- Anemia is a generic term defined as a decrease of total erythrocyte mass and can be caused by a diverse set of pathophysiological processes. Whatever the precise etiology, anemias result in decreased blood oxygen carrying capacity, often manifesting clinically as a vague complex of symptoms, typically centered around fatigue and dyspnea on exertion. Although anemias can be categorized in a large variety of ways, they are often clinically distinguished by the physical size of the patient's erythrocytes into microcytic, normocytic, and macrocytic anemias.
- As mentioned, anemia is defined as a decrease in the body's red cell mass; however, it is impractical to accurately measure the total mass of red cells in a patient. Consequently, the two proxy variables of hematocrit (Hct) and hemoglobin concentration (Hgb) are typically used. It is important to point out that, when possible, a patient's hemotocrit or hemoglobin should always be compared to a previous baseline for that patient. Frequently, a patient's Hct/Hgb may be within the lab normal range; however, the clinical suspicion for a pathological process should be high if there has been a significant fall from the patient's previous baseline.
- The basic pathophysiological consequence of reduced red cell mass is a reduction in the blood's oxygen carrying capacity. Simply put, regardless of the elegant mechanisms of oxygen-hemoglobin dissociation discussed in Oxygen Transport, if there is not enough hemoglobin within the blood to transport oxygen, the body's tissues will receive insufficient oxygen to meet their metabolic demands, ultimately yielding tissue hypoxia.
- Anemia often presents with a complex of vague symptoms such as fatigue, light-headedness, headaches, vague abdominal pain, and nausea. When severe, or occurring in the context of increased cardiac demand, anemia can result in myocardial ischemia and thus a symptomological picture similar to Ischemic Heart Disease involving chest pain and dyspnea on exertion.
- Classically the anemic patient displays paleness of the conjunctiva or other mucosa as well as sluggish capillary refill in the nail-beds. To increase oxygen delivery, the heart rate and ejection fraction may increase, potentially yielding tachycardia as well as a systolic ejection murmur, often referred to as a "flow murmur" best heard at the heart's base. The basis of this systolic ejection murmur is an increased volume of dilute, anemic blood flowing across a normal aortic valve, creating turbulent flow that is heard as a murmur similar in quality to that of aortic stenosis
- Anemias can be categorized in a number of ways; however, the mean physical size of the erythrocyte, measured as the "Mean Corpuscular Volume (MCV)" is often used as the first guide-post in narrowing the list of potential etiologies. Anemias characterized by lower than normal MCVs are referred to as "Microcytic Anemias", anemias with normal MCVs termed "Normocytic Anemias", and anemias with higher than normal MCVs termed "Macrocytic Anemias." These subtypes are discussed further within their own pages. Hemolytic Anemias are a special sub-group of normocytic anemias that are caused by red cell destruction and are discussed in their own section.