by

Scott Brown, Student Milwaukee School of Engineering

Chronic, major vessel thromboembolic pulmonary hypertension (CTEPH) is a rare form of pulmonary hypertension that approximately 450 patients, of the estimated 500,000 who experience a pulmonary embolus each year, suffer from [2]. Previously, this disease was difficult to diagnose and surgical intervention was very limited. But, with improved diagnostic techniques, including lung scanning and pulmonary angiography, in combination with improving surgical techniques, including the pulmonary thromboendarterectomy, it appears a cure for these patients is on the horizon.

The pathology of this disease begins in the venous system. The majority of the pulmonary thrombus originate in the veins of the lower extremities. When a thrombus breaks loose to form an embolus, it travels up through the venous system, through the right heart and into the pulmonary arteries. If the thrombus gets lodged in the pulmonary artery or segmental branches of the pulmonary circulation the result is an obstruction to flow. As the disease process progresses, the thrombus adheres to the artery wall while a "neo intima" forms over the thrombus. The "neo intima" is a whitish material which looks very similar to the normal intima. The thrombus then begins to fibrous and scar which results in a decrease in the elastance of the artery. Eventually the fibrous scar (the thrombus) will constrict the artery, significantly restricting the blood flow distal to the thrombus. The end result is severe pulmonary hypertension. It is important to note here that CTEPH is an uncommon end result of an acute pulmonary embolus [2].

As mentioned earlier, the diagnosis of CTEPH is very difficult to make. The primary symptom is dyspnea upon exertion. If this symptom is present, the diagnosis of CTEPH can not be ruled out and further investigation is warranted (e.g. lung scans and/or pulmonary angiography). Pulmonary angiography is the most definitive diagnostic tool in diagnosing a patient with CTEPH. If the results of the pulmonary angiography are questionable, fiberoptic angioscopy is used for direct visualization of the thrombus.

In order for a patient who is properly diagnosed with CTEPH to be considered a surgical candidate for a pulmonary thromboendarterectomy, there are four criterion which must be met. First and foremost, the thrombus must be accessible via current surgical approaches. If this is not met, the patient can not be considered a surgical candidate. The rule of thumb is that if the thrombus is located distal to the origin of the segmental arteries, it is considered to be inaccessible [2]. Second, the patient and his / her family must accept the risks of the surgical procedure. The majority of these patients would be considered candidates for lung transplantation, but because of the associated high risk of mortality with transplantation, they and their families are searching for other alternatives. Third, the patient’s pulmonary vascular resistance (PVR) needs to be greater than or equal to 300 dyne·sec·cm^-5. Lastly, there should be no other significant co-morbid disease present. If the patient does suffer from another such disease, they are not absolutely excluded from surgical intervention, but it is critical that the patient and their family understand the added risk of the procedure. An example would be if a patient also had coronary artery disease, coronary artery bypass grafting could be performed at the time of the thromboendarterectomy, but again, this would increase the risk of the overall procedure.

The essentials for successfully performing a pulmonary thromboendarterectomy include: 1.) the distinction of the thrombus versus the normal intima, 2.) meticulous dissection and removal of the thrombus, and 3.) a bloodless surgical field. The recognition of the thrombus and its meticulous dissection and removal is a very difficult task for the surgeon(s). It is the perfusionist’s task to provide the bloodless surgical field. Therefore, it is necessary to utilize hypothermic circulatory arrest in combination with added cerebral protection for a successful surgery.

Hypothermic circulatory arrest is necessary because there is usually a large amount of bronchial artery blood flow. Without circulatory arrest, it would be impossible for the surgeon to recognize the thrombus and dissect it out. Profound hypothermia (< 20EC) reduces both the basal and functional metabolic activity, therefore increasing the bodies ability to tolerate ischemic periods without suffering cellular damage. The hypothermia allows for safe periods of circulatory arrest. At a core temperature of 15EC, arrest times of 60 minutes can be tolerated (in theory) [1]. Clinically, it has been shown that cumulative arrest periods greater than 55 minutes or a single arrest period greater than 20 minutes results in an increase in incidence of delirium 72 hours post-operative [3]. A general rule to follow would be to break long arrest periods into shorter ones (< 20minutes each) and try and limit the number of arrest periods to 2-5. The number of arrest periods will correspond to the surgeons experience with the procedure and the extent of the patients disease. Each arrest period should be followed with 10 minutes of reperfusion with 18EC blood to replenish the oxygen deficit of the tissue. A benchmark for adequate reperfusion is a venous oxygen saturation greater than 90%.

As the temperature of the blood decreases there will be an increase in its viscosity which is detrimental when trying to homogeneously cool the patient while maintaining adequate tissue perfusion. In this situation, hemodilution to a hematocrit of 18-25% will help to decrease the viscosity resulting in an increase in capillary blood flow. The end result will be improved perfusion with a more homogeneous cooling.

As the patients core temperature decreases, shivering resulting in vasoconstriction may occur. This is also detrimental to the cooling and perfusion process. In such instances, a muscle relaxant such as pancuronium will alleviate the shivering and associated vasoconstriction resulting in a more uniform cooling of the patient.

It is necessary to utilize added forms of cerebral protection in cases of circulatory arrest. Extra-cranial cooling by packing the patients head in ice is an excellent adjunct to core cooling (to 15EC). This is necessary because the brain will generally cool slower than the rest of the body. Also, temperature gradients between 2-5EC may exist in the brain. Therefore, the utilization of extra-cranial cooling will help to ensure adequate cerebral cooling.

Pharmacologic interventions include: barbiturates such as pentabarbitol or thiopental which help to ensure cessation of electro-cerebral activity as well as precondition the brain for the ischemic period; steroids such as dexamethasone are also neuroprotective during ischemia and help prevent cerebral edema; and osmotic agents such as mannitol also help reduce edema and are free radical scavengers.

It is important that the use of glucose be limited during circulatory arrest procedures in adults. During ischemia, anaerobic glycolysis occurs which results in the production of lactic acid and acidosis. With excessive amounts of glucose, severe acidosis will occur and lead to cellular and membrane damage. The use of insulin in instances of high glucose levels will help to reduce the lactic acid production and acidosis from anaerobic glycolysis.

CTEPH is typically a bilateral disease, therefore these patients are approached with a median sternotomy. The surgeon will then dissect down to the pulmonary arteries. From there an incision is made into either the right or the left pulmonary artery and by gently manipulating the pulmonary artery and by using long instruments the surgeon has to look distal to the incision for the thrombus and carefully dissect it out. The thrombus can usually be identified by a reddish material just proximal to a whitish lesion which looks like normal intima [2]. This will be the thrombus covered in the neo-intima. Both the reddish material and the thrombus need to be removed in order to restore the patency of the artery.

As mentioned before, the surgeon needs to be extremely careful when dissecting out the thrombus. Not only does the thrombus appear like normal intima, but the underlying wall of the artery is usually thinned significantly and can be punctured easily. The hope is that when the thrombus is removed, the elastance of the artery will return and the artery will "bounce" back [2].

Immediately after the pulmonary thromboendarterectomy is complete there should be a decrease in both the pulmonary artery pressure and the pulmonary vascular resistance with a corresponding increase in cardiac output. In addition to the normal post-operative problems associated with cardiac surgery, pulmonary thromboendarterectomies are associated with reperfusion lung edema which results in poor gas exchange and hypoxemia [2]. Therefore, these patients are mechanically ventilated and are commonly on oxygen therapy for a period of time post-operatively.

As the symptoms, and consequently the diagnosis, of CTEPH becomes better understood and as the surgical techniques continue to improve, the number of pulmonary thromboendarterectomies performed each year will increase. For the perfusionist this will mean a more sophisticated approach to cardiopulmonary bypass combined with detailed attention to patient parameters and record keeping.

Gravlee, Glen P., Richard F. Davis, and Joe R. Utley, Cardiopulmonary Bypass; Principles and Practice. William & Wilkins, Baltimore Maryland. Ó 1993

Moser K.M.,Auger W.R., Fedullo P.F., Jamieson S.W., Chronic Thromboembolic Pulmonary Hypertension: Clinical Picture and Surgical Treatment. Eur Resp Journal 1992; 5:334-342

Winkler M.H., Rohrer C.H., Ratty S.C., Jamieson S., Dembitsky W., Moser K., Auger W., Perfusion Techniques of Profound Hypothermia and Circulatory Arrest for Pulmonary Thromboendarterectomy. Journal of Extra-Corporeal Technology, Vol. 22, No. 2, 1990.

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