Current Activity Guidelines for CABG Patients are too Restrictive: Comparison of the Forces Exerted on the Median Sternotomy during a Cough vs. Lifting Activities Combined with Valsalva Maneuver

Abstract

Background: The current activity guidelines for coronary artery bypass graft surgery (CABG) patients are overly restrictive, hindering recovery. As the sternotomy repair must withstand repeated coughs during convalescence, this provides a benchmark for the force tending to separate the incision that can be tolerated. Methods: Nine volunteers performed 5 weightlifting activities (lifting 5 lbs [2.3 kg], lifting a 25-lb simulated grandchild [11.4 kg], lifting a 30-lb suitcase [13.6 kg], lifting two 20-lb weights [18.2 kg], and lifting a gallon of milk to a counter [3.7 kg]), plus coughing. Valsalva forces were detected using a mouthpiece configured with an Ashcroft Inc. expiratory pressure gauge (model N10-120CMW). Three measurements were taken for each activity to calculate the mean internal forces while external forces on the sternotomy were calculated using vector algebra. Total force exerted on the sternotomy by the cough was compared to the total force exerted by each of the 5 activities using paired t-tests. Results: The cough exerted a significantly greater force across the median sternotomy (mean 27.5 kg-mass) than any of the five weightlifting activities (p < 0.05). The greatest difference was observed was for lifting a 5-lb weight (22.5 kg-mass), and the smallest for lifting two 20-lb weights (4.4 kg-mass). Conclusion: Lifting even 40 lbs puts less force on the median sternotomy incision than a cough. The strength of the repair is significantly greater than is implied by the recommendation to “not lift more than 5 lbs”.

coronary bypass surgery - cardiovascular surgery - thoracic surgery

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Addendum

Calculation of force on the median sternotomy

1. To calculate the force on the median sternotomy closure caused by a Valsalva maneuver or a cough.

We assumed that the chest was a cylinder and that the forces acting during coughing along the circular portion of the cylinder would be directed radially. The force across a sternotomy closure can be calculated as:

T = rlP

whereby T is the resultant force required to keep the sternum closed, r is the radius, l is the length of the cylinder, P is the distending pressure (difference between internal and external pressures which puts stress on the sternotomy closure) [[12]]. The force on the sternotomy closure is perpendicular to the line of the incision. This is because the stress is really a hoop stress, that is, a stress around the circumference of the cylinder (and thus perpendicular to the sternotomy closure).

The “cylindrical chest” model is required because the solution is known: it provides a necessary simplification that avoids the complexity created by the arbitrary variation in chest shape that prohibits the calculation of actual forces.

2. To calculate the external force on the sternotomy closure from an activity such as lifting half a gallon of milk.

These calculations were based on a planar model (which assumes the external force is applied in a plane) rather than a cylindrical model

Looking at the diagram presented in [Fig. 1], it is assumed:

• that the sternotomy is down the center of the breastbone, splitting it in half longitudinally;

• that the force from an activity is applied downward at the shoulder, and;

• that the force then has a component that would pull apart the closure, since the sternotomy closure is “open” at the top where it is held together only by sutures, and has a “pivot point” at the bottom where cartilage holds it together;

• that the dimensions of the body are such that the distance from the point of the shoulder to the centerline of the body is about the same as the distance from the assumed pivot point to the line parallel to the ground and even with the point of the shoulder.

Based on these assumptions, the force applied downward at the point of the shoulder results in a force of the same magnitude perpendicular to the closure (shown in the vector diagram in [Fig. 1])

3. To calculate the total force on the sternotomy closure.

Since the force from the external activity is in the same direction as the force from the cough or Valsalva maneuver (i.e., perpendicular to the closure), they are additive. Since the force resulting from the cough or Valsalva maneuver was calculated using a cylindrical model, and the force resulting form the activity (e.g., lifting the half gallon of milk) was calculated using a planar model, it is necessary to assume the applied forces do not cause the closure to separate. In this case, the wall of the chest can be assumed to be either a cylinder or a plane since there is no motion; this is an assumption commonly applied when calculating stresses and modeling complex structures.