TOTAL BODY WEIGHT (TBW) , LEAN BODY WEIGHT (LBW), IDEAL BODY WEIGHT (IBW) & ADJUSTED BODY WEIGHT (ABW) ; THEIR IMPLICATIONS IN ANESTHESIA AND CRITICAL CARE

🏀 Drug administration in obese patients is difficult because recommended doses are based on pharmacokinetic data obtained from individuals with normal weights

🏀 With increasing obesity, fat mass accounts for an increasing amount of TBW, and the LBW/TBW ratio decreases

🏀 TBW is defined as the actual weight 

🏀 IBW is what the patient should weigh with a normal ratio of lean to fat mass 

🏀 IBW can be estimated from the formula: IBW (kg) = Height(cm) − x ( where x = 100 for adult males and 105 for adult females).

🏀 LBM is the patient's weight , excluding fat

🏀Male LBM = 1.1(weight)-128(weight/height)^2 (Weight in Kg and Height in cm)

🏀Female LBM = 1.07 (weight) -148 (weight/height)^2

🏀Regardless of total body weight, lean body weight rarely exceeds 100 kg in men and 70 kg in women 

🏀 Below IBW, TBW and LBM are similar. 

🏀 Adjusted body weight (ABW) Takes into account the fact that obese individuals have increased lean body mass and an increased volume of distribution for drugs.

🏀 It is calculated by adding 40% of the excess weight to the IBW :

ABW (kg) = IBW (kg) + 0.4 (TBW (kg)   IBW (kg)

🏀 Drugs with weak or moderate lipophilicity can be dosed on the basis of IBW or more accurately on LBM. These values are not same in obese; because 20–40% of an obese patient's increase in TBW can be attributed to an increase in LBM. Adding 20% to the 'estimated IBW based dose' of hydrophilic medication is sufficient to include the extra lean mass. Non-depolarizing neuromuscular blocking drugs can be dosed in this manner. 

🏀 In morbidly obese patients, the induction dose of propofol can be calculated on IBW. 

🏀 In case of midazolam, prolonged sedation can occur from the larger initial dose needed to achieve adequate serum concentrations. 

🏀 Remifentanil dosing regimens should be based on IBW or LBM and not on TBW. 

🏀 When using succinylcholine in obese adults or adolescents, dosage should be calculated on TBW

🏀 The antagonism time of neostigmine has been shown to be independent of TBW and BMI. Therefore, TBW can be used to calculate the dose.

−128(weight/height)2

$$\begin{array}{rcl}& & \mathrm{M}\mathrm{a}\mathrm{l}\mathrm{e}\mathrm{L}\mathrm{B}\mathrm{M}1.1\mathrm{w}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t}\\ & & 128{\mathrm{w}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t}\mathrm{h}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t}}^2\end{array}$$

Female LBM = 1.07(weight)

$$\mathrm{I}\mathrm{B}\mathrm{W}\mathrm{k}\mathrm{g}\mathrm{h}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t}\mathrm{c}\mathrm{m}x$$

IBW (kg) = height (cm) − xm.ceaccp.oxfordjournals.org/content/4/5/152.full

Ref:Association of Anaesthetists of Great Britain and Ireland. Peri-operative management of the obese surgical patient 2015. Anaesthesia 2015, 70, pages 859–876.

http://bja.oxfordjournals.org/content/105/suppl_1/i16.full

MEASUREMENT OF CORE BODY TEMPERATURE: FAQs

❓What's the problem, if we place the temperature probe in upper 1/3rd or 2/3rd if the esophagus?

✔️Esophageal temperature should be taken from the lower third of the oesophagus; placed above this level, the probe may under-read due to cooling effect of inspired gases. It gives a good estimate of cerebral blood temperature. 

❓What's the advantage of nasopharyngeal temperature measurement over oesophageal measurement?

✔️The nasopharyngeal temperature probe is placed just behind the soft palate. The advantage is that it is more accessible compared to the oesophageal temperature measurement. The disadvantage is that it is less accurate in representating the core temperature.

❓What are the advantages of measurement of temperature @ Tympanic membrane?

✔️The tympanic membrane provides an accurate representation of hypothalamic temperature. It is less invasive, has a short response time and correlates well with oesophageal temperature. But it does not allow continuous measurements.

❓What is the best method for CONTINUOUS measurement of core temperature?

✔️Blood temperature measurement using a pulmonary artery flotation catheter 

❓What are the factors reducing the accuracy of Rectal temperature measurement?

✔️Rectal temperature is influenced by heat generated from gut flora, the cooling effect of blood returning from the lower limbs and the insulation of the probe by faeces. It is normally about 0.5–1.0 ° C higher than core temperature and has a slow response time. 

❓Can you say an e.g. of utilising the temperature gradients existing between different sites of the body for clinical advantage?

✔️The gradient between a skin temperature and a core temperature can be used as a marker of peripheral perfusion.

MONITORING OF NEUROMUSCULAR BLOCK : DO YOU FREQUENTLY FORGET THE NUMBERS RELATED TO TOF, DBS & PTC?

Train of four (TOF) 

💥four stimuli {T1-T4} are given at a frequency of 2 Hz (0.5 sec between the stimuli)

💥 Each stimulus in the train causes the muscle to contract and the 'fade' in the response allow us to evaluate the neuro muscular blockade

💥The ratio T4:T1 ( i.e. Dividing the amplitude of the fourth response by the amplitude of the first response ) indicates the degree of neuromuscular block. 

💥Non-depolarizing NMBAs produce a decrease in magnitude of the first twitch compared with a pre-relaxant stimulus, and a progressive reduction in magnitude of T1–T4. 

💥The number of elicited twitches indicates the degree of receptor occupancy. 

💥Disappearance of T4, T3, T2, T1 corresponds to 75%, 80%, 90% and 100% occupancy. 

💥With recovery of neuromuscular function the twitches appear in the reverse order. 

💥 Accepted values for TOF count are:

🔹 1 twitch for tracheal intubation

🔹1–2 twitches during established anaesthesia

🔹1 twitch for tracheal intubation

🔹1–2 twitches during established anaesthesia

🔹3–4 twitches before reversal of neuromuscular blockade is attempted.

Double burst stimulation 

💥 Consists of two bursts  ( the duration of each square wave impulse in the burst is 0.2 sec ) at 50 Hz with each triple burst separated by 750 ms. 

💥 DBS with 3 impulses in each of the two tetanic bursts is commonly used

💥 These manifest visually as two separate stimuli (T1 and T2). 

💥 The ratio is related to the TOF ratio and is easier for the operator to interpret reliably.

💥 Used under light paralysis where train of four ratio is difficult to distinguish

Post-tetanic Count (PTC) 

💥 PTC is used when there is no response to TOF stimuli and also when we want to eliminate sudden movements of the patient completely as during ophthalmic and neurosurgery

💥 Uses tetanic stimulation at 50 Hz for 5 s to mobilize presynaptic ACh (to ‘kick start’ the nerve under deep paralysis) . 

💥 After a recovery time of 3 sec , it's followed by 20 pulses at 1-2 Hz twitch stimulation 

💥 The number of twitches generated (i.e. the post-tetanic count) reflects the degree of neuromuscular blockade.  

💥 Shows fade response earlier than train of four 

💥 Used under deep paralysis to estimate time to recovery

Reference: frca.uk Anesthesia Monitoring Techniques , Miller's Anesthesia , 7/e

PHARMACOLOGICAL TREATMENT OF ACUTE SEVERE ASTHMA ( BASED ON 2014 BTS GUIDELINES)

✔️Supplementary oxygen to all hypoxaemic patients with acute severe asthma to maintain an SpO2 level of 94-98%

✔️Nebulisers for giving nebulised β2 agonist bronchodilators should preferably be driven by oxygen. A flow rate of 6 l/min is required to drive most nebulisers

✔️High-dose inhaled β2 agonists as first line agents in patients with acute asthma. Repeat doses of β2 agonists at 15–30 minute intervals or give continuous nebulisation of salbutamol at 5–10 mg/hour (requires appropriate nebuliser) if there is an inadequate response to initial treatment. Higher bolus doses, for example 10 mg of salbutamol, are unlikely to be more effective (2.5–5 mg salbutamol in children >2 years).

✔️There is no evidence for any difference in efficacy between salbutamol and terbutaline. Nebulised adrenaline (epinephrine), a non-selective β2 agonist, does not have significant benefit over salbutamol or terbutaline.

✔️Add nebulised ipratropium bromide (0.5 mg 4-6 hourly) to β2 agonist treatment for patients with acute severe or life-threatening asthma or those with a poor initial response to β2 agonist therapy. ( 250 micrograms/dose in children >2 years).

✔️Consider giving a single dose of IV magnesium sulphate (1.2-2 g IV infusion over 20 minutes) to patients with acute severe asthma who have not had a good initial response to inhaled bronchodilator therapy.

✔️Nebulised magnesium is not recommended for treatment in adults with acute asthma. Consider adding 150 mg magnesium sulphate to each nebulised salbutamol and ipratropium in the first hour in children >2 years with a short duration of acute severe asthma symptoms presenting with an oxygen saturation less than 92%.

✔️Routine prescription of antibiotics is not indicated for patients with acute asthma.

SECOND LINE TREATMENT OF ACUTE ASTHMA

✔️Consider early addition of a single bolus dose of intravenous salbutamol (15 micrograms/kg over 10 minutes) in a severe asthma attack where the patient has not responded to initial inhaled therapy.

✔️Consider aminophylline for children >2 years with severe or life-threatening asthma unresponsive to maximal doses of bronchodilators and steroids. A 5 mg/kg loading dose should be given over 20 minutes with ECG monitoring (omit in those receiving maintenance oral theophyllines) followed by a continuous infusion at 1 mg/kg/hour. Measure serum theophylline levels in patients already receiving oral treatment and in those receiving prolonged treatment.

NOTE:

✔️Give steroids in adequate doses in all cases of acute asthma attack.

✔️Prednisolone 40–50 mg daily or parenteral hydrocortisone 400 mg daily (100 mg six-hourly in adults and 4 mg/kg repeated four hourly in children >2 years ) are as effective as higher doses. Continue prednisolone 40–50 mg daily for at least five days or until recovery. ( In children >2 years, treatment for up to three days is usually sufficient).

✔️Following recovery from the acute asthma attack steroids can be stopped abruptly. Doses do not need tapering provided the patient receives Inhaled Corticosteroids

✔️In adults with an acute asthma attack, i.v. aminophylline is not likely to result in any additional bronchodilation compared to standard care with inhaled bronchodilators and steroids. Side effects such as arrhythmias and vomiting are increased if Iv aminophylline is used

✔️Heliox is not recommended for use in patients with acute asthma outside a clinical trial setting

✔️Although theoretically furosemide may produce bronchodilation, a review of three small trials failed to show any significant benefit of treatment with nebulised furosemide compared to β 2 agonists

Anesthesia implications of Von Hippel–Lindau disease (vHLD)

🎨vHLD usually presents in young adults with cerebellar, medullary or spinal haemangioblastomas, retinal angiomatosis, renal cell carcinoma and phaeochromocytoma

🎨The frequency of phaeochromocytomas is 7–20%

🎨About 25% of patients with CNS haemangioblastomas subsequently turn out to have vHLD.

🎨Erythrocytosis and a high haematocrit are common and has to be searched for.

🎨Surgery for one manifestation of the disease may be complicated by the presence of an undiagnosed phaeochromocytoma. In this situation, pharmacological control of phaeochromocytoma should get more priority and surgery may have to be carried out in two stages

🎨Spinal anaesthesia may be dangerous in the presence of an undiagnosed cerebral or spinal tumour. Another point is, spinal cord haemangioblastomas can occur at more than one level. An MRI if already done, can help us to take a proper decision.

🎨Pregnancy may worsen the disease, by increasing the vascularity of tumours. Urgent and life saving neurosurgical intervention may become necessary: for e.g. when a spinal tumor bleeds, when a tumor obstructs CSF flow and causes acute hydrocephalus. Sometimes elective procedures like removal of a phaeochromocytoma which became evident during pregnancy has to be removed. It may become necessary to carry out these procedures during pregnancy or along with a Caesarean section.

🎨Surgery may be required for more than one lesion at the same time.

🎨So careful assessment should be made for lesions other than the one for which anaesthesia is required, and in particular for any symptoms and signs of cerebral, cerebellar or spinal cord tumours and phaeochromocytoma.

🎨In the situation in which two lesions are present, decisions may have to be made as to whether to operate simultaneously or separately . During pregnancy the management of the delivery must be carefully planned in advance.

🎨Although 24-h urinary screening for catecholamines can be performed, plasma normetanephrines and metanephrines are the most sensitive tests for detecting phaeochromocytomas in patients with family predisposition

Reference: Anaesthetic management of a patient with von Hippel–Lindau disease: a combination of bilateral phaeochromocytoma and spinal cord haemangioblastoma. European Journal of Anaesthesiology 13: 81–3. , Anesthesia Databook, 3rd edition 

NICE guidelines for treatment of hypertension: really NICE❗️

💓NICE published guidelines in 2006, revised in 2011. 

💓)))) Step 1 : Choose either an ACE inhibitor, a thiazide diuretic or a calcium channel antagonist (A, D and C). 

💓An ACE inhibitor is more effective as first-line therapy in younger patients (< 55 years ) and Caucasians. 

💓Diuretics or calcium channel blockers are better in older patients and African / Caribbean patients of any age. 

💓 This trial of step 1 is run on for 4 weeks, and if blood pressure is not controlled, the opposite agent is added in:

💓 )))) Step 2 : An ACE inhibitor is added to a diuretic (A + D) or calcium channel antagonist (A + C), or vice versa. 

💓 )))) Step 3 : Ongoing poor control is then managed by the addition of the third agent (A + C + D). 

💓 )))) Step 4 : If a patient is established on triple therapy, and still not well controlled, they are probably aldosterone sensitive, so spironalactone would be a wise option.

National Institute for Health and Clinical Excellence. Hypertension: Clinical Management of Primary Hypertension in Adults. NICE Clinical Guideline 127, August 2011. 

#Hypertension , #NICEguidelines , #antihypertensives

HYPERTONIC SALINE VS MANNITOL FOR MANAGEMENT OF ACUTELY RAISED ICP

🔳 Like mannitol, hypertonic saline

🔻create an oncotic pressure difference that mobilises water across the blood–brain barrier through osmosis. 

🔻shrinks erythrocytes and makes them more deformable 

🔻increases the calibre of the microcirculation by dehydrating vascular endothelium

🔳 Advantages of Hypertonic Saline over mannitol:

🔻less haemodynamic instability

🔻less damage to the kidneys 

🔻easier therapeutic monitoring through tracking serum sodium concentration. 

🔻less inclined to produce the late rise in ICP sometimes found with mannitol when it is deposited in the brain parenchyma.

🔳 But no difference in survival between the two agents has been demonstrated.

LEARN THE CONCEPT OF CRITICAL & PSEUDOCRITICAL TEMPERATURE WITH THE EXAMPLE OF ENTONOX

▪️Entonox is Nitrous oxide mixed 50:50 with oxygen 

▪️It provides analgesia with maintenance of consciousness. 

▪️Usually administered via a demand valve for self administration. 

▪️Takes 30 seconds to act and continues for approx. 60 sec after inhalation has stopped 

▪️ For optimum effect inhalation should start when the contraction tightens. This will co-ordinate the maximal effect with the central painful part of the contraction. 

▪️20% N20 is equivalent to 15 mg of subcutaneous morphine. 

▪️ The optimal analgesic concentration was found to be 70% but some mothers lost consciousness at this concentration 

▪️ 50% N20 in oxygen is safer and this has become standard now

▪️ Entonox is the BOC trade name for this gas mixture.

▪️Poynting effect

The Poynting effect involves the dissolution of gaseous O2 when bubbled through liquid N2O, with vaporisation of the liquid to form a gaseous O2/N2O mixture.

▪️Critical & Pseudocritical temperature

🔹The critical temperature of a gas is the maximum temperature at which compression can cause liquefaction. Or it is the temperature above which a substance cannot be liquefied however much pressure is applied. Mixing gases may change their critical temperature.

🔹The pseudocritical temperature applies to a mixture of gases, such as Entonox, and is the temperature at which gas mixtures separate into their component parts. 

▪️The Poynting effect produces a 50:50 mixture which reduces the crtical temperature of N20 so Entonox has a pseudocritical temperature of -6 degree.

▪️Entonox 

 Highest -5.5°C @117 bar

 Cylinder -7°C @137 bar

 Pipeline -30°C @4 bar

▪️In cylinders it is supplied at a pressure of 137 bar and must be stored above its pseudocritical temperature of -6°C.  

▪️ Below this temperature the N2O liquefies in a process called lamination.  If this occurs a high concentration of O2 will be delivered first with little analgesic effect, but as the cylinder empties the mixture will become progressively more potent and hypoxic as it approaches 100% N2O. 

▪️If a cylinder has been exposed to cold below -6 degree C it should be warmed for 5 minutes in a 37 degree C water bath or for 2 hours in a room at 15 degree C. It should then be inverted three times before use.

▪️ When delivered via a pipeline at 4.1 bar the pseudocritical temperature is less than -30°C.

▪️Altitude per se has no effect on Entonox.

Reference: www.frca.uk

VEIN OF GALEN MALFORMATION: ANESTHESIA CONCERNS

▶️Vein Of Galen Malformation (VOGM) is a dilatation of the median vein of the procencephalon (which is a precursor of the Vein of Galen), caused by an arterio venous (A-V) shunt from the choroidal arteries of the anterior and posterior circulation. It can cause  A-V shunting and related systemic effects and hydrocephalus. 

▶️Incidence is less than 1% of cerebral vascular malformations.

▶️PROBLEMS:

✔️Most of the issues are due to the effect of A-V shunting

✔️Congestive Cardiac Failure and Pulmonary Hypertension : Due to A-V shunting . The etiology of cardiac failure may begin in utero. High output cardiac failure is associated with VGAM because 60 to 80% of aortic blood flow is directed through the VGAMS low resistant shunt. Blood flow travels through the VGAM during diastole creating a steal phenomenon. The reduction of diastolic pressure can lead to myocardial ischemia secondary to reduced coronary blood flow. 

 ✔️Ventriculomegaly : A-V shunting --> increased dural sinus pressure --> resistance to CSF entry to the sinus --> enlargement of ventricles

✔️Seizures

▶️INDICATIONS FOR DEFINITIVE MANAGEMENT: CCF, Hydrocephalus , Neurological symptoms

▶️TREATMENT

✔️Main goal of the treatment is to control the A-V shunt

✔️Endovascular embolization is the preferred treatment; other option is surgical clipping.

✔️Embolization of the feeding arteries and draining veins can result in reduction of blood flow through the VOGM which is the key to improved cardiac function and brain injury prevention.

✔️Embolization is less invasive and has a higher survival rate than open neurosurgical procedures. It provides better hemodynamic stability with minimal pain. Heart failure is an indication for urgent embolization. 

▶️ANESTHESIA CONCERNS

✔️Improving the cardiac function is the key to avoid multiple organ failure. The mortality rate for all neonates in heart failure undergoing transcatheter embolization can be up to 50%, mortality is much higher when pulmonary hypertension is present.

✔️Cardiac failure is difficult to treat and most beta agonists along with diuretics and Milrinone have been used with varying success. Although total systemic vascular resistance is reduced through the VOGM, the child in cardiac failure has an increased extracranial vascular resistance. So interventions that reduces extracranial systemic resistance is likely to improve systemic perfusion. Nitric oxide has been used to treat pulmonary hypertension in these patients.

✔️The goal is to achieve partial embolization with the endpoint of improving heart failure. So, the embolization may be staged. Each session is limited by the volume of contrast media delivered and the patient’s tolerance of the procedure. The ultimate goal of these sessions is to completely occlude the VOGM while avoiding neurological and cardiac injury

✔️Intracerebral hemorrhage due to venous hypertension is a potentially fatal complication of endovascular management. There is a thought that this complication can be avoided by staging the embolization procedure. Perforation of the venous sac can usually be managed by reversal of anticoagulation and continuation of coil embolization. Ischemic neurological deficits, Pulmonary embolization with embolic agents ( due to the high flow across the intracranial shunt that drains immediately into the central venous system) are other concerns. 

Reference: 

Anesthetic Implications of Neonatal Vein of Galen Aneurysmal Malformation (VGAM)  Ira S Landsman, Than Nguyen 

 

🗃A FEW POINTS WHICH CAN MAKE AN ANESTHESIOLOGIST's LIFE EASIER🗃

✔️CONVERSION OF SI UNITS TO NON SI UNITS

🔀 kilo Pascal to mm of  Hg ➡️multiply with 7.5

🔀 kilo Pascal to cm of H2O ➡️multiply with 10.2

🔀 blood sugar from mmol/l to mg/dl ➡️multiply with 18 

#SIunits , #nonSIunits

✔️AN EASIER FORMULA FOR CALCULATING 'IDEAL BODY WEIGHT'(IBW)

The Lemmens formula

Ideal weight (in kilograms) = 22 x (Height in meters)x2

✔️FORMULA FOR CALCULATING PLASMA OSMOLALITY (If using SI UNITS)

[2x (Na)] + [BUN] + [glucose]

✔️FORMULA FOR CALCULATING PLASMA OSMOLALITY (If using NON SI UNITS)

[2x (Na)] + [BUN / 2.8] + [glucose / 18]