Pt. Safety Risk

Swiss Cheese Model

The Swiss cheese model of accident causation is developed by John Reason . The concept behind this model is that catastrophic errors do not occur in isolation. Rather there are multiple opportunities for errors to occur. It is only when the systems align in a certain way, and the fail-safe mechanisms all fail, that the catastrophic event occurs.

In the Swiss cheese model, an organisation’s defenses against failure are modeled as a series of barriers, represented as slices of cheese. The holes in the slices represent weaknesses in individual parts of the system and are continually varying in size and position across the slices. The system produces failures when a hole in each slice momentarily aligns, permitting (in Reason’s words) “a trajectory of accident opportunity”, so that a hazard passes through holes in all of the slices, leading to a failure

Simply explained, if you look at the above figure , each slice of Swiss cheese has holes in it, but the hole location will not be consistent to allow a straight line to be drawn from the front to the back. There is a barrier preventing further passage through the cheese. One failure (hole) occurs but does not contribute to another failure (Duke, 2014).


The wrong patient is brought into the operating room and prepped. Before the surgery starts, a time out is called and someone realizes that this is not the correct patient for the procedure. This stops further errors from occurring.

It is only when the holes all line up one after another, that a catastrophic event occurs.

Continuing with our surgery example, a patient is brought into the surgery but:

  • No one checks the patient’s arm band, so they do not know if this is the correct patient.
  • The surgeon is in a hurry to start and rushes through the time out.
  • The OR staff are not really ready to start, but no one speaks up and the surgery starts.
  • The surgeon begins the surgery to remove the cataract in the right eye and is upset when the lens available is not the correct one to be inserted in that eye.
  • Someone runs to get the correct lens, which the surgeon implants.

Finally, when the patient is taken to the recovery room, the staff realize that the surgery should have been performed on the left eye.

All of these errors (holes) had to line up perfectly for this adverse event to occur.



Dr. Khalid Abulmajd

Healthcare Quality Consultant

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