Blood Pressure Monitoring Lecture 5

• Monitoring circulation and perfusion includes blood pressure and cardiovascular drug introduction.
• Blood pressure reflects force applied to artery walls.

Anesthetic Monitoring Textbook

• Chapter 6 of "Anesthesia & Analgesia for Veterinary Technicians" (5th edition, p. 180-187; 6th edition, p. 211-219) is relevant.

Non-Invasive Blood Pressure Monitoring

• Reflections and discussion on non-invasive blood pressure monitoring are integral.
• Non-invasive blood pressure monitoring is also called indirect blood pressure monitoring or NIBP.
• Palpation sites include: coccygeal, palmar/dorsal metatarsal, femoral, digital, palmar/dorsal metacarpal, aural, and lingual.
• Technicians can practice peripheral arterial pulse palpation in the lab.

Doppler and Pulse Points

• A Doppler probe should be used over the pulse point.
• Pulse palpation sites: palmar metacarpal distal to the metacarpal joint, palmar/dorsal metatarsal, dorsal pedal, and the tail (ventral side)/coccygeal area.
• Shaving the area when using IVC is recommended.

Cuff Placement Reminders

• The cuff is placed around the distal radius, distal tibia, or metatarsus in dogs and cats
• In large animals, the cuff goes around the metacarpus or tail.
• Do not place the cuff over joints or on the IVC leg.
• Must ensure it is snug with the artery against the pulse with the hose proximal and not kinked, and the cuff faces the correct way

Impact of Cuff Size and Position

• An inappropriately wide cuff can falsely lower the BP.
• An inappropriately narrow cuff can falsely increase the BP.
• A cuff that is too loose can falsely increase the BP.
• A cuff that is too tight can falsely lower the BP.
• The cuff is placed as close to the heart level as possible.

Physiological Factors

• Cardiac arrhythmias can cause erratic blood pressure readings.
• Prolonged and deep breaths are known to falsely lower BP, specifically with sinus arrhythmia in dogs.
• Movement can interfere with sensor signals, resulting in no reading or a false reading.

Monitoring Blood Pressure Goal

• The goal of monitoring BP is ensuring adequate blood flow to all tissues.
• Adequate oxygen also must be delivered to tissues to keep all organs functioning.
• Cardiac output (CO) and arterial oxygen content determine blood pressure.
• Cardiac output refers to the amount of blood pumped through the body per minute. (volume/min)

Blood Pressure Components

• Blood pressure relies on cardiac output and systemic vascular resistance.
• Blood Pressure = Cardiac Output + Systemic Vascular Resistance

Systolic, Diastolic, and Mean Pressures

• Systolic pressure (SAP or SBP) occurs when the left ventricle contracts to push oxygen-rich blood into the arterial system, and is the pressure peaks because of the ventricles.
• Diastolic pressure (DAP or DBP) reflects the pressure remaining in the arteries when the heart is in a resting phase in between contractions.
• Mean arterial pressure (MAP) represents the average pressure throughout the entire cardiac cycle, making it the main and best indicator of tissue perfusion. MAP = Diastolic BP + 1/3(Systolic BP – Diastolic BP)
• MAP is the average pressure throughout the entire cardiac cycle, because during one cardiac cycle the heart is in diastolic for a longer period, and is not the average of SAP and DAP.

Doppler and SAP/DAP readings

• Doppler systems effectively measure SAP.
• The Doppler method is a poor estimator of DAP.
• With SAP readings less than 100 mm Hg, the Doppler tends to underestimate SAP markedly but correlates well with MAP.
• Doppler readings less than 100 mm Hg could indicate MAP.

Oscillometric Readings

• The oscillometric method accurately provides DAP and MAP but slightly underestimates the SAP.
• Averaging 5 oscillometric BP values gives a more accurate reading.
• Arrhythmia, movement and improper cuff use reduce reliability.
• Oscillometric methods are less accurate than Doppler in patients experiencing hypotensive states.
• Overall, trends are most helpful.

Factors Influencing Blood Pressure

• Blood pressure is made up of systemic vascular resistance and cardiac output.
• Systemic vascular resistance is is measured by the vessel diameter and length, and the blood viscosity.
• Cardiac output is measured in heart rate and stroke volume.
• Stroke volume is measured by preload, contractility, and afterload.

Indicators of Perfusion

• Systemic vascular (peripheral) resistance is vessel squeeze.
• It reflects the resistance to blood flow in vessels via the systemic vasculature.

Cardiac output depends on

• Heart rate (beats/min) x stroke volume (mL/beat).
• Stroke volume is made up of preload, contractility, & afterload.
• Preload is the force that stretches the cardiac muscle before contraction, with volume filling the ventricles before pumping.
• Contractility is cardiac muscle contraction.
• Afterload is the amount of resistance the heart overcomes to open the aortic valve and eject blood.

Managing Hypotension

Initial Steps

• First, evaluate the animal and ensure they are not too light.
• Turn down the vaporizer if the anesthetic plane is too deep for the current level of stimulation, because inhalants decrease SVR causing vasodilation.
• Consider adding a MAC-sparing opioid to turn down the vaporizer, which can lead to increase in BP.
• Administer anticholinergic if an animal is bradycardic and hypotensive to increase HR
• Give a crystalloid fluid bolus and check BP to evaluate response if step one cannot be performed and you cannot reduce the inhalant

Additional steps

• Third, give blood pressure medication which targets the underlying cause.
• Administer BP drugs like vasopressors if vasodilation is the underlying cause, targets increasing afterload, increases stroke volume, cardiac output, and BP. This is very difficult to confirm during anesthesia.
• Administer BP drugs like adrenergic agonists for decreased contractility, strengthens contractility, increasing stroke volume, and blood pressure

Intravenous Fluid Therapy (IVF)

• IVF helps maintain intravascular fluid volume and osmotic pressure.
• IVF treats dehydration, hypovolemia, or hypoperfusion.
• It helps correct electrolyte imbalances.
• 60% of body weight is water (total body water, TBW) which can vary by species.
• TBW is greater than 75% at birth.
• TBW is lower in obese and old animals.

TBW percentages

• 100% BW is [Body Weight, BW].
• 60% BW is [Total Body Water, TBW].
• 40% BW is [Total Dry Matter].
• 20% BW is [Extracellular Fluid ECF].
• 5% BW is [Intravascular Fluid, IVF].
• 15% BW is [Interstitial Fluid, ISF].

IVF Considerations During Anesthesia

• All intubated anesthesia patients need balanced electrolyte fluid solutions due to potential fluid, electrolyte, and acid-base disturbances.
• Patients with open body cavities can receive IV fluids.
• Patients with a high-fresh gas flow driven fluid loss via a non-rebreathing system can receive IV fluids.
• Patients who are bleeding can receive IV fluids.
• Patients with drug induced polyuria can receive IV fluids.
• Patients with effects arising from drug-induced respiratory depression can receive IV fluids.

Reasons for Higher IVF Rates During Anesthesia

• Maintenance of total body fluid volumes
• Preventing adverse effects of anesthetic drugs like vasodilation and hypotension.
• Maintaining preload, and stroke volume, preventing likely hypotension.
• Counteracting peripheral vasodilation from drugs.
• Maintain IV catheter patency in case CPR is indicated.
• Correcting deficits like fasting and ongoing normal losses.
• Be cognizant of possible effects of fluid therapy.

Intravenous fluids help with preload

• IVF and blood products act as drugs that affect preload.
• Preload Increases venous blood volume to the heart, supporting ventricles at the end of diastole and also increasing BP.
• Vasopressin is works by vasoconstriction, which increase returning blood stimulating the sympathetic nervous system.
• Diuretics such as Furosemide increase preload.

Direct vs. Indirect Methods

• Indirect methods are non-invasive (NIBP, oscillometric, Doppler flow).
• Direct methods involve an arterial catheter or central venous line.
• IBP is also an indirect method.

Direct Blood Pressure Monitoring

• Provides precise measurement of blood pressure.
• Requires an invasive procedure consisting of inserting a catheter into a peripheral artery, and connecting it to a transducer.

Invasive Blood Pressure (IBP) Monitoring

• Reading obtained via an indwelling catheter inserted into a specified artery.
• The catheter is attached to a pressure transducer and monitor.
• IBP is most often used in referral medicine.
• Equine practices and in research depend on IBP.
• Waveform and numerical pressures are given through this monitoring system.
• IBP provides continuous, real-time readings.
• IBP is considered the most accurate method of monitoring
• An arterial catheter is placed in the dorsal pedal metatarsal artery most of the time.
• It's important to zero the system or sample blood for arterial blood gas.
• Risk of serious bleed from arterial line
• A video on hemodynamic monitoring can be found at the given URL.

Central Venous Pressure Monitoring

Central venous pressure monitoring is another invasive blood pressure monitoring method

Components

• CVP includes a manometer or multiparameter digital monitor to detect pressure in a large central vein.
• Video on set-up for CVP using an older method of a basic water manometer.
• In Central Venous Pressure, blood pressure is monitored in the anterior vena cava using a jugular catheter (a central line)
• The machine assesses blood return to the heart and functioning heart.
• It is a direct form of measurement utilizing a long jugular catheter.

Trend Monitoring

• The trends of the patient's blood pressure should be the primary focus rather than single continuous readings.
• It can detect any overhydration on the part of the patient for the administration of IV fluids

Case Example

• A 15 kg, two-year-old female beagle undergoing ovariohysterectomy has received acepromazine-hydromorphone premedication and propofol for anesthetic induction.
• Maintenance is performed with isoflurane.
• Twenty minutes into the procedure, these vitals are recorded: a temperature of 99.5 F, pulse of 45 bpm, respiratory rate of 12 breaths per minute, and Doppler blood pressure of 75 mmHg.
• Problem is hypotension due to bradycardia.
• The patient is at risk for reduced levels of CO.
• A DVM would likely prescribe atropine or glycopyrrolate.
• With a corrected HR, blood pressure should improve and cardiac output will return to normal.