Body Temperature

Body temperature is routinely measured to assess for signs of inflammation or infection. Even though the body’s skin temperature varies widely in response to environmental conditions and physical activity, the temperature inside the body, the core temperature, remains relatively constant—about 37° C (98.6° F), with a daily variation of ±0.5° C (1° to 2° F). Under normal circumstances the body is able to maintain this constant temperature through various physiologic compensatory mechanisms, such as the autonomic nervous system and special receptors located in the skin, abdomen, and spinal cord.

In response to temperature changes the receptors sense and send information through the nervous system to the hypothalamus. The hypothalamus, in turn, processes the information and activates the appropriate response. For example, an increase in body temperature causes the blood vessels near the skin surface to dilate—a process called vasodilation. Vasodilation, in turn, allows more warmed blood to flow near the skin surface, thereby enhancing heat loss. In contrast, a decrease in body temperature causes vasoconstriction, which works to keep warmed blood closer to the center of the body—thus working to maintain the core temperature.

At normal body temperature, the metabolic functions of all body cells are optimal. When the body temperature increases or decreases significantly from the normal range, the metabolic rate and therefore the demands on the cardiopulmonary system also change. For example, during a fever the metabolic rate increases. This action leads to an increase in oxygen consumption and to an increase in carbon dioxide production at the cellular level. According to estimates, for every 1° C increase in body temperature, the patient’s oxygen consumption increases about 10%. As the metabolic rate increases, the cardiopulmonary system must work harder to meet the additional cellular demands. Hypothermia reduces the metabolic rate and cardiopulmonary demand.

As shown in Figure 2-1, the normal body temperature is positioned within a relatively narrow range. A patient who has a temperature within the normal range is said to be afebrile. A body temperature above the normal range is called pyrexia or hyperthermia. When the body temperature rises above the normal range, the patient is said to have a fever or to be febrile. An exceptionally high temperature, such as 41° C (105.8° F), is called hyperpyrexia.

The four common types of fevers are intermittent fever, remittent fever, relapsing fever, and constant fever. An intermittent fever is said to exist when the patient’s body temperature alternates at regular intervals between periods of fever and periods of normal or below-normal temperatures. In other words, the patient’s temperature undergoes peaks and valleys, with the valleys representing normal or below-normal temperatures. During a remittent fever, the patient has marked peaks and valleys (more than 2° C or 3.6° F) over a 24-hour period, all of which are above normal—that is, the body temperature does not return to normal between the spikes. A relapsing fever is said to exist when short febrile periods of a few days are interspersed with 1 or 2 days of normal temperature. A continuous fever is present when the patient’s body temperature remains above normal with minimal or no fluctuation.

Hypothermia is a core temperature below normal range. Hypothermia may occur as a result of (1) excessive heat loss, (2) inadequate heat production to counteract heat loss, and (3) impaired hypothalamic thermoregulation. Box 2-1 lists the clinical signs of hypothermia.

Hypothermia may be caused accidentally or may be induced. Accidental hypothermia is commonly seen in the patient who (1) has had an excessive exposure to a cold environment; (2) has been immersed in a cold liquid environment for a prolonged time; or (3) has inadequate clothing, shelter, or heat. A reduced metabolic rate may compound hypothermia in older patients. In addition, older patients often take sedatives, which further depress the metabolic rate. Box 2-2 lists common therapeutic interventions for patients with hypothermia.

Induced hypothermia refers to the intentional lowering of a patient’s body temperature to reduce the oxygen demand of the tissue cells. Induced hypothermia may involve only a portion of the body or the whole body. Induced hypothermia is often indicated before certain surgeries, such as heart or brain surgery.

Rate

The normal pulse rate (or heart rate) varies with age. For example, in the newborn the normal pulse rate range is 100 to 180 beats per minute (bpm). In the toddler the normal range is 80 to 130 bpm. The normal range for the child is 65 to 100 bpm, and the normal adult range is 60 to 100 bpm (see Table 2-1).

A heart rate lower than 60 bpm is called bradycardia. Bradycardia may be seen in patients with hypothermia and in physically fit athletes. The pulse may also be lower than expected when the patient is at rest or asleep or as a result of head injury, vomiting, or advanced age. A pulse rate greater than 100 bpm in adults is called tachycardia. Tachycardia may occur as a result of hypoxemia, anemia, fever, anxiety, emotional stress, fear, hemorrhage, hypotension, dehydration, shock, and exercise. Tachycardia also is a common side effect in patients receiving certain medications, such as sympathomimetic agents (e.g., adrenaline or dobutamine).

Rhythm

Normally the ventricular contraction is under the control of the sinus node in the atrium, which generates a normal rate and regular rhythm. Certain conditions and chemical disturbances, such as inadequate blood flow and oxygen supply to the heart or an electrolyte imbalance, can cause the heart to beat irregularly. In children and young adults it is not uncommon for the heart rate to increase during inspiration and decrease during exhalation. This is called sinus arrhythmia.

Strength

The quality of the pulse reflects the strength of left ventricular contraction and the volume of blood flowing to the peripheral tissues. A normal left ventricular contraction combined with an adequate blood volume will generate a strong, throbbing pulse. A weak ventricular contraction combined with an inadequate blood volume will result in a weak, thready pulse wave. An increased heart rate combined with a large blood volume will generate a full, bounding pulse.

Several conditions may alter the strength of a patient’s pulse. For example, heart failure can cause the strength of the pulse to vary every other beat while the rhythm remains regular. This condition is called pulsus alternans. The practitioner may detect a pulse that decreases markedly in strength during inspiration and increases back to normal during exhalation, a condition called pulsus paradoxus that is common among patients experiencing a severe asthmatic episode. This phenomenon can also be heard when blood pressure is measured.

Finally, the stimulation of the sympathetic nervous system increases the force of ventricular contraction, increasing the volume of blood ejected from the heart and creating a stronger pulse. Stimulation of the parasympathetic nervous system decreases the force of the ventricular contraction, thus leading to a decreased volume of blood ejected from the heart and a weaker pulse. Clinically the strength of the pulse may be recorded on a scale of 0 to 4+ (Box 2-3).

For peripheral pulses that are difficult to detect by palpation, an ultrasonic Doppler device may also be used. A transmitter attached to the Doppler is placed over the artery to be assessed. The transmitter amplifies and transmits the pulse sounds to an earpiece or to a speaker attached to the Doppler device. The heart rate can also be obtained through auscultation by placing a stethoscope over the apex of the heart.

Blood Flow

Blood flow is equal to cardiac output. Cardiac output is equal to the product of (1) the volume of blood ejected from the ventricles during each heartbeat (stroke volume) multiplied by (2) the heart rate. Thus a stroke volume (SV) of 75 mL and a heart rate (HR) of 70 bpm produce a cardiac output (CO) of 5250 mL/minute, or 5.25 L/min (CO = SV × HR). The average cardiac output in the resting adult is about 5 L/min.

A number of conditions can alter stroke volume and therefore blood flow. For instance, a decreased stroke volume may develop as a result of poor cardiac pumping (e.g., ventricular failure) or as a result of a decreased blood volume (e.g., during severe hemorrhage). Bradycardia may also reduce cardiac output and blood flow. Conversely, an increased heart rate or blood volume will likely increase cardiac output and blood flow. In addition, an increased heart rate in response to a decreased blood volume (or stroke volume) may also occur as a compensatory mechanism to maintain normal cardiac output and blood flow.

Resistance

The friction between the components of the blood ejected from the ventricles and the walls of the arteries results in a natural resistance to blood flow. Friction between the blood components and the vessel walls is inversely related to the dimensions of the vessel lumen (size). Thus as the vessel lumen narrows (or constricts), resistance increases. As the vessel lumen widens (or relaxes), the resistance decreases. The autonomic nervous system monitors and regulates the vascular tone.

Table 2-5 presents factors that affect the blood pressure.

Abnormalities

Pulsus Paradoxus

Pulsus paradoxus is defined as a systolic blood pressure that is more than 10 mm Hg lower on inspiration than on expiration. This exaggerated waxing and waning of arterial blood pressure can be detected with a sphygmomanometer or, in severe cases, by palpating the pulse at the wrist or neck. Commonly associated with severe asthmatic episodes, pulsus paradoxus is believed to be caused by the major intrapleural pressure swings that occur during inspiration and expiration. The reason for this phenomenon is described in the following sections.