This is an interesting question and one which lacks a truly straight forward explanation. When you use the term fixed direct costs you are really combining two specific meanings from the world of cost accounting (which is still rather new in the field of healthcare finance). A cost can be either fixed or variable depending upon whether that cost has a relationship to the volume of use over a specific set of parameters. For example, if the cost is incurred on a monthly basis and the cost does not typically vary based on activity or volume changes we would consider the cost to be fixed in nature. If however, the cost is tied to volume changes such as typically seen for something like raw material usage in a manufacturing setting then the cost would be consider variable, that is, it varies in proportion to the raw materials consumed. Direct/Indirect costs on the other hand are concerned with assigning cost based on specific identifiable usage (direct) or by a method of allocation to fairly assign a portion of the cost to an overall cost structure (indirect). A direct cost again could be the raw material used in a manufacturing setting if the raw material could easily be assigned to a specific product or part. Indirect costs could be something like rent of the manufacturing facility which would need to be allocated in some fashion to all of the parts produced over a specific period of time. So using these definitions you could have a piece of equipment (depending upon how it is acquired purchased/lease or depending upon how the cost is tracked) which is a fixed direct cost or a variable indirect costs. Remember that cost accounting is also routinely referred to as managerial accounting and has far fewer hard and fast rules as does financial accounting. Managerial accounting is meant as a management tool and particularly costing methods must be designed to the specific activity at hand. In order to correctly indentify the type of cost you must have a clear understanding first of how the cost is incurred and next, how the cost will be assigned to the overall cost structure.
I will try and provide just a quick consideration on how a capital purchase could be a fixed cost. Remember when you purchase a piece of equipment you will be depreciating the equipment over some period of time. The purpose of depreciation is simply to allocate the expense of the equipment over a period the equipment is most likely to be useful, in other words it is not a valuation method it is an expense allocation method. You can also use this method to allocate cost on a per part basis (variable cost) if you have a method to track the specific usage. If however, the equipment had little relationship to a specific activity (such as a piece of HVAC equipment) then the cost could be allocated over a period of time and considered a fixed cost rather then variable. Remember YOU define the type of cost it will be by following the definitions of direct/indirect, fixed/variable, with consideration of the specific nature of the activity you are tracking.