When a drug is given via intravenous infusion, the plasma concentration rises exponentially as a wash-in curve until it reaches steady-state concentration (the point at which the infusion rate is balanced by the elimination rate or clearance). To reach this steady state, the drug will take 4-5 half-lives.

Cpss (target plasma concentration at steady state) and clearance (CL) in ml/minute or litre/hour are the two factors that determine the infusion rate or dose (ID) in mg/hour of a drug.

ID = Cpss × CL

We know the infusion rate is 20 ml/hour in this case. The drug’s concentration is 5 mg/mL. The patient is receiving 100 mg of the drug per hour, with a 20 L/hour clearance rate.

ID = Cpss × 20

Therefore,

Cpss = 100 mg/20000 ml

Cpss = 0.005 mg/mL or 5 mcg/mL

The action of class 1 anti-arrhythmic is sodium channel blockade. Subclasses of this action reflect effects on the action potential duration (APD) and the kinetics of sodium channel blockade.

Drugs with class 1A prolong the APD and refractory period, and dissociate from the channel with intermediate kinetics.

Drugs with class 1B action shorten the APD in some tissues of the heart, shorten the refractory period, and dissociate from the channel with rapid kinetics.

Drugs with class 1C action have minimal effects on the APD and the refractory period, and dissociate from the channel with slow kinetics.

The equation for calculating vaporiser output is:

Vaporiser output (VO) mL = Carrier gas flow (mL/minute) × SVP of agent (kPa)
Ambient pressure (kPa) − SVP of agent (kPa)

The saturated vapour pressure of sevoflurane at 1 atm (100 kPa) and 20°C is 21 kPa.

VO = (100 mL × 21 kPa)/(100 kPa − 21kPa) for sevoflurane,
VO = 26.6 mL

26.6 mL of 100% sevoflurane and 100 mL bypass carrier gas is being added to the fresh gas flow per minute.

2660 mL of 1% sevoflurane and 100 mL bypass carrier gas is approximately 2.7 L/minute.

The measure of drug potency or therapeutic response is the ED95. This is defined as the dose of a neuromuscular blocking drug required to produce a 95% depression of muscle twitch height. The ED50 and ED90 describe a depression of twitch height by 50% and 90% respectively.

The ED95 (mg/kg) of the commonly used neuromuscular blocking agents are:

suxamethonium: 0.27
rocuronium: 0.31
vecuronium: 0.04
pancuronium: 0.07
cisatracurium: 0.04
mivacurium: 0.08

Insulin is secreted from the ? cells of the pancreas. It consists of 51 amino acids arranged in two chains. It interacts with cell surface receptors (not the nuclear receptors and thus mechanism of action is not similar to steroids).
Since insulin can pass from plasma to interstitium and lymphatics, it can be measured in lymph but the concentrations here can be up to 30% less than that of plasma.

It decreases blood glucose by stimulating the entry of glucose in muscle and fat (by increasing the synthesis of Glucose transporters)

Salmeterol is a long-acting Beta 2 selective agonist. Therefore it is only used for prophylaxis whereas salbutamol is a short-acting Beta 2 agonist and is thus used for the treatment of acute attacks of asthma.

Salmeterol is 15 times more potent than salbutamol at the Beta 2 receptor but 4 times less potent at the Beta 1 receptor.

Tachyphylaxis to the unwanted side effects commonly occurs, but not to bronchodilation.

Oxytocin is structurally similar to Antidiuretic Hormone (ADH) and thus oxytocin can cause water intoxication (due to an ADH like action)

Oxytocin is secreted by the posterior pituitary along with ADH. It increases uterine contractions – the contraction of the upper segment (fundus and body) of the uterus whereas the lower segment is relaxed facilitating the expulsion of the foetus

Antidiuretic hormone (ADH) also called vasopressin is released from the posterior pituitary in response to hypertonicity and increases fluid reabsorption from the kidney.

Caudal analgesia using bupivacaine is a widely employed technique for achieving both intraoperative and early postoperative pain relief. 0.5 ml/kg of 0.25% plain bupivacaine is favoured by many practitioners who employ this fixed scheme for procedures involving sacral dermatomes (circumcision, hypospadias repair) as well as lower thoracic dermatomes (orchidopexy). However, there are other dosing regimens for caudal blocks with variable analgesic success rates: These include 0.75 ml/kg, 1.0 ml/kg and 1.25 ml/kg.

A study indicated that plain bupivacaine 0.25% at a dose of 0.75 ml/kg compared to a dose of 0.5 ml/kg when administered for herniotomies provided improved quality of caudal analgesia with a low side effects profile. There were consistently more patients with favourable objective pain scale (OPS) scores at all timelines, increased the time to the analgesic request with similar postoperative consumption of paracetamol in the group of patients who received 0.75 ml/kg of 0.25% bupivacaine.

Lorazepam is an intermediate-acting benzodiazepine that binds to the GABA-A receptor subunit to increase the frequency of chloride channel opening and cause membrane hyperpolarization.

Lorazepam has emerged as the preferred benzodiazepine for acute management of status epilepticus. Lorazepam differs from diazepam in two important respects. It is less lipid-soluble than diazepam, with a distribution half-life of two to three hours versus 15 minutes for diazepam. Therefore, it should have a longer duration of clinical effect. Lorazepam also binds the GABAergic receptor more tightly than diazepam, resulting in a longer duration of action. The anticonvulsant effects of lorazepam last six to 12 hours, and the typical dose ranges from 4 to 8 mg. This agent also has a broad spectrum of efficacy, terminating seizures in 75-80% of cases. Its adverse effects are identical to those of diazepam. Thus, lorazepam also is an effective choice for acute seizure management, with the added possibility of a longer duration of action than diazepam.

Phenobarbitone is a long-acting barbiturate that binds to GABA-A receptor site and increase the duration of chloride channel opening. It also blocks glutamic acid neurotransmission, and, at high doses, can block sodium channels. It is considered as the drug of choice for seizures in infants.

Phenytoin is an anti-seizure drug that blocks voltage-gated sodium channels. It is preferred in prolonged therapy of status epilepticus because it is less sedating.

In cases wherein airway protection is required, thiopentone and propofol are the preferred drugs.

Drugs cross the placenta by Simple, Ion channel and Facilitated diffusion; Exocytosis and Endocytosis, Osmosis, and Active transport (primary and secondary)

The following factors influence rate of diffusion across the placenta:

Protein binding
Degree of ionisation
Placental blood flow
Maternal and foetal blood pH
Materno-foetal concentration gradient.
Thickness of placental membrane
Molecular weight of drug <600 Daltons cross by diffusion
Lipid solubility (lipid soluble molecules readily diffuse across the placenta)

Rocuronium has a F/M ratios of 0.16, a 30% plasma protein binding, low lipid solubility, a low volume of distribution (0.25L/kg), and a high molecular weight (530Da).