What is the MICU?

A grab bag, "ICU of exclusion" - everything that doesn't go elsewhere, similar to internal medicine or general surgery

Eg surgical might patients go to SICU, neuro patients go to SICU, but if they're none of these, they go to MICU. If they developed a "pneumonia ICU," all those patients would leave the MICU.
Depends on how many specialty units you have - larger/more academic centers have more . If you don't have a CCU, your cardiac patients will be in MICU, for instance
What kind of patient other specialty units want to accept tends to depend on local culture and bed availability/volume - eg some SICUs want to take pancreatitis patients (even if non-surgical), some neuro units don't want to see routine non-operative intracranial hemorrhages because they don't have room for it, etc. Generally, if nobody else wants a type of patient, it's the MICU's.
In smallest centers, there is only one ICU which takes it all.

The MICU patient

A majority of MICU patients are elderly patients with chronic illnesses. This means:

Usually complex
Usually many additional/contributing comorbidities, not directly part of the reason for admission, but still complicating care

In the majority of cases, the presenting problem is not an isolated acute problem, but an acute exacerbation of a chronic process.

Usually an incurable organ failure (COPD, CHF, CKD, diabetes, liver failure) which is otherwise relatively stable/compensated at baseline
The chronic disease is prone to acute decompensations/exacerbations/flare-ups leading to instability, causing ICU admission

Common causes for exacerbation: med issues (non-adherence, lack of access), infection (local or systemic), other acute disease, progression of underlying disease

Even when a problem seems truly acute in the MICU, usually it's caused indirectly by a background of chronic frailty, aging, degenerative diseases, psychiatric factors, etc.

Note that patients with acute decompensations of chronic organ failures can be extremely sick, yet have a relatively good prognosis - they are fundamentally recoverable problems (otherwise they would not lead to chronic disease). But they can still kill if not treated promptly, and the end result after recovery is usually a little worse baseline than they started.

Largely not an ICU issue, but the highest risk time after an acute flare is right afterwards - they remain tenuous and there are often gaps in the transition back to outpatient care

The general vibe in the MICU

There are relatively few "medical mysteries" in most other ICU services, as their very admission to a specialty requires a diagnosis of some type

The "bang for buck" of care depends on how much acute versus chronic element there is. When the ratio is very high (a truly acute, reversible issue), you can affect outcomes a lot. When it's low, the benefit of even the best critical care is slim. Fundamentally, MICU has a lower ratio than other places like the SICU.
Need to embrace the care of elderly, chronically ill patients to enjoy the MICU. Palliative care and addressing goals of care plays an important role for most patients.
Even the best MICU "save" may still result in a patient with a relatively short life expectancy due to other problems.

Surgical patients - get better fast, lots of turnaround
Neuro patients - often very long stays due to trajectory of disease
Medical falls in between. Many who leave may come back (from floor or home). "Frequent fliers" are common.

Sepsis: pneumonia (including aspiration) in particular
Respiratory failure: infectious, CHF, COPD
Altered mental status: various causes
DKA, alcohol withdrawal, A-fib with RVR, liver disease may all be more or less common depending on catchment and other units (e.g. some centers may allow reasonable cases of RVR, DKA, alcohol withdrawal, etc onto a stepdown unit

Workflow

In surgical ICUs, surgical teams may be primary, or at least exercising heavy involvement in important decisions

Internal medicine or other residents
APPs may be involved, either woven into those teams as additional staff, or organized into separate teams
Fellows may be present

May be pure critical care, but fewer of these training programs around
Intensivists come to critical care from other pathways, but internal medicine to critical care is most common for most MICUs

Anesthesia, surgery, neurology, others - although some centers may try to put these intensivists in other ICU types

Closing thoughts

MICU is not for everyone, but is arguably the "oldest" or most fundamental flavor of critical care
It can be the broadest and require the most general knowledge base
It can inherently be less fast-paced/exciting/stimulating than some settings
Every ICU patient has "medical" problems; we all need to have a strong understanding of all aspects of critical care regardless of our setting.

Practicalities and workflows