Organización de la evaluación educativa

The structure of the BN was defined using the methodology described in Section 6.2.1. A domain expert (ZP) identified the variables that are found to have clinically significant effect in the meta-analysis. ZP described the mechanistic relation between each of the variables and the predicted outcome, which were modelled in a causal BN structure. Knowledge about the mechanistic relations helped us to identify the variables that are outside the scope of the BN. For example, nerve injuries were not included in the model even though it is found to increase the probability of amputation in the meta-analysis. The domain expert indicated that the outcomes related to limb function are outside the scope of the LEVT BN, and the amputations related to nerve injuries are often caused by pain and poor function outcomes.

Table 6.8 Observed and Latent Variables in LEVT BN Observed Variables Latent Variables Arterial Repair

Anatomical Site Multiple Levels (MAI) Soft Tissue Injury Associated Fracture Shock Ischemic Time Ischemic Degree Compartment Syndrome Repair Failure

Number of Injured Tibials Nonviable Extremity

Blood Supply Ischemic Damage Microcirculation Soft Tissue Cover

Several latent variables were introduced while the domain expert identified the mechanistic relations between the observed clinical factors and outcomes. These variables were clinically important but neither the dataset nor the reviewed studies contained them as they cannot be directly observed. For example, both graft repairs and soft tissue injuries have higher probabilities of amputation in the meta-analysis. However, each of these factors is related to amputation through a different pathway. Graft repairs can lead to amputation when the repaired artery bleeds or gets blocked, and thus cannot deliver enough blood to the lower extremity. A variable about the degree of blood supply is required to model this relation. Although the degree of blood supply can be estimated by several measurements, the precise state of this variable is difficult to observe and therefore is not in the dataset. Soft tissue injuries

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can lead to amputation if there is not enough viable soft tissue to cover the injuries and to repair the wounds. Similarly, a latent variable about the degree of soft tissue cover is required to model this relation into the BN model. Table 6.8 shows a list of the observed and latent variables in the LEVT BN structure. These variables are described in the remainder of this section.

The information in our dataset was more detailed, for some variables, compared to the information reported in the meta-analysis. For example, soft-tissue injuries were modelled with more detailed states in the BN as the dataset had more information about this variable. Similarly, information about the degree of ischemia were present in the dataset but not in the meta-analysis. Therefore, the BN contains more detail about some variables compared to the information obtained from the meta-analysis.

Model Structure

The LEVT BN is divided into 5 components, corresponding to the 5 boxes shown in Figure 6.7. The remainder of this section describes the LEVT BN by summarising the meanings of the variables and relations in each of these components:

Figure 6.7 LEVT BN Structure

 Lower Extremity Outcome: The aim of the LEVT BN is to predict the risk of failure of an attempted salvage for a lower extremity with vascular injury. The ‘Nonviable Extremity’ variable represents extremities that are amputated as a result of nonviable tissue. A lower extremity can sustain life if there is adequate blood flow from the vessels and enough viable soft tissue to cover

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the vessels. ‘Nonviable Extremity’ is the main outcome variable that the LEVT BN aims to predict.

 Ischaemia: Ischaemia is the deficiency of blood supply as a result of an arterial injury or obstruction. Ischaemia causes permanent damage to tissues if it continues for a prolonged time. Since our model is built for lower extremities with vascular injuries, most of the extremities within the scope of our model will be partly or completely ischemic until the vascular injury is repaired. The severity of ischaemic damage depends on the time elapsed since the beginning of ischemia (Ischaemic Time) and the degree of obstruction (Ischaemic Degree). A major cause of ischemia is a compartment syndrome, which causes complete obstruction of blood flow due to increased pressure in the muscle compartments of a lower extremity.

 Soft Tissue Damage: This part of the model predicts the projected amount of viable soft tissue cover in the lower extremity. Adequate amount of soft tissue cover is necessary to repair the tissues and protect them from infection. Therefore, soft tissue cover is one of the main factors affecting the viability outcome. Our model estimates the amount of soft tissue cover based on the amount of non-viable tissue due to the direct damage from the injury (Soft Tissue Injury) and ischemia (Ischaemic Damage).

 Success of Arterial Repair: This part of the model predicts the success of a vascular repair operation. ‘Arterial Repair’ variable represents the type of the repair operation, and have two states: ‘Graft’ and ‘Primary Repair’. ‘Graft’ represents bypassing of the injured artery by a vein harvested from the patient. ‘Primary repair’ represents a simpler repair operation such as stitching of a small laceration in the artery. ‘Graft’ repairs have higher rate of failure compared to ‘Primary Repair’ as this operation is more complicated and applied to more severe cases. Injury characteristics often define the type of the arterial repair. For example, an arterial injury cannot be treated by primary repair if a significant part of the artery has been torn apart and thus a graft is necessary.

The ‘Multiple Levels’ variable represent whether vascular injuries are present at multiple levels of the same extremity. Repair of such injuries have a higher

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probability of failure as clots are more likely to form when the artery is injured at multiple levels.

‘Anatomical Site’ variable represents the location of the main arterial injury. The injury can be at above the knee (femoral artery), at the knee (popliteal artery) or below the knee (tibial artery). Reconstruction of a femoral artery often has better outcomes compared to a popliteal or a tibial artery.

 Blood Circulation: ‘Blood Supply’ variable represents the degree of blood supply to the lower extremity. This variable essentially depends on the ‘Repair Failure’ variable. If the vascular repair fails, the extremity will not have adequate blood supply; so there is a deterministic relation between the negative repair failure and inadequate blood supply. However, a successful arterial repair may not guarantee adequate blood supply throughout the lower extremity; side factors including ‘Shock’ and ‘Microcirculation’ can also affect the outcomes. The ‘Shock’ variable represents an overall deficiency of blood supply throughout the body. ‘Microcirculation’ represents the severity of injury in the smaller vessels of the lower extremity.

Figure 6.8 LEVT BN Modification for Below the Knee

The main branch of artery that carries blood to the lower extremity divides into three branches below the knee. In other words, a single main branch of artery supplies blood for the tissues above the knee whereas three branches, called tibial arteries, supply blood for the tissues below the knee. In order to model this difference, we modified the BN structure for injuries below the knee by adding a variable about the number of injured tibial arteries. This

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modification is shown by the variable with dashed lines in Figure 6.8. Modelling of tibial arteries is important since the extremity is more likely to survive if all of the tibial arteries are not injured, even when the arterial repair fails. Apart from this difference, the BN models for above the knee and below the knee injuries are exactly the same. Table 6.9 shows the description and states of each variable in the LEVT BN.

Table 6.9 Description and States of Variables in LLVI BN

Variable Description States

Anatomical Site Level of arterial injury {Femoral, Popliteal, Tibial} Arterial Repair Surgical method for treating

arterial injury

{Primary, Graft, Ligation}

Associated Fracture Associated fracture at the level of arterial injury

{True, False}

Blood Supply Predicted degree of blood supply after repair

{Low, Medium, High}

Ischaemic Damage Degree of soft tissue damage caused by ischaemia

{Low, Medium, High}

Ischaemic Degree Degree of obstruction in blood supply

{None, Partial, Complete}

Ischaemic Time Duration of obstructed blood supply

{<1hr, <3hr, <6hr, ≥6hr}

Microcirculation Degree of microcirculation problems at the level of arterial injury

{Normal, Severe, Deranged}

Multiple Levels Presence of arterial injuries at multiple levels

{True, False}

Nonviable Extremity Presence of a non-survivable lower extremity

{True, False}

Number of Injured Tibials Number of tibial arteries injured

{0, 1, 2, 3}

Shock Presence of uncompensated

shock

{True, False}

Soft Tissue Cover Degree of soft tissue damage due to injury and ischaemia

{Low, Medium, High}

Soft Tissue Injury Degree of soft tissue damage cause by injury

{None, Mild, Moderate, Severe, Profound} Repair Failure Failure of arterial repair due

to occlusion or bleeding

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