Week 10 - Renal Physiology and Anesthesia Flashcards
What are the different homeostatic processes the renal system plays a vital role in?
– Regulation of H2O and electrolyte balance – Regulation of body fluid osmolality – Electrolyte composition in the body – Excretion of metabolic wastes – Regulation of arterial blood pressure – Secretion of hormones – Bone metabolism – Hematopoiesis (erythropoietin)
What are the two major layers of the kidney?
Outer Cortex and Inner Medulla
What is the nephron of the kidney?
Functional unit of the kidney
- each kidney contains 1 million nephrons
- body cant regenerate nephrons
- after age 40 the number of nephrons decreases by 10% for every 10 years of age
What are the 6 major components of a nephron?
– Glomerulus – Proximal convoluted tubule – Loop of Henle – Distal renal tubule – Collecting tubule – Juxtaglomerular apparatus
What are the characteristics of the glomerulus?
– Network of capillaries – High hydrostatic pressure of 60 mmHg – Covered with epithelial cells – Encased in bowman's capsule (Filtered fluid enters Bowman's capsule and then the proximal tubule)
What are the characteristics of the long tubule of the kideny?
– Proximal tubule lies in the renal cortex – Loop dips into the renal medulla and forms the loop of henle --- Thin walled – The ascending loop goes back into the cortex --- Thick walled – Terminates in short segment called the macula densa
What are the two classes of nephrons?
Cortical Nephron (80%): short nephron loop and glomerulus further from the corticomedullary junction -efferent arteriole supplies peritubular capillaries
Juxtamedullary Nephron (20%): has long nephron loop and glomerulus closer to the corticomedullary junction -efferent arteriole supplies vasa recta
What are the different mechanisms of renal excretion?
Glomerular Filtration: large amounts of protein free fluid are filtered into Bowman’s Capsule
Reabsorption: filtered substances are reabsorbed from the tubules and back into the blood
Secretion: some substances are secreted into the renal tubule so that the amount in the urine is greater than the amount found in the glomerular filtrate
What is glomerular filtration determined by?
Hydrostatic and Colloid Osmotic forces across the glomerular capillary basement membrane
What is the function of the glomerular capillary basement membrane?
- Restricts filtration of substances with a large molecular size
- It is negatively charged thus prevents the filtration of negatively charged substances
- some plasma proteins like albumin are negatively charged and the charge not the size prevents filtration
- early in some renal diseases, this negative charge is lost and proteinuria occurs
What are determinants of GFR?
- Hydrostatic pressure in the glomerular capillaries
- Hydrostatic pressure in Bowman’s capsule
- Glomerular capillary colloid osmotic pressure
- Bowman’s capsule colloid osmotic pressure — GFR = Pcap - (PBC + COPcap)
- Normal is 120mL/min
- life threatening when GFR is < 30 mL/min
What does the measurement of GFR tell you?
Renal Clearance – way of quantifying how efficiently the kidney clears the blood of various substances
- can use inulin (polymer of fructose that is not produced in the body and isn’t metabolized)
- Creatinine is a convenient way because no artificial substance is necessary
What percent of cardiac output supplies the kidneys?
20-25% of cardiac output (normal is 1200mL/min)
- renal artery comes off aorta and enters the hilum – branches to form the interlobar arteries, arcuate arteries, interlobular arteries and arterioles
- 90% renal blood flood is to the cortex and 10% is to the medulla
What determines renal blood flow?
The pressure gradient across the renal vasculature
RBF = (renal artery pressure - renal vein pressure) / total renal vascular resistance
What two mechanisms control autoregulation of the renal system?
Tubuloglomerular Feedback
Myogenic mechanism achieves autoregulation by changing renal vascular resistance as the arterial blood pressure changes
*urine output is NOT autoregulated
What pressure range is renal blood flow and GFR autoregulated between?
80 - 180 mmHg
What are the regulators of renal blood flow increase RBF or GFR?
- Prostaglandins: appear protective and prevent renal vasoconstriction/ischemia
- Nitric oxide: increases RBF and GFR
- Bradykinin: locally produced vasodilator (increases RBF and GFR)
- Atrial Natriuretic Peptide: secreted by heart with HTN and expanded blood volume (increases GFR)
- Dopamine: proximal tubule produces dopamine (increases RBF and inhibits renin release)
What are the regulators of renal blood flow decrease RBF or GFR?
- Sympathetic nerves: SNS stimulation = vasoconstriction in both afferent and efferent arterioles (decreases RBF and GFR)
- Angiotensin II: constricts afferent and efferent arterioles
- Endothelin: potent vasoconstrictor of both afferent and efferent arterioles (may be responsible for some renal damage in DMII)
- Adenosine: decreases RBF and GFR by causing vasoconstriction of the afferent arteriole
What are the three principles of membrane transport?
Passive Transport – doesn’t require energy, moves down concentration gradient
Facilitated Diffusion – involves a membrane transporter, moves with and against electrochemical gradients
Active Transport – requires energy in the form of ATP, sodium/potassium pump most common active transport in the kidney
What is a transport maximum?
Most substances that are actively secreted or reabsorbed have a limit on the rate of transport (occurs due to saturation of transport system)
- Glucose: transport max is approx 250-300mg/dL (glucosuria occurs when level exceeds this)
- Creatinine also has transport max
What are the functions of the proximal tubule?
- Sodium Reabsorption (Major function) – 67% of solute reabsorption, facilitated transport transport from tubule to cell
- Reabsorption of glucose, potassium, calcium, phosphate, uric acid and urea
- Water reabsorption – driving pressure is the osmotic gradient created as solute is reabsorbed
- Protein reabsorption – normally filtered and reabsorbed, easily saturated and found in urine if significant amount present (early sign of renal disease)
What can occur if a patient is taking two organic anions or two organic cations?
Excretion may be delayed
What is the function of the loop of Henle?
- Water reabsorbed in descending loop
- Thick ascending loop is impermeable to water but solute reabsorption occurs
- Loop diuretics work by inhibiting the reabsorption of Na in the ascending loop (reabsorption of K and Mg is also inhibited)
- Because solute and not water is reabsorbed in ascending loop the osmolality of tubular fluid is reduced
What is the juxtaglomerular complex?
Formed by the first part of the distal tubule
Provides feedback control of GFR and RBF
Divided into cortical and medullary portions
What is the function of the cortical tubules?
– Secrete K+
– Aldosterone mediated Na+ reabsorption
– Acid-base balance
– Freely permeable to urea
What is the function of the medullary tubules?
Normally impermeable to water but in the presence of ADH, water is reabsorbed
What are four factors that affect the kidneys ability to concentrate urine?
- Countercurrent mechanism
- Impermeability of thick ascending loop of Henle to water
- ADH: increases water reabsorption in the distal nephron
- Active reabsorption of NaCl in the ascending loop of Henle
What is the countercurrent multiplier in the renal system?
- Renders the renal medullary interstitium hypertonic — normal body osmolarity is 275-295 mOsm/L (renal interstitium is 1200)
- Loop of Henle has 2 parallel limbs – “Countercurrent”
- Fluid into medulla: IN: descending limb OUT: ascending limb
- Vasa recta reabsorbs water and transports solute
- Accumulation of NaCl in the interstitium is critical and a standing osmotic gradient is established
What is the lower limit of urine output in order to excrete necessary wastes?
500 mL/day (with maximum water reabsorption)
What SNS level is the renal cortex and vasculature innervated by?
SNS Fibers T4-L1
- mild alpha adrenergic stimulation = efferent arteriolar constriction (preserves GFR)
- intense alpha adrenergic stimulation = afferent arteriolar vasoconstriction (decreases GFR)
Where is renin produced? When is it released?
Produced in cells of JG apparatus
Released due to several stimuli:
- decreased renal artery perfusion pressure
- SNS stimulation
- decreased NaCl in macula densa
*Renin alone has no physiologic function (acts as proteolytic enzyme – cleaves angioteninogen to form angiontensin I which is cleaved to angiotensin II by ACE)
What are the effects of Angiotensin II?
Overall effect is increase water and Na reabsorption and return circulating volume to normal
Other effects:
- stimulation of aldosterone release by adrenal cortex
- arterial vasoconstriction which increases BP
- stimulation of ADH secretion and thirst
- enhances reabsorption of NaCl in prox tubule
- in severe stress (shock)- large amts of Angio II cause intense renal artery constriction and decreased RBF
- increases intrarenal prostaglandins
*ACE Inhibitors reduce renal vascular resistance
What is aldosterone secreted from? What is its function?
Secreted by Zona Glomerulosa of adrenal cortex in response to hyperkalemia, Angio II, and ACTH
Acts on ascending loop of Henle, distal tubule and collecting ducts – increases reabsorption of Na and Cl (causes water to passively follow)
Overall effects is increase Na and water reabsorption from distal tubule
Where is antidiuretic hormone synthesized? Where does it act? What does it cause?
Synthesized by anterior hypothalamus
When released it acts of receptors in collecting ducts – increases water reabsorption leading to more concentrated urine
Causes increased NaCl reabsorption in ascending loop of Henle – increases medullary osmolarity and further promotes water reabsorption
What effect does atrial natriuretic peptide have on the renal system?
- Released from the atrium in response to local wall stretch and increased atrial volume
- Increases GFR and promotes water and Na excretion
- Inhibits release of renin (blocks effect of renin on angio I and the vasoconstriction produced by angio II)
*overall effects of ANP are opposite and opposed to renin, angiotensin, ADH and aldosterone
Where are Dopamine-1 receptors located? What is the effect when low dose Dopamine is infused (1-3mcg/kg/min)?
Located in renal vasculature and proximal renal tubule
Low Dose (1-3 mcg/kg/min) = increased RBF and GFR and increased Na excretion and diuresis *>10 mcg/kg/min = renal vasoconstriction and decreased GFR
What spinal level innervates the ureters and kidneys?
Primarily T8-L2
*nerve supply to pelvic organs (bladder, prostate, seminal vesicles, and urethra) is primarily lumbosacral with some thoracic innervation
What are indications for a cystoscopy?
Hematuria
Recurrent UTIs
Renal calculi
Urinary obstruction
*most common urological procedure
What are intraop considerations for cystoscopy?
Lithotomy Position!
- failure to properly position pt can result in iatrogenic injuries
- leg position maintained using straps around ankles or special holders to support legs
*positioning is everyones responsibility
What is the most common nerve injury associated with the lithotomy position?
Brachial Plexus
What physiological changes are seen with cystoscopy?
- Decreased FRC (atelectasis and hypoxis, accentuated by trendelenburg >30*)
- Exacerbation of CHF (elevation of legs increases venous return acutely, MAP increases, but CO doesn’t change significantly)
- Hypotension (rapid lowering of legs acutely decreases venous return, vasodilation from either general or regional anesthesia may worsen it)
What are considerations of regional anesthetic management for cystoscopy?
Spinal vs Epidural – spinal most preferred (faster onset, shorter duration)
*adequate sensory blockade may take 15-20 min for Epidural vs 5 min for spinal
Sensory level to T10 necessary
Obturator reflex (external rotation and adduction of the thigh) - only reliably blocked by muscle paralysis during GA
What are the indications for a Trans Urethral Resection of Prostate (TURP)?
- Benign Prostatic Hypertrophy (BPH) - symptomatic bladder outlet obstruction (usually men >60yr)
- Severe urinary tract symptoms refractory to medical therapy
- Persistent gross hematuria
- Recurrent UTIs
- Renal insufficiency
- Bladder stones
What are complications of TURP procedures?
- Hemorrhage (more common w/ glands >40mL in size, difficult to control bleeding through cystoscope)
- Bladder Perforation
- Septicemia
- Hypothermia
- DIC
- MI
- Pulmonary edema
- Renal failure
- TURP syndrome
What is the pathogenesis behind TURP syndrome?
Open venous sinuses in the prostate allow for potential systemic absorption of the irrigating fluid – hypotonic (mostly H2O) irrigation
Absorption of >2L = TURP Syndrome
What are the symptoms of TURP Syndrome?
- CV and Respiratory
- CNS
- Metabolic
- Other
CV and Respiratory: HTN, brady/tachyarrhythmia, CHF, pulmonary edema, hypoxemia, hypotension
CNS: agitation/confusion, seizures, coma, visual disturbance/blindness
Metabolic: hyponatremia, hyperglycemia, hyperammonemia
Other: hypoosmolality, hemolysis, intravascular volume expansion
How do you treat TURP Syndrome?
- Early recognition is vital
- Absorbed H2O must be eliminated (fluid restriction, loop diuretic)
- Avoid hypoxemia and hypoperfusion
- Treat symptomatic hyponatremia (hypertonic saline, amount based on serum Na level)
- Treat seizure activity (small doses of versed, diazepam, or thiopental)
- Intubation (advisable to prevent aspiration until mental status returns to normal
What are intraop considerations for TURP procedures?
- Hypothermia – large amounts of irrigating fluid at room temp = heat loss
- Bladder Perforation – signaled by poor return of irrigating fluid, sudden HoTN, bradycardia
- Coagulopathy (DIC) – result from release of thromboplastins
- Septicemia – prophylactic antibiotic therapy prior to TURP
What are benefits of spinal or epidural anesthetic management for TURP procedures?
- Provides excellent anesthesia and good operating conditions
- Allows for more immediate detection of signs of TURP syndrome and bladder perforation
- Acute hyponatremia from TURP syndrome may delay or prevent emergence from GA
- contraindicated with metastasis to lumbar spine
- need T10 level
What are postop considerations for TURP procedures?
- If TURP syndrome: obtain serum osmolarity, CXR, EKG, electrolytes
- Fever/bacteremia/sepsis
- Prevent Hypothermia -Shivering (dislodges clots, promotes bleeding)
- Hemodilution from absorption of irrigation (transient decrease in hematocrit)
- Foot drop: peroneal nerve compression at lateral fibular head while in lithotomy position
What is extracorporeal shock wave lithotripsy (ESWL) a treatment for?
Treatment of kidney stones and stones in upper 2/3 of the ureters
-used more for larger stones (>4mm) that are intrarenal
Repetitive high energy shock waves focus on stone – stone is fragmented and usually passed through urinary tract
What are preop considerations for extracorporeal shock wave lithotripsy (ESWL)?
-Pt with cardiac arrhythmia hx or with pacemaker/ICD are at increased risk for arrhythmias induced by ESWL
Shock waves can damage components of pacemaker/ICD
Synchronization of the shock waves to the R wave from the ECG decreases the incidence of arrhythmias
What are disadvantages of using regional anesthesia for extracorporeal shock wave lithotripsy (ESWL) procedures?
Need T6 level to ensure adequate anesthesia
Inability to control diaphragmatic movement (may prolong the procedure) – GA allows for control of breathing
Bradycardia from high sympathetic blockade also prolongs the procedure – shock waves are coupled to ECG
What are contraindications to extracorporeal shock wave lithotripsy (ESWL) procedures?
- Inability to position patient so lung and intestine are away from the shock wave
- Urinary obstruction below the stone
- Untreated infection
- Bleeding diathesis
- Pregnancy
- Aortic aneurysm +/-
What drugs should you avoid in renal transplant surgery?
Succinylcholine if K >5 Morphine due to metabolite Meperidine due to metabolite NSAIDs COX-2 inhibitors Enflurane
*use rocuronium, vec, mivacurium, and pancuronium with caution
What is Serum Creatinine level a useful marker of?
Useful marker of GFR
-if creatinine doubles, GFR has decline by 50%
- product of skeletal muscle catabolism
- Normal = 0.8-1.2
- Creatinine Clearance normal = 120 mL/min
BUN level is useful in assessing what?
Prerenal vs Intrarenal abnormalities
- urea = most abundant metabolic waste product
- BUN increases when GFR is reduced
- Normal = 8-10
What is the minimum allowable urine output for adequate excretion of metabolic wastes?
0.5 mL/kg/hr
What are the three types of acute renal failure?
Prerenal - Disorder of systemic circulation that cause hypoperfusion
Infrarenal - Destruction of lining of renal tubules d/t nephrotoxins or renal ischemia (most common cause in critically ill patients)
Postrenal - Obstruction may occur anywhere from renal pelvis to distal urethra – Less than 5 % of cases
What are the different stages of chronic kidney disease?
Stage 1 – Kidney damage, but normal GFR (>90ml/min)
Stage 2 – Mild Kidney damage (GFR 60-89 ml/min
Stage 3- Moderate kidney damage (GFR 30-59 ml/min)
Stage 4- Severe kidney damage (GFR 15-29 ml/min)
Stage 5- Kidney failure (GFR <15 ml/min or ESRD – On renal replacement therapy)
What causes anemia of chronic renal failure? Which way does the oxyhgb dissociation curve shift?
Decreased erythropoietin and hemolysis
Shift oxyhgb dissociation curve to RIGHT due to increased 2-3 DPG
What coagulopathies are seen in chronic renal failure?
Decreased platelet adhesiveness due to metabolic acidosis interfering with Factor VIII
*treat with desmopressin or cryoprecipitate
What electrolyte disturbances are seen in chronic renal failure?
- Hyperkalemia
- Hypermagnesemia
- Hypocalcemia (decreased Ca absorption due to inability to activate vitamin D)
How does renal disease affect Propofol, Etomidate, Benzos, and Opioids?
Propofol: pharmacokinetics not altered, no drug accumulation, no unusual hemodynamic effects
Etomidate: decreased protein binding w/ hypoalbuminemia may enhance its effects
Benzos: highly protein bound and increased sensitivity w/ hypoalbuminemia
Opioids: inactivated in liver -p accumulation of active metabolites
What are manifestations of uremia?
- Confusion
- Lethargy
- Seizures
- Volume overload / CHF
- HTN
- Arrhythmia
- Hyperventilation / Dyspnea
- Metabolic Acidosis
- Hyperkalemia/ Hyponatremia/ Hypermagnesemia
- Anemia / Platelet dysfunction
What are indications for dialysis?
- Fluid overload
- Hyperkalemia
- Severe acidosis
- Metabolic encephalopathy
- Pericarditis
- Coagulopathy
- Drug toxicity
