II. EQUIPMENT, INSTRUMENTATION AND TECHNOLOGY
MONITORING DEVICES
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QUESTIONS
QUESTIONS
QUESTIONS
QUESTIONS
QUESTIONS
QUESTIONS
Central Nervous System
- A complication of the “wake-up” test is extubation.
- Wake-up test determines motor function.
- Sevoflurane will affect the bispectral index monitor greater than fentanyl, hydromorphone, and nitrous oxide.
Evoked Potential
- The advantage of motor evoked potentials as opposed to sensory evoked potentials for spinal cord monitoring is that motor evoked potentials monitor the ventral spinal cord.
- Somatosensory evoked potential monitoring assesses the dorsal cord and cranial nerve VIII.
- Opioids have no effect on SSEPs.
- Nitrous oxide will not change the latency during SSEP monitoring.
- Hyperthermia most depresses SSEP monitoring.
- Rocuronium has the least depressant effects on SSEPs than sevoflurane, versed, and desflurane.
- Visual evoked potentials monitors cranial nerve II.
Intracranial Pressure
Modified EEG Monitor (BIS, PSArray)
Bispectral Index Monitoring
- The normal range for Bispectral Index Monitoring for general anesthesia is 40-60.
- 100: Awake
- 90-70: Light/Moderate Sedation
- 70-60: Deep Sedation (low probability of explicit recall)
- 60-40: General Anesthesia
- 40-10: Deep hypnotic state
- 10-0: Flat-line EEG
PSArray
Cerebral Oximetry
Cardiovascular
Electrocardiogram (3 Lead and 5 Lead)
- Right coronary artery infarction is suspected with there is ST elevation in leads II, III, and aVF.
- A stent placed in the left anterior descending coronary artery reestablishes perfusion to the anterior/septal area of the heart.
- Leads I, aVL, and V4- V6 are the best leads to monitor the lateral region of the heart, which is perfused by the left circumflex artery.
- When using a 3-lead ECG monitor and positioning the electrodes in a modified V5 placement, the appropriate lead selection for monitoring is lead I.
- V5 monitors the anterolateral region of the heart on an electrocardiogram.
- Prominent U waves are associated with a K+ < 3 mmol/L.
- Peaked T waves are associated with K+ > 6 mmol/L.
- Shortened QT segment is associated with Ca++ > 11 mg/dL.
- The electrical axis of lead II is approximately 60O from the right arm to the left leg, which is parallel to the electrical axis of the atria, resulting in the largest P wave voltages of any surface lead.
Arterial Pressure Monitoring
- The most cannulated artery for blood pressure monitoring is the radial artery. The dorsalis pedis can be used as well. Other sites include: femoral, brachial, ulnar, and axillary.
- The arterial transducer should be placed at the level of the heart, which is roughly 10 cm above table. Above this level and it will show a lower blood pressure. Below this level and it will reflect a higher blood pressure.
- Patients with incomplete palmar arches are around 5%. These patients are at increased risk for hand ischemia secondary to radial artery catheterization.
- The natural frequency of the system is decreased with the addition of tubing, stopcocks and air in the line. When the frequency response is too low, the system will be overdamped and will not accurately portray the arterial waveform, underestimating the systolic pressure.
- The location of the site measured greatly affects the arterial blood pressure. Systolic and pulse pressures become exaggerated as the pulse moves peripherally, and the wave reflection alters the pressure waveform.
- A damping coefficient of 0.6 – 0.7 is optimal to prevent both over- and under-dampening of the arterial waveform. The high-pressure flush test can help determine the damping coefficient.
- Strong contractility is usually the cause of a rapidly rising anacrotic limb of the arterial waveform.
Bisferiens pulse Pulsus tardus Pulsus parvus Pulsus alternans Pulsus paradoxus | Pulse waveform with two systolic peaks Delayed upstroke Diminished pulse wave Alternating smaller and larger pulse waves Inspiratory fall in systolic arterial blood pressure > 10 mm Hg |
Pulsus alternans Pulsus tardus Pulsus paradoxus Bisferiens pulse | Severe left ventricular dysfunction Aortic stenosis Cardiac tamponade Aortic regurgitation |
Noninvasive Blood Pressure Monitoring
- The formula for mean arterial pressure (MAP) is ((SBP) + 2(DBP)) / 3.
- The accuracy of any method of blood pressure measurement depends on proper cuff size. The cuff’s bladder should extend at least halfway around the extremity and the width of the cuff should be 20 – 50% greater than the diameter of the extremity.
- Automated blood pressure monitors measure the pressures at which oscillation amplitudes change. From this, a microprocessor derives systolic, mean, and diastolic pressures.
- Falsely low blood pressure measurements result when cuffs are too large, the extremity is above the heart level or after quick deflations. Falsely elevated measurements result when cuffs are too small, applied too loosely or the extremity is below the heart level.
- Although the doppler probe offers more sensitivity than palpation of the artery, only systolic pressure can be reliably determined by this method.
Central Venous Pressure Monitoring
- The central venous catheter tip should lie just above or at the junction of the superior vena cava and the right atrium.
- The “A” wave on a CVP tracing represents right atrial contraction.
- The “C” wave on the CVP wave form correlates with RV systole.
- Tricuspid regurgitation: incompetence of the tricuspid valve abolishes the x descent resulting in a prominent cv wave on a CVP tracing.
- Measurement of CVP is made with a water column or transducer. The pressure should be measured during end expiration.
Pulmonary Artery Pressure Monitoring / SvO2
- The flotation of a PA catheter can be enhanced by maneuvers that increase cardiac output, increase venous return, or elevate the position of the pulmonary artery.
- Examples:
- Deep inspiration
- Head-up position
- Right lateral tilt all improve flow direction
- Injection of iced saline stiffens the catheter and reduces coiling thereby improving flow direction.
- As the catheter passes through the right ventricle, arrhythmias are common.
- Catheterizing the pulmonary artery carries the potentially lethal but rare complication of rupturing the artery. The most common sign is hemoptysis.
- Distances from the right internal jugular vein:
- Right atrium: 20-25 cm
- Right ventricle: 30- 35 cm
- Pulmonary artery: 40- 55 cm
- ** If the central line is placed in the left internal jugular vein, add about 5- 10 cm.
- ** If the central line is placed in the femoral vein, add about 15 cm.
- ** If the central line is placed in the antecubital vein, add about 30-35 cm.
- The right BBB with a left anterior hemiblock will progress to 3rd degree block in about 1/10 cases. However, the right BBB with a left posterior hemiblock OFTEN leads to 3rd degree heart block.
- The catheter should be withdrawn slightly if the balloon is wedged before maximal inflation. This signals an over-wedged position and carries a 50-70% mortality rate.
- The PCWP is overestimated in mitral stenosis.
- The PCWP is underestimated in aortic regurgitation.
- It is recommended to retract the pulmonary artery catheter 2-3 cm during cardiopulmonary bypass.
- Examples:
SvO2
Hemodynamic Monitoring
- Electrocautery will decrease the accuracy of thermodilutional cardiac output reading.
- Plotting the temperature change as a function of time produces a thermodilution curve. Cardiac output is determined by the integration of the area under the curve.
- Pulmonary hypertension leads to a higher CVP than PCWP.
Precordial / Esophageal Stethoscope
Transesophageal Echocardiogram
- The tip of a single orifice catheter should be placed 3 cm above the junction of the superior vena cava and right atrium for aspirating air.
- Best way to determine diastolic function is pulmonary capillary wedge pressure.
- The presence of less than normal ventricular motion is hypokinesis.
- The doppler should be placed between 3-6 intercostal spaces to detect a venous air embolism.
- The most sensitive for detecting myocardial ischemia is transesophageal echocardiography.
Respiratory
Capnography
- Capnography detects carbon dioxide that is exhaled.
- Referring to the capnograph, dead space plus alveolar ventilation is expiration.
- The capnograph waveform that is responsible for the alveolar plateau portion is C-D.
- COPD can result in a progressive prolongation of the expiratory phase on capnography. CO2 is exhaled more slowly from diseased portions of the lungs (with more significant airway narrowing) than from areas with less severe narrowing.
- A kinked ETT may result in a progressive prolongation of the expiratory phase. An increase in peak inspiratory pressure without any change in plateau pressure signals an increase in airway resistance or inspiratory gas flow rate.
- Cardiac oscillations on capnography are recognized as regular, sawtooth waves within the expiratory phase at a rate equal to the heart rate. This is the result of the contraction of the heart and great vessels forcing gas in and out of the lungs and a common occurrence in pediatric patients, owing to the relative size of the heart to the thorax.
- Rippling effect
- A waveform that fails to return to baseline sign rebreathing of CO2, which occurs as a result of inadequate fresh gas flow or a depleted or ineffective soda-lime absorber.
Airway Gas Analysis
- Mass spectrometry detects vaporizer malfunction, air embolism, and anesthesia malfunction.
- Mass spectrometry detects nitrogen when a patient has an air embolism.
Pulse Oximetry
- Pulse oximeters have their greatest accuracy (+ 2 – 3%) in the range of 70 – 100 %.
- Carboxyhemoglobin gives a false high reading.
- Because carboxyhemoglobin and oxyhemoglobin absorb light at 660 nm identically, pulse oximeters will register a falsely high reading in patients with carbon monoxide poisoning.
- Methemoglobinemia gives a false low reading, low oxygen saturations, excessive room ambient light or overhead infrared lights, tremors or vibrations of the patient, methylene blue die, etc.
- Methemoglobin has the same absorption coefficient at both red and infrared wavelengths causing a reading of 85% regardless of the actual hemoglobin saturation.
- To eliminate the effects of light absorption from tissues and venous blood, arterial pulsations are identified by plethysmograph.
- In contrast to the pulse oximeter, which can detect very small changes in saturation, visual detection of cyanosis occurs only when more than 5 g of hemoglobin is desaturated or at about a SpO2 of 80%.
Airway Pressure
- The breathing circuit is the most common single source of injury (39%); nearly all incidents were related to misconnects or disconnects.
Blood Gas Analysis
- The normal gradient between PaCO2 and ETCO2 is 2 – 5 mmHg and reflects alveolar dead space. Any significant reduction in lung perfusion increases alveolar dead space, diluting expired CO2 and increasing the gradient.
Peripheral Nerve Stimulator (Qualitative and Quantitative)
- Double burst most accurately assesses fade.
- Train-of-four ratio > 70% is present with peripheral nerve stimulation after depolarizing neuromuscular blockade.
- Associated with nondepolarizing blockade:
- Fade during tetany
- Post-tetanic potentiation
- 70% of receptors can be blocked with no fade on train-of-four stimulation.
- Most nerve stimulators provide a 5-second tetanic stimulation at 50 Hz. This frequency was adopted because at frequencies of 100 Hz or greater some fade may be seen even in the absence of neuromuscular blockade.
- All stimuli from the peripheral nerve stimulator are 200 µs in duration, of square-wave pattern and of equal intensity.
- Tetany at 50 or 100 Hz is a sensitive test of neuromuscular function. Sustained contraction for 5 seconds indicates adequate reversal. As stimulus frequency increases, fade is more pronounced. As a result, a 100 Hz stimulus is more likely to produce fade than a 50 Hz stimulus.
- To deliver a supramaximal stimulus to the underlying nerve, peripheral nerve stimulators must be capable of generating at least 50 mA across a 1000 Ω load.
Temperature Monitoring
- There are three phases of temperature drops during general anesthesia:
- Drop of 1-2O C during the first hour and results from the redistribution of heat to cooler peripheral tissues.
- The next 3-4 hours due to heat loss from the environment.
- There is little change in temperature because heat production and environmental loss reach a steady-state equilibrium.
- Heat loss from the body is primarily by radiation.
- Major route of heat lost by a patient suffering from burns is evaporation.
- To maintain normothermia, insulate the body by using reflective coverings, like Bair Huggers, heating blankets, or plastic bags (insulation reduces body heat loss by radiation and convection).
- Pediatrics, geriatrics, and those with hypothyroid are at risk of intraoperative hypothermia.
- Normothermia during the maintenance phase of anesthesia decreases morbidity (infection), decreases blood loss, and minimizes postoperative shivering.
- Forced air warmers are the most effective warming system but require electricity.