Non Invasive Cardiac Output is the amount of blood pumped by the heart in one minute. Cardiac output – It is the product of heart rate x Stroke volume and is expressed in absolute terms of liters / minute (l/min). Cardiac output can be normalized for body size by divided cardiac output by body surface area (BSA) and is named cardiac index. The advantage of normalizing cardiac output for BSA is that the normal range would apply to all patients, regardless of size. For example, a large male might have a non invasive cardiac output of 6.8 l/min and a small female might have a cardiac output of 3.4 l/min. However, when normalized for body size, both might have a cardiac index of 3.0 l/min.
One of the major factors which affect Non Invasive Cardiac Output is the metabolic rate of the body. Consequently, factors that affect the metabolic rate also affect the Non Invasive Cardiac Output. For example:
Age: metabolic rate is highest in youth and gradually decreases with age. For example, a cardiac index of 4.5 l/min/m2 is normal for a 7 year old, while 2.5 l/min/m2 is normal for a 70 year old. Cardiac Index decreases by about 5% per decade from age 20 to age 70.
Posture: cardiac output will decrease by approximately 10% when a person changes from a lying to a sitting position and approximately 20% from a lying to a standing position.
Exercise: In well-conditioned elite athletes, cardiac output may increase six-fold from rest to maximal exercise. In other words, the resting cardiac output of 5 l/min may increase to 30 l/min at maximal exercise.
Body Temperature: High body temperature or fever will cause an increase in cardiac output. Low body temperature will result in a lowering of cardiac output (unless the patient begins to shiver, which increases metabolic needs of the muscles and will result in increasing cardiac output). In some surgical procedures, such as coronary artery bypass surgery, the patient’s body temperature is purposely lowered to decrease the metabolic and, consequently, cardiac demands during the procedure.
Body Size and Composition: The larger a person’s body size, the greater the cardiac output will be. For example, a cardiac output of 10 l/min may be normal for a person weighing 120 kg, while 4 l/min may be normal for a person weighing 50 kg.
Gender: Because males typically have more muscle mass than females, the resting cardiac output of males will normally be higher than the cardiac output of females. For example, a male who weighs 80 kg would normally have a higher resting cardiac output (approximately 10%) than a female who also weighs 80 kg.
Environmental Temperature and Humidity: As the body’s core temperature increases or decreases, the cardiovascular system responds to maintain the desired core temperature. Reflex changes occur not only in cardiac output, but also in many other parameters including systemic vascular resistance. To achieve the desired body temperature, cardiac output is either increased or decreased and blood flow to the skin is either increased to cool the body or decreased to maintain body temperature.
Critically Ill / Post Surgery: The body’s metabolic demands increase significantly when the body is ill or traumatized. In critically ill or post-surgical patients, it is very common to see cardiac output higher than normal (increases of 50% or more are not uncommon).
Diseases: Diseases, such as hyperthyroidism or hypothyroidism (abnormal function of the thyroid gland), will also affect cardiac output.
Psychological Factors: Strong emotions, such as fear and anxiety, also result in increased cardiac output, even though the body’s metabolic demands have not appreciably changed. This is known as the “fight or flight” syndrome, since the body is physiologically preparing to fight or flee. For example, if you are very nervous about demonstrating the NICaS to a group of doctors and nurses and if you hook yourself up to the NICaS, your hemodynamic parameters will be much higher than they would normally be at rest. Another very common example of psychological factors affecting physiological parameters is the “anticipatory response.” When a person begins thinking about strenuous exercise, the cardiovascular system begins to increase its function to prepare the body for the increased metabolic demands of exercise. An excellent example of this is a sprinter preparing for a 100-meter dash. When he is in the starting blocks prior to the start of the race, his metabolic demands are very low; however, his heart rate, blood pressure, cardiac output are extremely high as his body anticipates the increased metabolic demands that will occur when the race starts.
Cardiac output is an extremely informative physiological parameter. Changes in cardiac output can provide a much earlier warning of significant changes in the body’s function or metabolic needs than changes in blood pressure. For example, cardiac output may change as much as 30% before any changes are noted in blood pressure. This is due to reflex cardiovascular changes (vasoconstriction and vasodilation) that attempt to maintain a stable blood pressure.
While monitoring critically ill patients, it is important to remember that the metabolic needs of these patients are much greater than healthy persons of the same size. Consequently, a cardiac index of 2.0 – 3.0 l/min for a critically ill patient could be life-threatening, even though 2.5 l/min is usually considered within normal limits. In addition, a sudden reduction in a patient’s cardiac output/index could be life-threatening. In general, cardiac index values of less than 1.8 l/min/m2 are considered extremely serious and values less than 1.0 l/min/m2 are not sufficient to sustain life.
Cardiac output may be increased by modifying any of the underlying parameters that affect the cardiac output. These include increasing heart rate, increasing stroke volume by increasing preload (end diastolic volume), contractility (speed and strength of contraction of the left ventricle) or decreasing afterload (systemic vascular resistance).
Unlike the “gold standard” of measuring cardiac output that require the insertion of a catheter into the heart, NICaS offer a simple and non-invasive way to measure cardiac output. This enable clinicians at all levels of healthcare to measure cardiac output.