Anesthesia monitoring: Part II (Proceedings)

• Anesthesia monitors are only as good as the people who use them. It is imperative to know what is normal so that you can recognize when something is outside of normal.

Common anesthesia monitors

• Pulse-oximeter (pulse-ox), capnograph, non-invasive blood pressure monitor (NIBP), invasive blood pressure monitor (IBP), electrocardiography (ECG)

Pulse-oximetry

• Estimates oxygen saturation of hemoglobin by measuring pulsatile signals across perfused tissues

• Main function

o To detect low hemoglobin oxygen saturation which alerts us to hypoxemia, the final pathway for many life-threatening events

• How it works

o Utilizes wavelengths of light

• Deoxygenated blood absorbs red light

• Oxygenated blood absorbs infrared light

→ The ratio of red to infrared light provides us with an SpO2 value

• Terminology

o PaO2 is the measure of oxygen dissolved in plasma

• Determined by arterial blood gas measurement

• Tells us the efficiency of the lungs to deliver oxygen to the blood

• Measured in mmHg (millimeters of mercury)

• Normal PaO2 depends on inspired O2 concentration

→ Multiply inspired concentration by 4-5

→ Room air is ~ 21% O2 (normal PaO2 is 80-110 mm Hg)

o SpO2 is peripheral oxygen saturation of hemoglobin

• Measured as a percent

• Estimates PaO2

→ Numbers of concern are different

o PaO2 of 100 mm Hg = an SpO2 of 98%

o PaO2 of 80 mm Hg = an SpO2 of 95%

o PaO2 of 60 mm Hg = an SpO2 of 90%

o PaO2 of 40 mm Hg = an SpO2 of 75%

→ These numbers are based on a normal oxyhemoglobin dissociation curve

• Oxyhemoglobin dissociation curve and hypoxemia

o Shows the relationship between PaO2 and SpO2

o SpO2 of 95% and above represents adequate oxygenation

o Precipitous drop in PaO2 at SpO2 of 94% and below

• Indicated early hypoxemia

• SpO2 of 90% indicates a PaO2 of 60 mm Hg and warrants treatment

• Five reasons for hypoxemia

o Low inspired oxygen, Hypoventilation, V/Q mismatch, Diffusion impairment, Shunt

• Low inspired oxygen

→ Usually not an issue when using 100% O2

→ Check O2 tank and flowmeter

→ If using N2O, check ratios or discontinue mixture

• Hypoventilation

- Problematic with room air

→ Common in spontaneously breathing patients

o Most anesthetics are respiratory depressants

→ An animal that appears to be breathing adequately may not be

o Adequacy of ventilation can be determined with ETCO2 monitor or blood gas analysis

o May need to assist breaths or mechanically ventilate

→ Not a common cause of hypoxemia under anesthesia due to 100% oxygen

• V/Q mismatch

→ Ventilation (V)- perfusion (Q)

o Alveoli are inflated but not perfused

• Ventilation good, blood flow bad

o Alveoli are perfused but not well inflated

• Blood flow good, ventilation bad

o Common causes of V-Q mismatch

• Anesthesia, inadequate ventilation, hypotension, body positioning, atelectesis

• Diffusion impairment

→ The thickening of the alveolar wall making gas exchange difficult

→ Or an increase in the distance that oxygen has to travel from the alveoli to the capillary

o Pulmonary fibrosis

o Pulmonary edema

• Shunt

→ A true anatomic shunt is defined as oxygen bypassing the alveolar exchange at either the heart or the lungs

o Ventricular septal defect

• Troubleshooting the Pulse-ox

o Prone to malfunction, bias and variation

o Potential reasons for flawed readings

• Tissue, venous/capillary blood, and skin pigment all absorb infrared light

• Motion, location, wetness/dryness, tissue thickness, electrical/optical interference

o When inaccurate, it is usually inaccurately low- Check patient status first, then troubleshoot monitor

o A poorly functioning monitor may indicate poor systemic perfusion

• Limitations

o The pulse-ox does not assess the adequacy of ventilation

o Anemic patients may have an SpO2 reading of 100% (available hemoglobin is saturated) when in fact their tissues are hypoxic

Capnography

• Gives us end-tidal carbon dioxide (EtCO2)

o EtCO2 estimates PaCO2

• PaCO2 is the amount of CO2 dissolved in arterial blood

• It tells us the adequacy of ventilation

o CO2 is important in two ways

• It defines the respiratory cycle

• It is a by-product of cellular metabolism (cells make it, lungs excrete it)

o As an anesthetic monitor

• Non-invasively gauges the adequacy of ventilation (normal is 35-45 mm Hg)

• Helps guide assisted breaths and mechanical ventilation

• Increases our ability to detect potential problems

• Helps to determine proper intubation and inadvertent extubation

• Alerts us to leaks within the breathing system (ET-tube or machine)

• Alerts us to apnea and changes in respiratory rate and pattern

• Alerts us to changes in chest compliance (pneumothorax, poor positioning...)

• Alerts us to changes in cardiac output

• Tells us how effective our chest compressions are during CPR

o Capnography and respiration

• Normal expired CO2 is 35-45 mm Hg (30-45 in cats)

• Normal inspired CO2 is 0-3 mm Hg

→ Any value greater than 3 may indicate a problem

o Leak in system, excessive dead space, exhausted soda lime, inadequate fresh gas flows (non-rebreathing)

• Elevated EtCO2 (hypercapnia or hypercarbia)

→ Respiratory- hypoventilation (increase RR or TV), rebreathing of CO2

o May lead to respiratory acidosis (EtCO2 greater than 60 mm Hg)

o vasodilation

→ Metabolic- hyperthermic, excessive production of catecholamines

• Low EtCO2 (hypocapnia or hypocarbia)

→ Respiratory- hyperventilation (decrease RR or TV)

o May lead to respiratory alkalosis (EtCO2 less than 20 mm Hg)

o Decreased cerebral blood flow and cerebral oxygenation from vasoconstriction

→ Metabolic- hypothermia, increase muscle relaxation, increased anesthetic depth, decreased cardiac output

• Common causes of hypercapnia

→ Hypoventilation, excessive depth, inappropriate ventilator settings, exhausted soda lime, machine malfunction, hyperthermia, airway obstruction, abdominal or thoracic restrictive disease, pleural space filling

• Common causes of hypocarbia

→ Hyperventilation, light level of anesthesia, hypoxemia, pain, hypothermia, inappropriate ventilator settings, decreased cardiac output

o Capnometry vs. capnography

• Capnometry is just a numerical value (EtCO2 and RR only)

• Capnography is also a graphical representation of each breath (tons more info!)

→ No wave = apea, obstruction or disconnection

→ Increased baseline = rebreathing of CO2

→ Increased plateau = hypoventilation, increased rate of CO2 production

→ Decreased plateau = hyperventilation, hypothermia, airway leak, tachypnea, decreased cardiac output...

Blood Pressure Monitoring

• Simply put, adequate blood pressure is necessary to deliver oxygen to tissues

o Components of blood pressure (systolic, diastolic, mean)

• Systolic- peak pressure during contraction- comprised of stroke volume and arterial compliance

• Diastolic- minimum pressure during relaxation- comprised of systemic vascular resistance and heart rate

• Mean- not just the average... Algorithm required: (systolic – diastolic) ⅓ + diastolic

→ Physiologically the most important value because it represents the mean driving pressure for organ perfusion.

o Normals under general anesthesia (awake values are higher)

• Systolic 100-140 mm Hg, Diastolic 50-100 mm Hg, Mean 70-120 mm Hg

o Hypotension

• Systolic blood pressure less than 80 mm Hg

- 80 mm Hg is the MINIMUM (some prefer 100 mm Hg as hypotension)

• Diastolic is a more subjective measurement (using non-invasive techniques)

→ Keep diastolic above 50 mm Hg but focus more on systolic and mean

• Mean arterial pressure (MAP) 60 mm Hg and below is hypotension

→ 60 is the MINIMUM pressure thought to perfuse kidneys and brain- aim higher

o Consequences of hypotension

• Hypoxemia, reduced drug metabolism, worsening of V/Q mismatch, delayed anesthetic recovery, renal failure, CNS abnormalities (blindness, neurologic deficits), cardiac arrest, death

o Blood pressure monitoring

• Non-invasive (Oscillometric)

→ Pros

o Gives you values for systolic, diastolic, mean and heart rate (oscillometric)

o Easy to use

→ Cons

o Can be less accurate in small patients and in the face of hypotension

o Only monitors trends

o Can be expensive

• Non-invasive blood pressure (NIBP) cuffs

→ Must be of appropriate size to get an accurate reading

o 40% of the circumference of the limb

o Too small and readings will be low

o Too big and readings will be high

o The Doppler readings

• Systolic vs. mean

• Some literature says the value given by the Doppler is 14 mm Hg above the mean blood pressure in small patients

→ Dogs over 7 kg = systolic. Keep between 80 and 140 mm Hg

→ Dogs less than 7 kg and cats = somewhere between mean and systolic.

→ Aim higher in smaller patients- make your low end 75 mm Hg instead of 60 mm Hg

o NIBP vs. IBP

• NIBP gives an estimation of blood pressure, it is a trend monitor (always look at the bigger picture)

→ If BP is trending up, evaluate and make adjustments

→ If BP is trending down, evaluate and make adjustments

→ If BP spikes, evaluate and either recheck or make adjustment

→ If BP drops, evaluate and either recheck or make adjustment

• IBP requires that a catheter be placed into an artery (dorsal metatarsal, coccygeal, lingual, femoral) and connected to a pressure manometer (mean only) or pressure transducer (sys, dias, mean, hr and wave form)

→ Gold standard in blood pressure monitoring

→ Requires technical skill, aseptic technique, special equipment

→ Can also be used to monitor BP in critical care setting and for blood gas sampling

Electrocardiography (ECG)

• Displays the electrical activity of the heart

o Does not tell us anything about mechanical function!

o Useful as an alert to potentially life-threatening cardiac arrhythmias

• Lead placement

o White (right forelimb)

o Black (left forelimb)

o Green (right hind if present)

o Red (left hind)

• If surgery prevents you from placing leads correctly, just keep their orientation the same

• If you do this, establish a baseline because your complexes may look atypical

• Always know what is normal so you can recognize what is abnormal!

Anesthesia monitors and YOU

• Monitors are truly only useful if you know what they are telling you, and if you know what to do with that information.

• Make note cards with normal values for each species and attach them to each monitor so you have them handy when you need them

• Add a few things that you can do if the values are out range...

• Stay organized (cords, cords, fluids lines, cords, hoses, cords...)

• Breathe

References

Dorsch JA, & Dorsch SE. Understanding Anesthesia Equipment. Baltimore: Williams & Wilkins, 1999

Haskins, SC. Monitoring Anesthetized Patients. In: Tranquilli WJ, Thurmon JC, Grimm KA, eds. Lumb & Jones Veterinary Anesthesia and Analgesia. Iowa: Blackwell, 2007