What is the role of kidney in acid base balance? What are the anaesthetic considerations in a dialysis patient?

The role of kidney in acid base homeostasis

Renal compensation in Metabolic Acidosis

1) Increased Reabsorption of HCO₃

2) Increased excretion of titrable Acids

3) Increased production of ammonia

Increased Reabsorption of HCO₃

almost 70-80% of all HCO₃ filtered by the kidneys is reabsorbed in proximal convoluted tubules

H⁺ is titrated into the tubular fluid and carbonic anhydrase catalyses the production of H₂O and CO₂which is rapidly reabsorbed and carbonic anhydrase within the cell converts CO₂ back into HCO₃which exits into the peritubular blood

Increased Excretion of Titratable Acids

The H⁺ that was secreted into the tubular fluid to recover the HCO₃⁻ combines with urinary buffers as HPO₄²⁻ to form H² PO⁴ which due to its charge can't be absorbed back and is excreted from body 

Increased production of Ammonia

The Deamination of Glutamine in the proximal convoluted tubules is the principle source of NH₃ production in the kidneys which is secreted into the tubular fluid as a buffer and combines with H⁺ to form NH₄⁺ which is readily not absorbed back and excretion of NH₄ in urine effectively eliminates H⁺

Renal compensation in Metabolic Alkalosis: The kidneys have good capacities of reabsorption or excretion of HCO₃which protects against metabolic alkalosis per se hence metabolic alkalosis  generally occurs in association with sodium depletion or mineralocorticoid excess. Sodium depletion enhances the Na⁺ reabsorption into the tubular blood  which is coupled with Cl⁻ to maintain neutrality which again causes H⁺ secretion and HCO³reabsorption causing metabolic alkalosis

The anaesthetic concerns in dialysis patients

1. Hypervolumia
2. Acidemia
3. hyperkalemia
4. Cardiac : Hypertension is most common
5. Pulmonary congestion and edema 
6. Hematological : Anemia due to insufficiente production of erythropoietin,platelet dysfunction leading to prolonged bleeding and clotting time
7. Hypoalbuminemia; due to hemodilution, impaired synthesis and increased loss leading to alterations in drug effect of high protein binding drugs like diazepam, warfarin and phenytoin but anaesthetic drugs are less albumin binding and free fraction of drug increases modulation needful in drug dosage
8. Patients weight need to be recorded so dry weight can guide to dialysis
9. Last dialysis time as immediately patients are hypovolumic with increased risk of hypotension
10. Use of Heparin in dialysis can potentiate blood loss during surgery so titres need to be done and antagonist used if more
11. Care of AV fistula 
12. Fluid therapy should be guarded on intra and post operative periods 

Anesthesia for total laryngectomy

Total laryngectomy is the removal one or more of the laryngeal structures including the epiglottis,hyoid,and a variable amount of upper trachea with selective neck dissection which aims to remove a portion of nodes most at risk of metastasis.
PREOPERATIVE ASSESSEMENT
Standard preliminary investigations with stress on Hb levels, anaemia if present should be optimised.Most patients may require grouping and saving of blood but pre-op transfusion is rare.
Comorbid health conditions: 1) Ischaemic Heart Disease is a frequent comorbidity and cardiology opinion has to be sought if unstable angina,recent stenting,decompensated heart failure or severe aortic stenosis.
2) Respiratory Disease Laryngectomy patients are frequent smokers.Thorough evaluation of the respiratory system with associated imaging studies, if laryngeal narrowing is significant then Pulmonary Function Tests are not very reliable If Pulmonary Hypertension and Right Heart Failure are associated then serious consideration to optimise as they fall under high risk category.
Dynamic assessment of the functional status of the cardiopulmonary status of the patient can be evaluated with the estimation of METS score if < 4 then associated with increased risk
Preoperative Optimisation Majority patients are chronic alcoholics and smokers, careful planning and optimisation to proceed  1 to 2 weeks ahead of surgery is warranted.Hospital admission and controlled alcohol withdrawal with NG feeding to improve nutrition.Smoking cessation.
Risk Stratification Discussion about risk and consent to be obtained on individual needs
Planning of airway Management
Airway Assessment
History Taking in detail with emphasis on voice change, dysphagia,breathlessness and stridor . if the patient can lie flat? and if he get"s up in sleep due to breathlessness?
Bedside examination: The patients natural resting position is to be noted.
 Airway assessment usual mouth opening, tongue protrusion, mallampati scoring, thyro-mental distance, neck movement is done
Neck Examination  For neck masses, any previous radiotherapy, tracheal deviation, previous scars.
Any anticipated difficulty with front of neck access should be noted
Imaging CT and MRI are the mainstay of preoperative imaging, The axial sections can be used to see the narrowing of airways and the coronal sections are used to estimate the length of narrowing.
Nasendoscopy imaging can be done prior to the airway management with the surgical team to note any points of narrowing, rigidity or fragility of tissue, or the presence of tumour which may obscure the laryngoscopic view.
Thorough evaluation is done in view of the following steps
Preoxygenation
Positioning the patient in the most restful position
High Flow Nasal Oxygenation (HFNO) as a means to pre oxygenate and apnoiec ventilation until definitive airway is obtained has been highlighted  in the Transnasal Humidified Rapid-insufflation ventilator exchange(THRIVE) study and is widely used. It can provide CPAP. If conventional Nasal prongs are used than can be augmented with face mask (NODESAT) technique
Preinduction 
These patients are always induced in OT table in presence of consultant surgeon
A anaesthesia and surgical briefing is done and planA,B,C and D is discussed, prepared and written on white board so all members of the team are prepared.
Awake intubation or conventional induction depends on the ease of attaining a definitive airway
Video-laryngoscopy has been the first line of choice in DAS guidelines for laryngoscopy
Awake Fibre-Optic intubation (AFOI) is the method of choice in supraglottic obstruction as in epiglottis and tongue base obstuction
In patients with critical narrowing of laryngeal inlet the AFOI is of lesser use as because of

• Topicalisation is difficult and there is Copious Secretion 
• Sedation has be used very judiciously as maintenance of spontaneous ventilation is needful
• "Cork in the Bottle" analogy as the critical narrowing is completely closed with the introduction of the FOB scope and the airway is lost.

FONA Front of neck access is ideal in the above conditions. Trantracheal cannulation for jet ventilation which can later be converted to tracheostomy if needful.Prior assessment of neck for FONA with landmarking and LA infiltration and prior ascertaining of who will perform FONA  can buy time in hour of need.The choice of cricothyroidectomy or tracheostomy is a individualised.

Perioperative Management

Monitoring

Standard monitors with invasive blood pressure monitors to assess the fluid and blood chemistry as the surgery is usually long and blood loss is anticipated.

Urinary Catheter and temperature probe.

Short acting agents are preferred due to long duration to allow early emergence

TIVA is preferred as there is loss of volatile anaesthetic during surgery

Meticulous care in positioning to avoid pressure points

Post operative care

Admission into HDU with team handlingl aryngectomy patients

Pain management with judicious use of opioids and NSAIDS and Patient controlled Analgesia

Patients are stepped down to specialist wards in the next 48hrs.

what are the adjuncts in anaesthesia

Histamine receptor Antagonists

H₁Receptor Antagonists 

H1 antagonist also have antimuscarinic or antiserotonergic which can be used for a multitude of therapeutic purposes

• suppression of allergic symptoms, cough, nausea and vomiting.
• sedation

Used as premedicaton because of antiemetic and mild hypnotic 

They maintain the ventilatory drive during sedation and their sedative effects can potentiate other CNS depressants such as Benzodiazepines,barbiturates and opioids.

H₂ Receptor antagonists

• Reduce gastric acid output ad raise gastric pH
• As a premedication to reduce peri operative risk of aspiration pneumonia
• Dosage is at bedtime and again at least 2hr before in the morning of surgery
• Rapid iv injection rarely associated with hypotension and arrhythmia
• Mild dose reduction in signifiant kidney impairment as renal elimination
• Weak inhibitor of Cytochrome P-450 hence no significant drug interaction

Antacids

• Neutralize the gastric acidity by giving  base to react with hydrogen ion to form water
• Unlike H2 blockers have an immediate effect
• Present in 2 forms- Particulate and Non-Particulate
• Non particulate are widely used as less damaging to heart 
• Dosage is 0.3M solution of Sodium Citrate 15-30ml orally 30min prior to induction
• Drug interaction by alteration in gastric pH like slowing absorption and elimination of Digoxin,Rantidine whereas phenobarbital elimination s prolonged

Metoclopramide

It is a Prokinetic Drug and increases the Lower Esophageal Sphincter tone, speeds gastric emptying and lowers gastric fluid volume

• It is cholinomimetic at intestinal smooth muscles and dopamine antagonist in CNS
• It is a antiemetic 
• Rapid injection may cause abdominal cramping and hypertensive crisis with pheochromocytoma 
• Sedation, nervousness and extra pyramidal signs with dopamine antagonism
• Concurrent usage with phenothiazines can increase extrapyramidal S/E

Proton Pump Inhibitor

• Pantoprazole inhibit the secretion of hydrogen ions and reduce gastric volume 
• Dosage 40mg 2hr prior to induction 
• Interferes with P-450 enzymes and decreases the clearance of diazepam, warfarin and phenytoin
• Repeat doses with caution in severe liver impairment as liver elimination

5-HT₃ Receptor Antagonists

Ondansetron is a antiemetic  due to blocking of serotonin 5HT3receptors at CTZ 

• Effective antiemetic in post op nausea and vomiting
• Prophylaxix in high incidence of N/V in Laparoscopy Surgery, Neurosurgeries
• Can Slightly prolong QT interval in ECG
• Given either prior to induction or at the end of the surgery

NSAIDS

Ketorolac and Diclofenac provide analgesia by inhibiting prostaglandin synthesis

• alternative to opioids in post operative pain management as they do not cause sedation, respiratory depression or N/V
• Inhibit platelet aggregation and prolong bleeding time cautious use in post op haemorrhage
• Dose reduction in renal impairment 
• Contraindicated in patients allergic to aspirin and NSAIDS

Clonidine

Alpha adrenergic agonist which  decreases sympathetic activity, enhance parasympathetic tone and reduce catecholamines

• Used as anti hypertensive
• Adjunct to Local anaesthetic agents in Epidural anaesthesia 
• As a premedication 
• Side effects sedation, bradycardia, dizziness, dry mouth
• Dose reduction in renal impairment

Dexmedetomedine

• Alpha agonist with selective activity
• Dose dependent sedation,anxiolysis and some analgesia
• Opioid Sparing role with no significant respiratory depression
• Side effects are bradycardia hypotension

Draw a labelled diagram of larynx. Mention the nerve supply of larynx. Describe Block for awake intubation?

Nerve Supply of Larynx

The larynx is innervated by

1. Superior Laryngeal Nerve⇨external and internal branches
2. Recurrent laryngeal nerve
3. Sympathetic nerves

• The Internal Laryngeal Nerve is sensory
• The External Laryngeal Nerve is Motor
• The Recurrent Laryngeal Nerve is mixed

The Internal Laryngeal Nerve is sensory down to the vocal cord and Recurrent Laryngeal Nerve is sensory below the vocal cords with overlap of territories
All the intrinsic muscles of the larynx is supplied by the Recurrent Laryngeal Nerve except the Cricothyroid which is supplied by the External Laryngeal Nerve

AWAKE INTUBATION
Awake intubation is the mainstay os ASA's difficult airway algorithms
Advantages of Awake over Anesthetised are

1. Maintenance of Spontaneous ventilation
2. increased size and latency of the pharynx
3. relative forward placement of the base of tongue
4. posterior placement of larynx
5. Patency of the redropalatal space
6. awake state offers better sphincter tone of oesophageal sphincters

Contraindications 
Patient Refusal
Inability to cooperate
Allergy to local anaesthetics

Procedure of awake intubation

1)Patient counselling with explanation for the need of  awake airway and the techniques briefed

2)Medication to allay anxiety can be done with judicious dosing of small amounts of sedatives like           midazolm or opioids like fentanyl can be titrated,avoid poly pharmacy and reversal agent in hand

Dexmedetomidine a highly selective centrally acting alpha 2 adrenergic agonist causes sedation without respiratory depression

3)Administration of antisialogogues is important to clear airway secretions for optical instruments

4)Vasoconstriction of nasal passages if instrumentation required

5) Supplemental Oxygen by nasal cannula

6) Local Anesthetics are the cornerstone of awake airway control techniques  The areas for topical anesthesia are nasal cavity /nasopharynx,pharynx /base of  tongue,hypopharynx/larynx/trachea

The Nasal cavity is supplied by Greater and Lesser palatine from sphenopalatine ganglion  and the anterior ethmoid nerve

The Sphenopalatine Ganglion Block 

Noninvasive technique of cotton tipped applicators soaked in local anaesthetic and passed along upper border of the middle turbinate left in place for 10min

Invasive technique is by the oral approach ,needle is introduced through the greaterpalatine foramen which is palpated at the posterior lateral aspect 1cm medial to the second and third maxillary molars

Anesthetic solution of 1-2ml is injected with spinal needle at supero-posterior direction to a depth of 2-3 cms

Oropharynx is innervated by branches of vagus, facial, and glossopharyngeal

The Glossopharyngeal has three branches with sensory innervation a wide variety of techniques are involved like aerosolised local anaesthetic solution or a voluntary swish and swallow

Glossopharyngeal nerve block esp when topical techniques do not block the gag reflux

Standing opposite to the side to be blocked the operator displaces the extended tongue to the contralateral side and a 25G spinal needle is inserted into the membrane near the floor of the mouth or the posterior approach is where the needle is inserted behind the palatopharyngeal arch but the risk of carotid insertion is high

Superior Laryngeal Nerve Block is done by injecting at the local anaesthetic at the space between the thyrohyoid membrane and the pharyngeal mucosa landmark is the hyoid bone

trans tracheal injection of local anaesthetic is done with lidocaine 4ml of 2% or 4%

Describe the regulation of cerebral blood flow? and what the factors which affect the intracranial pressure peri -operatively

Regulation of cerebral blood flow

Total cerebral blood flow in adults is 750ml/min (15 to 20% of cardiac output) 

Average around 50ml/100gm/min

Cerebral Perfusion Pressure 

CPP is the difference between Arterial Pressure and Intracranial Pressure or central venous pressure whichever is higher

CPP = MAP -ICP(CVP)

CPP is 80 -100 mm of Hg 

Increases in ICP over 30 mm of Hg can  compromise CPP with values below 25 mm of Hg can cause irreversible brain damage

Autoregulation

The cerebral vasculature is CPP dependent which in turn is regulated by MAP
Decrease in CBF causes vasodilation and increases in CBF causes vasoconstriction
CBF remains normal between 60 to 160 mm of Hg
The cerebral auto regulation curve is shifted to right in chronic hypertension
Both Myogenic and Metabolic mechanisms control cerebral auto regulation
Myogenic mechanisms involve the intrinsic smooth muscle in cerebral arterioles to changes in MAP
Cerebral metabolic demands determine the CBF when the tissue demands exceeds the blood flow then there s release of tissue metabolites which cause vasodilatation.
EXTRINSIC MECHANISMS
Respiratory Gas Tensions
The significant extrinsic factor is partial pressure of  Co2
⇧PaCO2 ⟶⇧CBF
Temperature
Changes of 5-7% in CPP per 1 degree centigrade change in temperature
Hypothermia decreases both CMR/CBF
Hyperthermia increases CMR/CBF
Viscosity 
Changes in blood viscosity doesnt cause much changes in CBF
The most imp determinant of blood viscosity is haematocrit
⇧Haematocrit  ⟶⇩viscosity ⟶⇧CBF
           ↓
⇩Oxygen Carrying capacity
Autonomic Influences
Intra cranial vessels are innervated by sympathetic ,parasympathetic and non cholinergic non adrenergic fibres
⇧Sympathetic stimulation ⟶⇧ Vasoconstriction ⟶⇩CBF
Autonomic innervation plays an imp role in cerebral vasospasm
Perioperative Factors Affecting Intracranial Pressure

Point of Care Coagulation Tests and perioperative implications

Point of care coagulation are the tests performed near the patient which are non lab tests with rapid results reducing the turnaround time,  requires minimal amount of whole blood and reduces inappropriate blood transfusion.

POC tests in the peri operative period can be broadly classified as

• Functional assay to monitor heparin anticoagulation 
• viscoelastic measures of coagulation
• platelet function monitoring
• clotting factor tests

Functional assays to monitor heparin Anticoagulation

Activated Clotting Time         

     The ACT is used to measure heparin therapy as in cardiac surgery, hemofiltration, extracorporeal     oxygenation etc with normal range between 90-150 secs and values of 480 secs are considered safe in  setting of CPB

Advantage  low cost and linear response with high heparin conc.

Disadvantages lack of sensitivity at low heparin conc
false prolongation at hypothermia, coagulation factor deficiency, warfarin
 
 2.   High Dose Thrombin Time

Viscoelastic measures of coagulation

These tests measure the whole spectrum clot formation and the coagulation parameters are assessed in real time on whole blood
Devices based on viscoelastic principles are

1) Thromboelastography

 2) Rotational thromboelastography

  3) Sonoclot

Thromboelastography
Analyses and graphically displays the changes in viscoelastography across all stages of clot formation and resolution
TEG is more sensitive to fibrinolysis
The shape of the TEG helps in assessment of different coagulation states
                             
                                     

R/CT indicate conc of soluble clotting factors in plasma / K-time indicates clot kinetics / alpha angle  indicate rapidity of fibrin build up/ MA indicate number and function of platelets and fibrinogen conc./CL30 and 60 indicate the clot stability and fibrinogen conc

• Viscoelastic test are rapid and overall coagulation status of patient is Assessed with derived parameters helping in administration of blood and blood products
• TEG analyses all stages of coagulation like initiation, amplification and propagation reflecting the interaction of plasma and cellular components of coagulation and fibrinolysis
• TEG and ROTEM based transfusion algorithms reduce the rates of transfusion of blood and blood components 
• viscoelastic measures have been shown of significance in hyper coagulable states in post operative period
• TEG cartridges are extremely sensitive to residual heparin which may benefit in inadequate heparin reversal

PLATELET FUNCTION TESTS

Whole blood point of care tests of platelet function are significant to assess the effectiveness of anti platelet drugs and the recovery of platelet function when they are stopped in the setting of widespread usage of such drugs
1) PFA 100  measures the adhesion and aggregation of platelets under high shear stress
the CT is 5-8 secs which is prolonged in Von Willebrandt Disease both (Epi CT and ADP CT)
 while only Epi CT is prolonged in Aspirin therapy but ADP CT remains normal
2) Platelet works cheap rapid assessment of all classes of antiplatelet  agents
3) Verify Now
4) TEG Platelet mapping

COAGULATION TESTS

Point of care coagulation tests of coagulation were useful in patients taking Coumarin Derivatives are Prothrombin Time(PT), International Normalised Ratio(INR), aPTT

Choosing the correct point of care coagulation test

the selection of POCT devices should be tailored to clinical situation like in hyperfibrinolysis and plasma coagulation defects in liver Transplant,Trauma

Limitations of POCT

• Differences with Lab based tests
• Experience of operator
• Regent sensitivity
• Thorough familiarity with device functioning.