Basic Intraoperative Monitoring Flashcards
Which standard states that the CRNA must monitor the patient’s physiologic condition as appropriate for the type of anesthesia and specific patient needs?
Standard V
What are the 5 components of monitoring stated in Standard V?
Monitor ventilation continuously
Monitor cardiovascular status continuously
Monitor body temperature continuously
Monitor neuromuscular function continuously
Monitor and assess patient positioning
When did the AANA create standards of monitoring for the CRNA?
1974
What does vigilance mean in anesthesia?
a state of clinical awareness whereby dangerous conditions are anticipated or recognized and promptly corrected
What is the most important monitor in anesthesia?
the vigilant anesthetist
What is the fundamental goal of ventilation and oxygenation?
avoidance of hypoxia
How can we determine if the patient has optimal oxygenation?
Oxygen analyzer Pulse oximetry Skin color Color of blood ABG (when indicated)
Where does the oxygen analyzer work?
Measures inspired gas on the inspiratory limb of the circuit, determines if pipeline is truly O2
When does the oxygen analyzer alarm?
Should alarm of concentration <30%
At what oxygen percentages do you calibrate the oxygen analyzer?
21% and 100%
Is the oxygen analyzer required for any general anesthetic?
Yes
What is the O2 percentage on the O2 analyzer useful for calculating?
PaO2
Alveolar gas equation: PAO2 = FiO2 x (BP - 47) - PaO2
What type of sensor is the oxygen analyzer and how does it work?
- electrochemical
- has cathode and anode embedded in electrolyte gel separated from gas by O2-permeable membrane
- O2 reacts with electrodes and generates electrical signal proportional to O2 pressure (mmHg) in sample gas
Which component of oxygenation monitoring provides early warning of hypoxia?
pulse oximetry
How does the pulse oximeter work?
- measures arterial oxygen saturation combining principles of oximetry and plethysmography (pulsatile measurement)
- requires pulsatile arterial bed
- continuous measurement of pulse rate and oxygen saturation of peripheral hemoglobin (SpO2)
- produces SpO2 measurement by changes in light absorption during arterial pulsations
Which law applies to pulse oximetry?
Beer-Lambert Law of spectrophotometry, oxygenated and reduced Hgb differ in their absorption of red and infrared light; comparison of absorbances of these wavelengths enables oximeter to calculate O2 saturation (ratio of infrared and red transmitted to a photodetector)
At what wavelength does HgbO2 (saturated Hgb) absorb infrared light?
960 nm
At what wavelength does Hgb (unsaturated) absorb red light?
660 nm
What are factors that can affect the accuracy of the pulse oximeter?
- high intensity light
- patient movement
- electrocautery
- peripheral vasoconstriction
- hypothermia
- cardiopulmonary bypass (no pulsatile bed)
- presence of other Hgbs (COHgb or MetHgb)
- IV injected dyes (dec. with methylene blue)
- Hgb < 5 (will not register)
What is the rule of thumb for estimating PaO2 from pulse oximeter percentages?
PaO2 30 = SaO2 60
PaO2 60 = SaO2 90
PaO2 40 = SaO2 75
(30 is 60…60 is 90…45 is 75)
What are different ways to monitor ventilation?
- Continuous auscultation
- Observation of chest excursion
- End-tidal capnography
- Spirometry
What is the purpose of the precordial stethoscope?
Easily detect changes in breath sounds or heart sounds; ability to quickly detect airway/circuit disconnect, endobronchial intubation, and anesthetic depth (increased heart rate or contractility means decreased anesthetic depth)
Where do you listen with the precordial stethoscope?
suprasternal notch or apex of left lung
What is the purpose of the esophageal stethoscope?
Allows better quality heart and breath sounds with incorporated temperature probe
What must a patient have in order to use an esophageal stethoscope?
ETT
Where is correct placement of the esophageal stethoscope?
distal 1/3 of esophagus
What are some of the primary principles of capnography?
- confirms ETT placement
- Confirms adequate ventilation
- average adult produces 250 mL CO2/min that changes with patient’s condition, anesthetic depth, and temperature
How does capnography work?
Uses sidestream sampling (most common) and aspirates airway gas and pumps it through measuring device
What are the sampling flow rates of capnography?
50-250 mL/min
What are some limitations of capnography?
- H2O condensation can contaminate the system and falsely increase readings
- lag time between sample aspiration and reading
What is the normal PACO2 - PaCO2 gradient?
2-10 mmHg
What are some causes for an abnormal PACO2 - PaCO2 gradient?
- gas sampling errors
- prolonged expiratory phase
- VQ mismatch
- airway obstruction
- embolic states
- COPD
- hypoperfusion
What does phase 1 of the capnograph correspond to?
- inspiration
- at baseline (should be 0 unless rebreathing) and indicates anatomic/apparatus dead space devoid of CO2
When would the baseline be elevated on capnography?
- CO2 absorbent exhausted
- Expiratory valve incompetent/missing
- Bain circuit
What does phase 2 of the capnograph correspond to?
- early exhalation, steep upstroke
- mixing of dead-space with alveolar gas
What does a prolonged upstroke of phase 2 indicate on a capnograph?
- mechanical obstruction, kinked ETT
- slow emptying of lungs due to COPD or bronchospasm
What does phase 3 of the capnograph correspond to?
- horizontal plateau with mild upslope
- CO2 rich alveolar air
- steepness is function of expiratory resistance (COPD or bronchospasm)
What does phase 4 of the capnograph correspond to?
Steep decline, inspiration of fresh gas, return to baseline
What is the purpose of the anesthetic gas analysis?
Measures volatile agents
How does the anesthetic gas analysis work?
Obtains sample with sidestream sampling, uses mass spectrometry to ionize gas sample by electron beam and pass it through magnetic field; ions are then identified by own unique trajectory across magnetic field
What are different alarms on the ventilator that alert you to inadequate ventilation?
- tidal volume (integrated spirometry)
- airway pressure (in-circuit pressure gauge, peak inspiratory pressure, sustained elevated pressure)
- disconnect alarm (low airway pressure)
How can you monitor adequate circulation/cardiovascular status?
- BP
- HR
- EKG
- heart sounds
What abnormalities can an EKG detect?
- cardiac dysrhythmias
- conduction abnormalities
- MI / ST depression
- electrolyte changes
- pacemaker function/malfunction
What lead do we typically monitor in a three lead EKG?
Lead II
What is a disadvantage of monitoring a three-electrode EKG?
limited in detection of MI
What leads does a five electrode EKG monitor?
Six standard limb leads (I, II, III, aVR, aVL, aVF) and one precordial lead (usually V5)
What are some advantages of using a five electrode EKG monitor?
- Better in detecting myocardial ischemia
- Allows better differential diagnosis of atrial and ventricular dysrhythmias
What are the 2 most commonly used EKG leads?
Lead II and V5
Why is lead II a commonly monitored lead?
- Yields max P wave voltages
- Superior detection of atrial dysrhythmias
- Detects inferior wall ischemia/ST depression
Why is lead V5 a commonly monitored lead?
- 5th ICS/anterior axillary line
- Allows detection of anterior and lateral wall ischemia
What are some advantages of automated indirect blood pressure monitoring (NIBPM)?
- easy and accurate
- versatile in children and obese
- may be used on calf or thigh
How does noninvasive arterial blood pressure monitoring work?
Is oscillometric device that uses air pump (microprocessor) to inflate cuff and then deflation valve opens to sample oscillations
What can cause errors in oscillometric blood pressure monitoring?
- surgeon leaning on cuff
- inappropriate cuff size (small cuff –> high reading; large cuff –> low reading)
- shivering/excessive motion
- atherosclerosis and HTN (systolic low; diastolic high compared with invasive arterial pressure)
What are some indications for invasive arterial BP monitoring?
- any patient requiring BP measurement more frequently than minute to minute
- critically ill
- anticipated rapid blood loss
- major procedures (cardiopulmonary bypass, aortic cross-clamping, intracranial surgery, carotid sinus manipulation)
- frequent ABG sampling
Where are possible sites for arterial lines?
- radial artery (most common)
- ulnar artery (not often used, could compromise circulation in hand, technically more difficult)
- brachial artery (complications may risk limb, predisposed to kinking/location)
- femoral artery (prone to pseudoaneurysm and atheroma formation)
- dorsalis pedis (may have distorted waveform)
- axillary artery (potential for plexus/nerve damage from hematoma or traumatic cannulation)
What are some indications for CVP monitoring?
- fluid management of hypovolemia and shock
- infusion of caustic drugs
- aspiration of air emboli
- insertion of pacing leads
- TPN
- venous access in patients with poor peripheral veins
Where are possible sites for CVP monitoring?
- internal jugular (right preferred)
- subclavian
- external jugular
- antecubital (requires special kit with long catheter)
What are some indications for a PA cath?
- poor LV function (EF < 2L/min)
- valvular heart disease
- recent MI
- ARDS
- massive trauma
- major vascular surgery
- evaluate response to fluid administration, vasopressors, vasodilators, inotropes
What are some factors affecting temperature?
Ambient room temperature Scope and length of surgery Hypothalamic depression Intraoperative fluid replacement (not warming fluids) Vigiliance in maintaining core temp
What are the 4 mechanisms of heat loss?
Convection, conduction, radiation, evaporation
What is radiation heat loss?
heat radiated from patient into room
What is convection heat loss?
Heat loss due to air velocity over patient
What is conduction heat loss?
Heat lost by contact with OR table
What is evaporative heat loss?
heat loss due to dry inspired gases
What is hypothermia?
Environmental heat loss outpaces metabolic heat production
Who is at greatest risk for hypothermia?
elderly, burn patients, neonates, and patients with spinal cord injuries
What are some adverse effects of hypothermia?
Delay awakening or cause shivering that increases body’s O2 need by 400%
At what temperature would you consider the patient hypothermic?
<36 degrees Celsius
What is mild hypothermia and what can it cause?
Mild: 33-36 degrees celsius (reduced enzyme function and coagulopathy)
What is moderate hypothermia and what can it cause?
Moderate: </= 32 degrees Celsius (fibrillatory threshold)
Is hyperthermia usually seen under anesthesia?
No, and it is a late sign of malignant hyperthermia
What are some other causes of hyperthermia under anesthesia besides malignant hyperthermia?
- Endogenous pyrogens
- Thyrotoxicosis or pheochromocytoma (increased metabolic rate)
- Anticholinergic blockade of sweating
- Excessive environmental warming
Where are some monitoring sites for temperature?
- Esophagus (lower 1/3) accurately reflects body temperature
- Nasopharynx
- Rectum
- Bladder (integrated into Foley)
- Tympanic membrane (risk of perforation)
- Blood (PA cath)
- Skin
What are some superficial warming modalities?
- Forced air warmer (Bair hugger; most effective because decreases radiant and convective heat loss; decreases postoperative shivering and PACU stay)
- Warming blanket (minimally effective; Arctic sun water circulating)
- Radiant heat unit (no role in OR; no impact on mean body temp)
- Heated liquids (iv bags or bottles on patient; very dangerous and can cause burns)
What are some core warming modalities?
- IV fluid warmers
- Gastric lavage (warms body core but impractical to perform during surgery)
- Peritoneal irrigation (encourage use of warm irrigation during intraabdominal procedures)
What are some passive warming modalities?
- Increase ambient temperature (has greatest effect on maintaining body heat; if >24 degrees Celsius most adults remain normothermic without requiring other measures)
- Insulation (extremities and head)
- Heat and moisture exchanger (“artificial nose”; retains moisture)
- Coaxial breathing circuit (“King” circuit; warms and humidifies inspiratory gases)
What is a peripheral nerve stimulator?
monitors status of neuromuscular junction when using NM blocking drugs
delivers electrical stimulation to a peripheral motor nerve mechanically evoking a response
permits titration of drug to optimal effect
quantifies recovery from NMB
What is the onset sequence of a NMB drug?
Eyes Extremities Chest Abdominal muscles Diaphragm
Where are the monitoring sites for a PNS?
Ulnar nerve
Facial nerve
Posterior tibial nerve
Peroneal nerve
What muscle does the ulnar nerve innervate?
Adductor pollis
Where are electrodes placed to innervate ulnar nerve?
placed at wrist or elbow with the negative (depolarizing) mode placed distally
If monitoring ulnar nerve, what is it not an accurate reflection of?
degree of diaphragm or airway muscles (muscles are less sensitive to NMB)
could have adductor pollis paralysis but still have coughing, breathing, and vocal cord movement
Where does the facial nerve lie?
Within the parotid gland, if doing excision of that gland should not use NMB
Where do you place the electrodes to monitor the facial nerve?
infront of tragus of ear and below; avoid direct muscle stimulation
What is the facial nerve a better indicator of?
NM blockade of diaphragm and airway; better to use facial nerve for induction
What muscle does the facial nerve innervate?
orbicularis oculi
Where do you place the electrodes to monitor the posterior tibial nerve and what do you see when nerve is innervated?
behind medial malleolus of tibia; plantar flexion
Where do you place the electrodes to monitor the peroneal nerve?
lateral aspect of knee with response of dorsiflexion of the foot
What are the patterns of stimulation for a PNS?
Single 0.5-1 second twitch (0.5-1 Hz) TOF ratio Tetanic stimulation Post-tetanic stimulation Double-burst stimulation
What is single twitch stimulation?
Single pulse delivered every 10 secs; increasing block results in diminished response
What is TOF stimulation?
most common
4 repetitive stimuli
ratio of responses to 1st and 4th twitches are sensitive indicator of ND relaxation
What are the T4:T1 ratios and what do they mean?
Loss of 4th twitch = 75% of receptors blocked
Loss of 3rd twitch = 80% of receptors blocked
Loss of 2nd twitch = 90% of receptors blocked
How many receptors need to be blocked for clinical relaxation?
75-95% blocked
What type of NMBD would you see fade on a TOF?
non-depolarizers
What is tetanic stimulation?
Tetany at 50-100 Hz
5 seconds at 50 Hz evoked tension approximates tension developed during maximal voluntary effort
Sustained response occurs when TOF >70%
When is post-tetanic count used?
when all twitches are suppressed
What is post-tetanic count?
applies tetanus at 50 Hz for 5 seconds, waits 3 seconds then applies single twitches every second up to 20
What is the relation between number of twitches and depth of block with post-tetanic count?
inversely related, less anesthetic means more twitches present
What is double burst stimulation?
less painful than tetany
3 short 50 Hz impulses followed by 750 msec then another 3 bursts
When is double burst stimulation more helpful than TOF?
more sensitive for visual evaluation of fade
What modes on the PNS are used during induction?
single twitch
TOF
What modes on the PNS are used during maintenance?
TOF
post-tetanic count
What modes on the PNS are used during emergence?
TOF
double-burst stimulation
Which nerves do you monitor for onset and recovery of NM monitoring?
onset - facial
recovery - ulnar
How long will reversal take with 1 out of 4 twitches?
30 minutes
How long will reversal take with 2-3 out of 4 twitches?
10-12 minutes with long-acting relaxants, 4-5 minutes after intermediate relaxants
How long will it take to recover with 4 out of 4 twitches?
Within 5 mins of Neostigmine, 2-3 mins of edrophonium
What is the bispectral index score (BIS)?
used to assess depth of anesthesia
Is BIS monitoring required or optional?
optional, not currently under standard of care
What are some advantages of using BIS monitoring?
- Reduced risk of awareness
- Better management of responses to surgical stimulation
- Faster wake up (controversial)
- More cost effective use of anesthetics
What is the BIS range and what does it mean?
EEG signal with index ranging from 0-100
0 = isoelectric EEG
100 = awake CNS
What does a BIS of 80-100 mean?
responds to normal voice
What does a BIS of 60-80 mean?
Can respond to loud commands or mild shaking
What does a BIS of 40-60 mean?
*General anesthesia
low probability of explicit recall and unresponsive to verbal stimulus
What does a BIS of 20-40 mean?
Deep hypnotic state
What does a BIS of 0-20 mean?
Burst suppression or flat line EEG
What affects BIS readings?
electro-cautery EMG pacer spikes EKG signal patient movement
What numbers on EEG monitoring are associated with recall?
no absolute or guarantees, but research indicates that levels >70 have increased risk of recall
What are routine monitors?
NIBP stethoscope EKG pulse ox O2 analyzer EtCO2 Et agent
