Pathophysiopathy of anesthesia: cardiovascular and other systems Flashcards
Medulla
-controls sympathetic and parasympathetic NS output
>receives feedback from various systems to maintain appropriate CO
Sympathetic NS
-release catecholamines (EPI/NE)
-changes mainly affects arteries, arterioles, great veins
Parasympathetic NS
-Release Ach
-Changes mainly affects cardiac rate and rhythm
Ultra short Acting BP control- autoregulation
**Basal vasomotor control
NOTE: not affected by anesthesia, but can by disease
-Organs sense tissue oxygen demands= increase/decrease blood flow to match
-Sense accumulation of K, H, CO2, adenosine, lactate
Ultra short Acting BP control- endothelium derived factors
-Local vasodilators: NO, PGI2
-Local vasoconstrictors: endothelins, thromboxane A2
Short term BP control
**Reflexes- important for anesthesia because reflexes are blunted by inhaled and injectable anesthetic drugs
- Baroreceptor reflex
- Chemoreceptor reflex
- Bainbridge reflex
- Frank-Starling relationship
Baroreceptor reflex
-stretch receptors in carotid sinus and aortic arch
-sense changes in BP and result in changes in HR
Chemoreceptor reflex
-Carotid and aortic bodies: perfusion, CO2, O2 blood levels and blood pH
-Feedback to brainstem to adjust ventilation and sympathetic activity
Bainbridge relfex
-stretch receptors in R atrium
-sense increased pressure in the right atrium (increased venous return) and results in increased HR
Frank-Starling Relationship
-increased venous return stretches myocardium
-leads to increase myocardial contractility and increased SV
Tissue perfusion pressure
Perfusions pressure is what provides adequate blood flow to tissues
-MAP minus pressure within the tissues
-MAP >60mmHg to provide adequate tissue blood flow to major organs
**Changes in perfusion pressure leads to immediate effects on organ function
Brain perfusions
Cerebral perfusions pressure (CPP)= MAP- intracranial pressure (ICP)
Kidney perfusion
Renal perfusions pressure= MAP- glomerular capillary pressure
Regulation of brain and kidney perfusion
Autoregulation zone= range of MAP where organs maintain perfusion pressure to optimize tissue perfusion
-Map< 60mmHg = results in being outside autoregulatory zone, perfusion becomes dependent on systemic BP
GOAL: maintain MAP >60mmHg
Coronary artery perfusion
Myocardial perfusion occurs in diastole= depends on diastolic arterial pressure (DAP)
**Coronary artery perfusion=DAP- right arterial pressure
DAP<40 = reduced coronary artery perfusions (<20mmHg) leading to myocardial ischemia
GOAL: maintain DAP >40mmHg
Anesthesia effects on vasculature (vasodilators)
Reduce systemic vascular resistance
- inhaled anesthetics= dose dependent decrease in SVR. increasing with depth of anesthesia
- Acepromazine= alpha 1 adrenergic blockade causes decrease in SVR
- Meperidine/morphine IV= histamine released leading to vasodilation
- Propofol and alfaxalone= preferential venodilation. Seen after IV boluses
**Effects are additive when used together
What can anesthesias vasodilator effects cause?
Hypotension
Anesthesia effects on vasculature (vasoconstrictors)
Increase SVR
1. alpha 2 adrenergic agonists= direct action on peripheral alpha 2 adrenergic receptors causing SVR
2. Ketamine and NO= sympathomimetic action that released endogenous catecholamines which increase SVR
What can anesthesias vasoconstrictors effects cause?
Can lead to hypertension
*often seen in premedication, but is ultimately reduced by inhaled anesthetics under general anesthesia
Which drugs have minimal effects on vascular?
-Opioids and benzodiazepines
Epidurals
Local anesthetic can travel to thoracic region of spinal cord= block sympathetic vasomotor nerves
Can result in:
-reduced sympathetic control on vasomotor tone= vasodilation and decreased SVR
**compounded by other vasodilatory anesthetic drugs
How to avoid epidural effects?
- lowest volume to provide adequate nerve blockade
- Slow injection speed= reduced injection pressure to slow forward spread which also prevents bradycardia
Note: effects are temporary but extra case should be taken with large animals (horses can drop!)
Drugs decreasing contractility
-inhaled anesthetics, alfaxalone, propofol= decreased Ca availability
-Acepromazine and alpha2 agonists
**dose dependent effects
What other states can reduce contractility?
-Hypoxemia
-depleted catecholamines (sepsis)
-Acidosis (pH <7)
-Electrolyte imbalances (increased K, decreased Ca)
-Hypovolemia
Stages of hypertension
Prehypertension (SAP 140-150mmHg)
Hypertension (SAP> 160-179 mmHg)
Severe Hypertension (SAP>180mmHg)
Acute hypertension
Can lead to a decreased in CO, edema, hemorrhage (brain/lungs)
Sustained hypertension
Results in myocardial remodeling, retinopathy, retinal detachment, encephalopathy, renal disease
Treatment of hypertension
**depends on underlying reason
- due to Sympathetic activation- light plane of anesthesia, nociception, hypercapnia, hypoxemia
- due to Diseases- chronic kidney disease, hyperadrenocorticism, hyperthyroidism, pheochromocytoma
Stage of Hypotension
**most commonly seen under anesthesia
SAP< 80mmHg, MAP <60 mmHg, DAP < 40 mmHg
Results in reduced vital organ perfusion and O2 delivery
What is significant hypotension?
Take into account:
- Severity
-Mild/moderate MAP <60 mmHg
-Severe MAP <35-45 mmHg - Time frame
How quickly is it falling? How long are thye hypotensive?
Sequelae of hypotension
-renal damage and poor urine production
-hepatic and GI damage
-increased lactate concentration
-myocardial ischemia, arrhythmias, cardiac arrest
Heart rate and rhythm control
Controlled by electrical activity balance of autonomic nervous system
Parasympathetic impact on heart rhythm
Sympathetic impact on heart rhythm
Bradycardia
Less than half the resting normal heart rate
Dogs <50bpm
Cats <90 bpm
Bradyarrhytmias= increased parasympathetic tone (usually drug or disease induced)
Tachycardia
Dogs >180 bpm
Cats >200 bpm
Tachyarrhythmias= incrased sympathetic tone (nociception, anxiety, hypercapnia, hypoxemia, hypovolemia, drugs)
Xylazine and ketamine effects on heart rhythm
Stimulate alpha and beta 1 receptors= tachyarrhythmias
Opiods and alpha 2 adrenergic agonists effect on heart rhythm
increase vagal tone =bradyarrhythmias
Acepromazine effect on heart rhythm
Anti arrhythmic
What drugs have minimal to no effect on rhythm of heart?
-inhalant anesthetics (until deep plane reached)
-propofol, alfaxalone
-benzodiazepines
Monroe-Kellie Doctrine
Cranium= brain + blood +CSF
If one increases, one of others must decrease
Cerebral metabolic rate
Determines cerebral blood flow (including internal carotid, vertebral arteries, circle of willis)
How is Cerebral blood flow regulated?
What does neuronal function depend on?
Inhaled anesthetics on CNS
Decreases CMR by 60%
-dose dependent effects on CBF due to blunting autoregulation
*minimal changes= <1.0 x MAC
*Direct cerebral vasodilation= >1.0 x MAX= increase in CBF and ICP
Ketamines effect on CNS
Cats unique blood flow
-Cortex and retinal blood supply is mainly due to maxillary artery. None from vertebral artery
-Use of spring loaded mouth gags to keep jaws open. This was causoing compression of maxillary artery= reduced blood flow= blindness
**instead use needle caps to make small mouth gags
Renal blood flow
-Receives 20-25% CO
-controlled by sympathetic NS through renal artery vasoconstriction
>vasodilation controlled by prostaglandins (COX2)
-NO parasympathetics
Autoregulation of renal blood flow
Autoregulated over MAP 80-180 mmHg
-protects against hyper and hypotensive states to maintain GFR
Importance of kidney function in anesthesia
Inhaled anesthetics effects on renal system
Dose dependent decrease in RBF and GFR
-post general anesthesia oligouria (urine output <0.5)
IV anesthetics, acepromazine, opioids, benzodiazepines effect on renal system
Minimal RBF and GFR effects
NSAIDs effect on renal system
Decreasing COX-2 production of prostaglandins
-blunts vasodilation of afferent arterioles
-inhibits ability to control RBF in the face of hypotension
Animals under anesthesia effects on renal system
Can see prolonged drug effects= hangover
Hepatic system Blood flow
Liver recieves 25% CO= majority of blood flow from portal vein (70-75%), rest is from hepatic artery (30%)
Importance of hepatic system during anesthesia
-blood reservoir
-carbohydrate metabolism
-plasma oncotic pressure (albumin and plasma proteins)
-coagulation factor production
-Xenobiotics-biotransformation and biliary excretion
-thermoregulation
Inhalant/injectable anesthetics and sedatives effect on hepatic system
minimal changes in hepatic blood flow
What causes low hepatic perfusion?
-increased sympathetic tone from stress
-surgical procedures near liver (up to 60% reduced blood flow)
-positive pressure ventilation (reduced venous return)
Drugs and liver metabolism
Effects of liver disease on hepatic system when under anesthesia
Use drugs that have reversal agents or short effects
Premedication drugs causing vomiting
Regurgitation during anesthesia
From decreased lower esophageal stricture pressure
-from sedatives, opioids, IV anesthetics, inhalant anesthetics, anticholinergics
What puts animals at high risk for regurgitation?
-brachycephalics, ruminants, or increased intra-abdominal pressure
-high risk procedures: intra-abdominal and orthopedic
Complications from regurgitation
- esophagitis
- Esophageal structure
3.aspiration pneumonia
Anesthesias effect on skeletal muscle
Disrupts normal control of blood flow and can lead to poor skeletal muscle perfusion
-Post-anesthesia myopathy or neuropathy seen in large animals, from prolonged anesthesia or hypotension
-Treatment: supportive care (fluids, analgesics, slings)
Types of post anesthetic myopathy or neuropathy
- Compartmental (local) = local hypoperfusion (1-2 muscles)
- Generalized (whole body)= general hypoperfusion (many muscles)
Support and positioning during anesthesia
-Best way to avoid myopathy and neuropathy
*important to remember that animals lose muscle tone under anesthesia which means they remove protective guarding at sites of injury
-ensure no strain on delicate structures (nerve plexus, eyes)
