The Markov model is used to follow the disease progression of all members of a cohort simultaneously on 5 different disease paths. For the simulation, transitions between states take place at discrete time intervals 1 year apart. Thus, at the end of each 1 year period, portions of the cohort can move from one disease state to another or stay in the same disease state. The simulation program determines what proportion of the cohort will move from one state to another on the basis of the transition probability. In several cases, an individual can experience a complication event that the patient either dies of or survives during the period. The Markov model keeps track of the number of patients who are in each state in each period. It also keeps track of the cumulative incidence of patients who have undergone complication events such as lower extremity amputation (LEA), angina, cardiac arrest (CA) or myocardial infarction (MI), or stroke.
The 5 disease paths diagramed here are, from top to bottom, coronary heart disease (CHD), stroke, nephropathy, neuropathy, and retinopathy. Each disease path starts with a "normal" disease state and moves from left to right through subsequent disease states and complication events to death (or, in the case of retinopathy, blindness).
Patients on the CHD disease path proceed from a normal state to a CHD event. From there, patients move to 1 of 3 disease states: angina, a history of CA/MI, or death. Patients who move to angina may either remain or move either directly to death or to a CA/MI event. Patients who move to a CA/MI event may continue either to death or to a history-of-CA/MI state. Patients may also move from a CHD event directly to a history-of-CA/MI state where they either remain or move to death.
Patients on the stroke disease path move from a normal state to stroke (stroke events are noted as "within-year events"). From the stroke event, they may move to a history-of-stroke state and either remain there or move to death. Or they may move from the stroke disease directly to death.
Patients on the nephropathy disease path go from normal to a state of low or high microalbuminuria state where they either remain or move to a clinical nephropathy state, where they either remain or move to an ESRD state where they may remain or move to ESRD death.
Patients on the neuropathy disease path go from normal to a peripheral neuropathy state where they either remain or move to an LEA event. From an LEA event, patients either move directly to LEA death or enter a history-of-LEA disease state. Patients in a history-of-LEA state may remain, or move to subsequent LEA events. From subsequent LEA events, patients may move back to a history-of-LEA state or move to LEA death.
Patients on the retinopathy disease path go from a normal state to a photocoagulation disease state then remain there or go on to a blind state, where they remain.