Coronary bypass surgery

Group 3: Goh Han-Ming Jasper, Liu Rijing, Fong Hui Fang Michelle, Huang Hansheng, Ng Mei Ying, Mohamad Azhar B Ibrahim, Nah Jiaming Joseph, Oi Seng Wee, Chen Yunxu

1. Introduction

Coronary bypass surgery is a surgical procedure involving an operation of the heart. Sometimes, arteries that bring blood to the heart can become blocked due to building up of fats, cholesterol, or other substances. These built up forms plaque that breaks open eventually thereby forming a blood clot which blocks the flow of blood to the heart. As a result, heart pain or heart attacks occur. Coronary bypass surgery is a procedure that allows blood to flow to the heart despite the blockages. This is done usually by taking a healthy blood vessel from other parts of the body to go around or "bypass" clogged coronary (heart) arteries. As such, blood can flow through the new vessels to the heart muscle.

1.1 How is it done (in general)

Before the surgery is commenced, small metal disks called electrodes will be attached to your chest to monitor the heart's rhythm and electrical activity. Also, an intravenous line will be inserted to give you anesthesia throughout the operation. Once the person is completely asleep, a tube will be inserted through the windpipe and connected to the respirator, which will take over the breathing process. A heart-lung machine is normally used for bypass operations. This machine requires two hook-ups. One is to a large artery where fresh blood can be pumped back into the body. The other is to a major vein where "used" blood can be removed from the body and passed through the machine. After the body is hooked up to the heart-lung machine, the heart is stopped and cooled.

As shown by Figure 1 below, A long piece of vein from your leg (the saphenous vein) may be removed. This piece of vein is called a graft. One end of the graft will be attached to the ascending aorta, the large artery that carries oxygen-rich blood out of the top of the heart to the body. The other end of the graft will be attached to a coronary artery below the blocked area. The surgeon may choose to use an artery from the inside of your chest wall (the internal mammary artery) instead.

 
    Figure 1. Coronary bypass using the internal mammary artery and the saphenous vein

 
 Figure 2. Actual Coronary bypass surgery

1.2 Types of bypass operations

There are two types of bypass operations : on-pump and off-pump bypass. In on-pump bypass surgery, the heart is stopped and the blood supply to the body is taken over by the cardiopulmonary bypass machine also known as the heart-lung machine. Some of the complications arising from this technique are stroke, kidney or liver failure, decrease in higher mental function, and bleeding. This is attributed to the use of the pump and the manipulation of the heart and lung to place the patient on the system. However with improved technology the heart-lung machine is now very safe and with good knowledge of the causes of the complications, surgeons are able to identify them and take precautions.

Despite the downside of on-pump bypass, it is superior over off-pump bypass in term of the graft quality. This is because in off-pump bypass the grafting is done on a moving heart and this increases the difficulty, therefore off-pump bypass grafting is done by an experience surgeon. However the complications are reduced for off-pump bypass grafting.

1.3 Fluid dynamics of narrowed/blocked artery

When blood flows through a blocked and narrowed artery, it will cause an increase in flow velocity through the narrowed section. Since mass is conserved, therefore the flow velocity has to increase in order to ensure that the mass flowing out at the narrowed section is equivalent to the mass flowing in from the unblocked section of the artery. This will lead to an increase in shear stress at the blocked section as rate of velocity change increases and also a higher Reynolds number which leads to turbulence effects.

Consequently, an increase in velocity at the blocked section will lead to a drop in pressure as pressure energy is converted to kinetic energy and this will cause the blocked section to collapse further under the influence of the surrounding pressure acting on the artery. 

1.4 Other alternatives 

Coronary artery bypass surgery over Angioplasty 
Someone who experiences frequent or severe chest pain may need either a coronary artery bypass surgery- creates a different blood vessel to supply blood to the heart, or angioplasty- widens the narrow artery.

Angioplasty

Figure 3. Angioplasty 

(A and B) A balloon-tipped catheter is positioned in the narrowed coronary artery and inflated.

(C) When the balloon is inflated, the stent expands and presses against the arterial wall. The balloon is then deflated and removed.

(D) The stent remains permanently in place, helping to keep the artery open.

 
Figure 4. Close-up of a stent

These patients generally need to take blood thinning medication (glycoprotein IIb/IIIa inhibitors) to prevent clotting. Many will have to take long term aspirin as well to prevent blood clot from developing on the stent. This could also be seen as a potential factor as well when deciding between Bypass surgery and Angioplasty.

Which method is to be favoured is usually voiced down to preferences of the doctor and/or the patient. Having said that, there are however also specific medical circumstances to warrant why a bypass is preferred over Angioplasty.

Bypass surgery is preferred over Angioplasty in these cases

1. The left main coronary artery is severely narrowed ( several other arteries branch from this main one, and thus it is too risky if angioplasty were to fail)
2. Severe narrowing of any three arteries in a person who also suffers from a weak pumping heart.
3. Severe narrowing of the left anterior descending artery and other coronary arteries
4. Narrowings in too many different places in the coronary arteries that it is impractical to widen all of them
5. Pre existing illness such as diabetes is present
6. Symptoms such as chest pains or short of breath, indicating patient has a weak pumping heart
7. Position of the blockage is too risky for Angioplasty to be effective

Although angioplasty is less stressful on the body, and an easier procedure as compared to a coronary bypass surgery, angioplasty patients may need to undergo the same procedure after a few months due to the gradual narrowing of the same artery after it has been widened (restenosis). Furthermore, certain cases of restenosis must be treated with coronary artery bypass surgery as angioplasty is ineffective and risky.

While these two treatments are the major treatments for narrowing of the vessels, there are cases where the patient has severe narrowing of small vessels. In these cases, bypass surgery or angioplasty may not be beneficial and medication is the only option for these patients.

2. Types of grafting and graft patency

2.1 Types of graft

The 2 most frequently used conduits in a Coronary artery bypass surgery (CABG) are the internal thoracic artery (ITA) (also known as the internal mammary artery) and the greater saphenous vein (GSV).

  !Internal_mammary_branch.jpg|width=342,height=278!      Figure 3. CPG conduits - Internal thoracic artery

    !Great_saphenous_vein.jpg|width=216,height=576!               Figure 5. CPG conduits - Saphenous vein  

The internal thoracic artery is an artery which starts at the subclavian artery and runs along the anterior chest wall and the breasts. The left internal thoracic artery (LITA) is used most commonly and is usually grafted onto the left anterior descending artery or the marginal arteries on the circumflex. The right internal thoracic artery could also be utilized for the anterior descending artery or attached to the right coronary artery.

The greater saphenous vein is another choice for a conduit during CABG. It is the large superficial vein that runs along the length of the leg and thigh. Nowadays, however, the internal thoracic artery is the preferred choice among Cardiovascular surgeons for a graft, due to its improved survival rate as compared to the greater saphenous vein.

Other possible conduits used in CABG include the lesser saphenous vein, gastroepiploic artery, and the inferior epigastric artery. Arm veins have also been utilised in CABG, but they tend to perform poorly in general.

Besides the above grafts mentioned, there exists a type of grafts that are synthetic. These type of synthetic grafts are most commonly made from Dacron (Polyethylene terephthalate, PET) or Teflon (polytetrafluroethylene, PTFE). It is deemed necessary as the autografts, or host grafts mentioned above (internal mammary artery or the saphenous graft), may not be of the correct diameter or available even though it is nonthrombogenic and compatible. Synthetic grafts are then flexible in size, nonthrombogenic and durable. In designing the new graft, considerations of its mechanical properties are taken into account. For example, diameter and compliance, with that of the host artery.

2.2 Graft patency

The term graft patency is used to refer to the chance that a graft would remain sufficiently open over a period of time. This is due to the fact that occlusion of the graft can often occur in the years after CABG is performed due to the grafts becoming diseased and/or other complications. To be considered patent, a graft must have adequate flow through it without any significant (>70% diameter) stenosis (narrowing) occurring within it.

Patency in the years after CABG depends on a variety of factors. It is primarily dependent upon the type of graft used, but is also affected by the size of the coronary artery connected with the graft, as well as the ability of the surgeon and whether the surgery was carried out properly. In general, the internal thoracic artery graft has a far higher patency as compared to the saphenous vein graft. The 10 year patency of a saphenous vein graft is approximately 50%, while that of the internal thoracic artery graft is close to 95%.

Figure 6. Graph of graft patency rate vs years of surgery

The key reasons for the saphenous vein graft losing patency include intimal hyperplasia during the initial phase (after 1 month), as the vein adapts and remodels itself to the artery. This results in decreased flow from the vein graft to the recipient artery and results in graft occlusion occurring. In later stages, atherosclerosis (hardening) of the vein graft might also occur, causing the graft to lose its patency. Saphenous vein graft patency is not stable; it declines with time from development of vein graft arteriosclerosis. From 1 to 5 years after surgery, saphenous vein grafts occlude at a rate of 1% to 2% per year, accelerating to 4% to 5% per year by 6 to 10 years. By 10 years, only 50% to 60% of these grafts are patent, and only one-half of these are free of angiographic arteriosclerosis. Unlike saphenous veins, internal thoracic artery grafts rarely develop arteriosclerosis (<4%), and only 1% develop important arteriosclerotic luminal narrowing. This resistance to arteriosclerosis results in close to 95% of internal thoracic artery grafts being patent 10 years after surgery.

 An important factor affecting early arterial graft patency is native coronary artery blood flow. When an arterial graft is used to bypass a coronary artery with only moderate proximal stenosis, the need for bypass blood flow is low, causing the graft to constrict and fail. This is consistent with the physiology of arteries. Unlike saphenous veins, arteries are muscular and can autoregulate their lumen in response to blood flow demand. This phenomenon influencing arterial graft patency is referred to as native coronary artery competitive blood flow, but because it is difficult to measure, maximum coronary artery stenosis is often used as its surrogate.

As for the other types of grafts used as conduits, the gastroepiploic artery and the inferior epigastric artery are comparable to the internal thoracic artery in terms of patency, while the lesser saphenous vein is similar to the greater saphenous vein. Arm veins have an especially poor patency, thus accounting for their poor performance in CABG.

2.3 Vascular resistance of graft

     

                                                                      Figure 7: model of Y-shaped graft

In the graft model (figure 7), the three vessels that make up the graft are assumed to be identical ( same length and diameter) and the pressure drop in each of the vessel will be the same. In order to evaluate the vascular resistance in the net direction of flow, the pressure drop in the side vessels has to be resolved in the direction of net flow given by ,
The net pressure drop in the direction of flow and the corresponding vascular resistance will be  
Therefore the overall vascular resistance of the graft will be in the range   R_{vertical vessel} < R_overall < 3R_{vertical vessel} depending on the angle.

3. Flow measurement techniques







3.1  Model for flow in coronary bypass graft
In practice, the estimation of flow is given by

 

whereby Q: Flow rate of fluid, A: Cross sectional area of flow, and V: Velocity of flow

It is based on the assumption that the spatial velocity profile is constant and parabolic in a straight linear tube. However, these assumptions are not applicable in CBG as the fluid dynamics in the graft are much more complex due to the shape of the graft which is usually Y-shaped and, changing flow conditions at the inflow cross section of the graft, as shown in Figure 2. 
!inlet flow conditions.JPG! 
Figure 8 Graph of fluid flow vs time, t during one cycle of the pulse of a period, tp

According to the Figure 7, it is deduced that the inlet flow conditions change during the process of a single heartbeat, unlike what was assumed previously. Therefore, in order to account for the changing flow conditions, Raffaela et al [1] proposed a model by linking the maximum velocity, V_max to the mean velocity, V_mean with the following equations:

                                                                          

 whereby W is the Womersley number, that accounts for unsteadiness in the flow. Different branches of the graft are associated with different W value as shown in Figure 8.



Figure 9. Womersley number values for different types of graft






It is pertinent to measure the coronary graft flows routinely at the completion of CABG because the flow rate in the graft would determine the quality of the graft and whether there will be enough blood flowwing to the heart. Therefore, it is vital to choose the most appropriate measurement techniques in evaluating the graft flow. The two techniques for graft flow measurement will be covered in the next section.

With respect to synthetic vascular grafts, the two major concerns for its modeling are: 1)compliance mismatching, in which each graft material has a different elastic modulas than that of the natural artery, and 2)its geometric mismatching, due to different graft lumen radius and wall thickness. The aorta is modeled based on the tapered piecewise linear approximation as shown below.
 
Figure 10. Tapered aortic model

The pressure and flow waveforms were calculated for each section of the model using the transmission line type equations as shown, 

  
where, P(z,t) and Q(z,t) are the pressure and flow at distance z and time t, respectively. The forward and reflected portions of the pressure and flow waveforms are identified by the subscripts f and r, Zo is the characteristic impedence, ? is the angular frequency and, ? the propogation constant, which describes the translation properties of the individual model segment. The influence of the graft material on the pressure and flwo wave formations were examined as a result of a substituions of specific sections of the model.

3.2 Ultrasound flow measurement

The ultrasonic flowmeter can measure flows non-invasively and advanced types of these devices can also measure flow profiles. The working principle behind the above method is Doppler effect. The mathematical formulation for this well-known effect is a bit complex but the principles behind it are easy enough to understand. Doppler effect states that there is a change in frequency and wavelength of a wave for an observer moving relative to the source of the waves. The total Doppler effect may therefore result from motion of the source, motion of the observer, or motion of the medium. In this case, motion of the medium(blood) is the most important aspect contributing to the Doppler effect.
 The transmitter exerts a pulse which travels in a single packet to the source. The wave reflects off of RBCs in the blood stream and is received by the transmitter with a time delay proportional to the distance travelled. Analyzing the Doppler shift at various delays creates the velocity profile across the vessel.


                      Figure 11: Working principles behind the ultrasound blood flow meter.

With this method, we can produce accurate assessment of the direction of blood flow and flow profiles, the velocity of blood and cardiac tissue at any arbitrary point using the Doppler effect

3.3Electromagnetic waves measurement

Calibration of the electromagnetic flow meter must be done prior to each flow measurement. Firstly, the flow meter is calibrated to measure the volumetric flow rate of water through a pipe. The device is later calibrated using a saline solution to simulate the electrical conductivity of blood.
Electromagnetic flow meters measure the instantaneous flow of blood, rather than the average flow.
Blood flowing with velocity, u, through a magnetic field, B, through length(distance between recording electrodes), L, induces an electromagnetic flux, e, governed by the formula:

e=BLu

The above formula is only applicable for uniform magnetic field and uniform velocity profile. It is easily derivable from Faraday's Law of Electromagnetic Induction.

With the above method, we can then determine instantaneously, the  amount of blood flowing through a section of vessel. 

3.4 Comparison between ultrasound and electromagnetic waves measurement

From the Journal of Cardiovascular surgery, it is discovered that the electromagnetic flow measurements are higher than ultrasonic flow (USF) in all coronary bypass locations. Furthermore, it is observed that the electromagnetic waves tend to be unsteady in small blood vessels and are affected by serum concentration and the angle of the probe. Another consideration to account, is that a longer preparation time is required for electromagnetic measurement as it is harder to obtain a proper contact within the vessels wall and finding the suitable size probes when measuring the ITA graft flow.

Table 1: Advantages of ultrasound method over electromagnetic method.

 Hence, ultrasonic flow measurement is preferred over electromagnetic waves measurement.

4. Evaluation of coronary grafts

The mean of evaluating the patency of grafts for coronary artery bypass surgery can be done through transient time flow measurement (TTFM). TTFM is a reliable and effective method for flow measurement as it is independent of blood vessel size, shape and doppler angle used. The two blood vessels that are used for the bypass are saphenous vein (SV) and internal thoracic artery (ITV) also known as internal mammary artery.  SV runs from the ankle to the groin of the body and ITA is found inside the chest wall.


                                                          Figure 12:Transient time flow(TTF) normal curve


In a patent graft, the TTF curve obtained is similar to that for flow through healthy coronary blood vessel as the haemodynamics are expected to similar. Figure 12 shows the typical TTF curve for circulation in normal coronary artery/patent graft. During the time when the heart contracts, the blood will flow backwards through the blood vessel(a process known as systole). On the other hand, the process at which blood flows into the heart is known as diastolic filling.
Usually for a patent graft the systolic peak will be small that is the negative flow is small. However for stenoic anastomosis that is connection between SV or ITA and the blocked coronary blood vessel which becomes narrow easily, the systolic flow during heart contraction is larger, therefore leading to turbulence in the blood flow, and occurs more frequently as in Figure 13 below. In addition, the epicardial vessels are slightly compressed during contractions resulting in positive  'spikes' in the TTF diagram as blood flows into the coronary due to the squeezing of the epicardial vessels (refer to Epicardium) during contraction.

                                        Figure 13: TTF for severely narrowed SV graft to right coronary artery (RCA)
                                                                                                                      
The TTF curves have to be interpreted with the Electrocardiogram, the mean flow rate and the pulsatile index (PI), which is the difference between the maximum and minimum flow divided by the mean flow rate, in order to evaluate the quality of connection between the graft vessel and coronary vessel (anastomosis). The mean flow rate alone is not a good indicator of the quality of anastomosis as it depends greatly on the quality of the revascularised coronary vessel and generally the PI value for a good anastomosis should range between 1 and 5. The electrocardiogram is needed to identify the diastolic and systolic processes.

5. Complications related to CABG

As of any surgery, there is always a risk of complications involved. Some of the complications with specfic reference to the CABG, are discussed in this section.


1.Myocardial Infarction(MI) is the medical term for the lack of blood supply to the heart. This happens as the bypass area is also subjected to deposition. Once the bypass channel gets choke up again, heart attack as a complication will set in.

2.Nonunion of the sterum bone might also occur. Nonunion is the complication related to fracture of the bone. The inability to heal effectively can be a result of excessive movement of the fracture bone area, simply a lack of blood supply or infection has occured.

3.Postperfusion syndrome or pumphead can result. It is a neurological impairments associated with heart surgery. Symptoms are subtle and defects associated with attention, concentration, short term memory and motor responses are detected. These deficits are however not permanent in most cases.
 

6. References

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Raffaele Ponzini, Massimo Lemma, Umberto Morbiducci, Franco M. Montevecchi, Alberto Redaelli (2008). Doppler derived quantitative flow estimate in coronary artery bypass graft: A computational multiscale model for the evaluation of the current clinical procedure. Medical Engineering & Physics,  30 (7):809-816. Retrieved January 16, 2009, from http://www.sciencedirect.com/science/article/B6T9K-4R2HKWJ-1/2/a37651aabd030df3ce3c817bebdd4b0b

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Texas Heart Institute Heart Information Center (2009). Coronary Artery Bypass Surgery. Electronic resources. Retrieved January 26, 2009, from http://www.texasheartinstitute.org/hic/topics/proced/cab.cfm

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Texas Heart Institute Heart Information Center (2009). Coronary Artery Bypass Surgery. Electronic resources. Retrieved January 26, 2009, from http://www.texasheartinstitute.org/hic/topics/proced/cab.cfm

Source for Figure 4: http://www.camstent.com/angioplasty.html

Source for Figure 2: http://en.wikipedia.org/wiki/File:Coronary_artery_bypass_surgery_Image_657B-PH.jpg