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Standard Patient Monitors
- 3. Pulse Oximetry Limitations to pulse oximetry: vasoconstriction, movement, shivering, methemoglobin, carboxyhemoglobin, methylene blue Non-pulsatile (venous) flow is discounted Light is absorbed at different wavelengths 940 nm (infrared) 660 nm (red) Probe emits light across a tissue bed OxyHb DeOxyHb S = (AC/DC)660 (AC/DC)940 Normal SpO2 > 94% Target SpO2 > 90%
- 4. Pulse Oximetry False Low Reading False High Reading Vasoconstriction Carbon Monoxide Poisoning Dysrhythmias Shivering Electrocautery Injectable Dye (methylene blue, ICG) Methemoglobinemia
- 5. EKG • 3-Electrode System • Leads I, II, and III • Lead II is best for detecting P waves and sinus rhythm • 5-Electrode System • V5 is 75% sensitive for detecting ischemic events; II + V5 is 80% sensitive; II + V4 + V5 together is 98% sensitive
- 6. Noninvasive Blood Pressure • MAP is primary measurement; SBP and DBP are derived from algorithms • Cuff too small = falsely HIGH BP. Cuff too big = falsely LOW BP
- 7. Arterial Pressure Monitoring • Indications • Beat-to-beat BP monitoring • Repeated blood sampling • Complications • infection, thrombosis, hematoma, distal ischemia, and air embolism, and hemorrhage Transducer Setup • Zeroing = exposes the transducer to air-fluid interface at any stopcock, thus establishing Patm as the “zero” reference pressure. • Leveling = assigns the zero reference point to a specific point on the patient; by convention, the transducer is “leveled” at the right atrium. ** Every difference of 10cm = 7.5mmHg If transducer reads a MAP of 75 at the level of the heart but the brain is 20 cm above that level, the brain has a MAP of 60
- 8. Capnography • Measures exhaled CO2 • With normal respiratory physiology, you can expect PaCO2 to be a value 6 higher than ETCO2 due to dead space ventilation • Capnogram Phases I. Dead space gas exhaled II. Transition between airway and alveolar gas III. Alveolar plateau IV. Inspiration
- 10. References • Adriano, A and Skanchy, J. 2018 CA-1 tutorial textbook. 12th Edition. Stanford University Medical Center Department of Anesthesiology. 2018. Retrieved from http://ether.stanford.edu/ca1_new/Final-%202018%20CA- 1%20Tutorial%20Textbook.Smartphone%20or%20Tablet.pdf • Mark JB, and Slaughter TF. Cardiovascular monitoring. In Miller RD (ed), Miller’s Anesthesia, 6th ed. Philadelphia: Elsevier Churchill Livingstone, 2005. • Moon RE, and Camporesi EM. Respiratory monitoring. In Miller RD (ed), Miller’s Anesthesia, 6th ed. Philadelphia: Elsevier Churchill Livingstone, 2005. • Morgan GE, Mikhail MS, and Murray MJ. Clinical Anesthesiology, 4th ed. New York: McGraw-Hill Companies, Inc., 2006. • Narang J, and Thys D. Electrocardiographic monitoring. In Ehrenwerth J, and Eisenkraft JB (eds), Anesthesia Equipment: Principles and Applications. St. Louis: Mosby, 1993. • Marino, P. Marino’s the ICU Book, 4th Edition. Philadelphia: Wolters Kluwer/Lippincott, Williams, & Wilkins, 2014. • Torp, K. Pulse Oximetry. In Faust’s Anesthesiology Review, 4th ed. Philadelphia: Elsevier Saunders, 2015.
Editor's Notes
- Vigilant monitoring is key to critical care and patient safety. ICU bedside providers must have a good understanding of how monitors work in order to interpret the data and make sound clinical decisions.
- Oxygenation, ventilation, circulation and temperature are the key parameters that must be monitored during intensive care.
- Pulse oximeters have two components, a probe and a computerized unit. The probe is placed on the patient’s finger or toe. In small infants the probe may cover the entire hand or foot. The probe has a LED light and photodetector. The light is shone through the patient’s tissues. Oxygenated and deoxygenated hemoglobin absorb the light at different wavelengths - oxyhemoglobin at 940 nanometers and deoxyhemoglobin at 660 nanometers. The ratio of wavelength absorption is converted to a percentage and displayed on the pulse oximeter and referred to as the “SpO2”. Normal SpO2 is > 94%. Since it the arterial oxygen saturation that is of interest, the pulse oximeter discounts any non-pulsatile flow such as that from venous blood, capillaries and tissues.
- Pulse oximetry requires a regular pulse for proper data collection and accurate recording of the SpO2. Disruption of a regular pulse detection can lead to erroneous SpO2 recordings. Examples include: Vasoconstriction from hypothermia, vasoactive medications, or hypotension Dysrhythmias such as atrial fibrillation, ventricular tachycardia or other abnormal cardiac rhythms Shivering and electrocautery can also interfere with accurate SpO2 readings Disruption of the wavelength absorption ratio can also lead to false SpO2 recordings, for example injectable dyes lead to falsely low values whereas carbon monoxide poisoning can lead to falsely elevated SpO2 values
- Electrocardiograms are standard monitors for ICU patients. Using a 3-electrode setup: Allows monitoring of Leads I, II, and III, but only one lead (i.e. electrode pair) can be examined at a time while the 3rd electrode serves as ground. Lead II is best for detecting P waves and sinus rhythm Using a 5-electrode setup: Four limb leads + V5 (left anterior axillary line, 5th intercostal space) V5 is 75% sensitive for detecting ischemic events; II + V5 is 80% sensitive; II + V4 + V5 together is 98% sensitive
- Most automated blood pressure cuffs use a technique called oscillimetry where cuff inflation leads to occlusion of blood flow, slow release of cuff pressure allows resumption of blood flow, and oscillations associated with turbulent resumption of flow are recorded. The point of maximal oscillation amplitude corresponds to mean arterial pressure. With oscillemetry, the systolic and diastolic blood pressures are derived from algorithms. If the blood pressure cuff is too small, the BP will be falsely high. If the cuff is too large, the BP will be falsely low
- Arterial lines are invasive and can be painful for patients during placement. Indications for placing an arterial line include close BP monitoring (for example when using vasopressor or inotrope medications) and repeated blood sampling (such as arterial blood gas samples). Complications include, pain, bleeding, hematoma, infection, thrombosis and ischemia. Arterial lines are placed most commonly in the radial artery but can also be placed in femoral, brachial, or dorsalis pedis arteries. Sterile technique should be used. A-line monitor systolic and diastolic blood pressure directly by detecting the pulse beat against fluid filled pressure tubing which is connected to a transducer. The transducer converts the pressure sensed into systolic and diastolic values which are displayed on a monitor. The arterial line set-up system must be ”zeroed” which exposes the system to atmospheric pressure and establishes Patm as zero. When the system is again closed to the atmosphere, any pressure encountered is arterial pressure. Since water columns can also exert pressure when affected by gravity, the transducer is typically kept level with the body area of interest. For BP, this is usually the level of the heart.
- Capnography is the measure of exhaled carbon dioxide. Capnography is displayed as a waveform on the monitor with each phase of the wave corresponding to each phase of the respiratory cycle or breath. The beginning of exhalation is dead space. This is followed by the transition between airways and alveolar gas. As the waveform levels off, it corresponds to the alveolar plateau, or end-tidal carbon dioxide. Finally, as inspiration starts again, the waveform should reverse flow and drop to zero.
- Routinely monitored in the ICU Sites Pulmonary artery = “Core” temperature (gold standard) Tympanic membrane - correlates well with core; approximates brain/hypothalamic temperature Esophagus - correlates well with core Nasopharyngeal - correlates well with core and brain temperature Rectal - not accurate (temp affected by LE venous return, enteric organisms, and stool insulation) Bladder - approximates core when urine flow is high Axillary - inaccurate; varies by skin perfusion Skin - inaccurate; varies by site Oropharynx – good estimate of core temperature; recent studies show correlation with tympanic and esophageal temperatures