissection or rupture, should undergo earlier intervention. Gott et al  recommend intervention in patients with Marfan syndrome at an ascending aortic diameter of 5.0 to 6.0 cm. Coady et al recommend intervention at 5.0 cm. Ergin et al recommend a measured to expected size ratio of 1.3. Patients with chronic dissection should be considered to have similar intervention criteria as those with Marfan syndrome. Patients with bicuspid and unicuspid aortic valves are probably at intermediate risk, and Ergin et al recommend intervention at a ratio of 1.4 in these patients. Pseudoaneurysms are at a high risk of rupture and should be treated when discovered.

Aortic root

The majority of dilemmas with regard to root replacement occur in the setting of aortic valve disease. Symptomatic aortic insufficiency or stenosis may be the primary indication for operation. 25% of patients undergoing surgery for aortic insufficiency who had ascending aortic diameters greater than 4 cm require subsequent operation for aortic replacement. Prenger et al reported a 27% incidence of aortic dissection following aortic valve replacement in patients with aortic diameters greater than 5 cm. Based on these findings it is recommended that aortic diameters of 4 to 5 cm be dealt with at the time of aortic valve surgery. 

Marfans Syndrome, remains a special situation where aggressive replacement at 4 to 4.5cm is recommended, especially if there is a family history of dissection.

Ascending Aorta

The need to replace the ascending aorta depends on age, BSA, and the pathology of the patient. The use of the ascending aorta calculator is highly recommended for this.

Factors involved include:

Marfans
Bicuspid aortic valve
Needs AVR
Degenerative with AR
Bicuspid aortic valve not needing surgery
Degenerative no valve problem, may be associated with coronary artery disease

Threshold for Replacing Ascending Aorta Calculator

Threshold diameter of ascending aorta in cm that needs replacement = Index BMA x Entity Factor x BSA

BSA = Sqrt [Weight(Kg) x Height (cm)/3600]

Index BMA 

Entity Factor

Ascending aorta calculator

Arch Aorta

Generally taken as 7cm, but opinion varies. Age and co morbid factors play a large part in decision making.

Descending Aorta

Generally taken as 6cm, but opinion varies.

1. increasing age, 
2. diameter of the descending thoracic aorta, 
3. diameter of the abdominal aorta, 
4. chronic obstructive pulmonary disease, and 
5. the presence of symptoms.

 The Mount Sinai group performed a multivariable analysis that included data from computer-generated three-dimensional computed tomographic reconstructions of the thoracoabdominal aorta. The resulting formula determines the probability of rupture within 1 year based on 

1. patient age, 
2. the presence of pain
3. chronic obstructive pulmonary disease, and 
4. the maximum true diameters of the descending thoracic and abdominal aortic segments.

Increasing age and preoperative renal insufficiency have remained major risk factors for early mortality throughout the history of TAAA repair. Both were among the predictive variables determined by Svensson et al's multivariable analysis of Crawford's complete experience with TAAA surgery in 1509 patients treated between 1960 and 1991. 

The recent report by Acher et al confirms that, along with acute presentation, age and elevated creatinine levels remain important predictors of early death

Aortic Aneurysm Expansion Rates

Estimate Rate of Rupture post CT Scan

Ln Lamda = -21.055 + 0.093 x age + 0.841 x pain + 1.282 x COPD + 0.643 x descending aortic diameter in cm + 0.405 x abdominal aortic diameter in cm

Pain and COPD are 1 if present 0 if absent

Probability of rupture within 1 year = 1- e ^-Lamda(365)

SUMMARY

Natural history of thoracic aneurysmal disease is incomplete.

Prognosis without intervention is poor in large aneurysms.

Factors that affect expansion and rupture are both aneurysm and patient related.

Aneurysm size is the most important factor determining expansion rates and risk of rupture.

Factors that affect interventional outcome are dependent on site, extent, pathology and co-morbidity.

Surgical intervention still carries a significant risk of mortality and permanent neurological impairment.