1. Estimate core temperature that is independent of ambient fluctuations because of hypothalamic regulation.

1. Indications are extensive and include infection, temperature dysregulation syndromes (autonomic dysfunction, neuroleptic malignant syndrome, malignant hyperthermia, endocrine syndromes), certain toxidromes (anticholinergic, serotonin syndrome), and environmental causes of temperature dysregulation.

1. Measure at least every 4 hours, continuously when <36°C or >39°C, or temperature-altering interventions (cooling blanket, active rewarming) are applied. Probes in the mouth, axilla, auditory canal, or on the skin overlying the temporal artery allow intermittent monitoring. Probes in the pulmonary artery, rectum, esophagus, or bladder allow continuous monitoring.

1. Rectal site can transmit resistant enteral bacteria. Axillary temperature is considered 1°F cooler than core temperature.

1. Assure adequate perfusion pressure for sufficient substrate/oxygen delivery and metabolic waste/carbon dioxide (CO₂) removal.

i. Allows continuous monitoring. A fluid column continuously transmits arterial pressure to a transducer, where it is converted into electrical signals through diaphragm deformation-induced resistance changes in a Wheatstone bridge.

i. Allows intermittent monitoring. Analyzes magnitude/shape of arterial pressure oscillations with intermittent, controlled, slow blood pressure cuff inflation/deflation.

a. Indications for continuous monitoring are patients who are on vasoactive medications or who require frequent blood gas analyses.

a. Indications for intermittent monitoring are patients at risk for hypo- or hypertension, which includes all patients admitted to an intensive care unit.

a. Seldinger technique is used for insertions at the radial, femoral, and axillary sites. Once the catheter is placed, it is connected to the fluid column that is in specialized pressure tubing. The system is zeroed at the level of the right atrium.

a. Cuff bladder width should equal 40%, and the length should equal/exceed 60% of extremity circumference. The cuff bladder is inflated until the distal pulse is obliterated. Air is slowly released from the cuff. When systolic pressure is reached, blood flow begins and is detected by the monitor as arterial wall vibrations. Arterial vibrations stop when diastolic pressure is reached.

a. Complications include distal vessel occlusion, hemorrhage, and infection.

b. Erroneously high pressures from small air bubbles/heart rates close to transducer system’s resonant frequency (overshoot).

c. Erroneously low pressures from large air bubbles and catheter thrombus/heart rates above transducer system’s resonant frequency (damping).

a. Intermittent measurements do not reflect rapidly changing hemodynamics.

b. Inadequately sized cuffs (width/length) overestimate.

c. Poor correlation with intra-arterial values at pressure extremes.

d. Complications include distal limb ischemia and venous stasis with prolonged/frequent cuff inflation or deflation failure; do not perform on limbs with compromised arterial/venous/lymphatic circulation.

e. Mean arterial pressure is a more reliable indicator of tissue perfusion than systolic and diastolic pressures.

1. Monitor endogenous cardiac electrical impulses to detect malignant arrhythmias or changes in electrical morphology as a result of structural
damage to the myocardial tissue. Computerized arrhythmia detection is based on heart rate variability, electrocardiogram (ECG) intervals/segment durations, and ECG morphology.

1. All critically ill patients should have an ECG as a baseline upon admission. Other indications are a high likelihood of malignant arrhythmia and/or cardiac ischemia.

1. Artifacts arise from muscle activity or poor transmission (obesity, poor skin preparation, improperly positioned electrodes).

1. Assure adequate gas exchange.

a. Measures thoracic impedance changes due to respiration-induced alterations in thoracic geometry and thus quantifies respiratory rate.

a. Measures inhaled/exhaled airflow versus time; derives respiratory rates, tidal volumes, and minute ventilation.

a. Uses photoplethysmography to measure the difference in light absorption spectra of oxygenated/deoxygenated hemoglobin across pulsatile tissue bed over time; calculates absorption ratio change over time, which estimates arterial oxygen saturation.