Tonicic equivalent

Method 3 If no iso-osmotic concentration is known, it can be calculated with the aid of the sodium chloride equivalent, also known as tonicic equivalent or E-value. The sodium chloride equivalent is defined as ... 

Step 4 From Step 3, 0.231 g of sodium chloride is required to make the preparation isotonic. But the prescription calls for boric acid as the tonicity agent. Boric acid has an E value of 0.52. This means that 1% boric acid is osmotically equivalent to 0.52% NaCl or 1 g of boric acid is equivalent to 0.52 g of NaCl. 

The osmolality of tears is of prime importance, since optical integrity of the cornea is significantly influenced by the tonicity of the tears. The normal osmolality of tears varies from 290 to 310 mOsmkg-1, which is almost equivalent to that of normal saline solution. Variations in osmotic pressure between 100-640 mOsmkg 1 appear to be well tolerated by the eye beyond these values irritation takes place, eliciting reflex tears and reflex blinking. 

The combination of active drug, preservative, and vehicle usually results in a hypotonic formulation (< 290 mOsm). Simple or complex salts, buffering agents, or certain sugars are often added to adjust osmolarity of the solution to the desired value. An osmolarity of 290 mOsm is equivalent to 0.9% saline, and this is the value sought for most ophthalmic and intravenous medications.The ocular tear film has a wide tolerance for variation in osmotic pressure. However, increasing tonicity above that of the tears causes immediate dilution by osmotic water movement from the eyelids and eye. Hypotonic solutions are sometimes used to treat dry eye conditions and to reduce tear osmolarity from abnormally high values. 

Marked deviation of pH and tonicity from that of tears provokes reflex lacrimation, diluting the drug and accelerating its elution from the eye. H) ertonic solutions may be further diluted by fluid drawn from subconjunctival tissues. Animals will tolerate a pH range of 3.5-10.5 and tonicity from 0.5% to 2% sodium chloride concentration equivalents (Miller 1992). 

Ophthalmic products instilled into the eye may be tolerated over a fairly wide range of tonicity (0.5-1.5 percent NaCl equivalents Hind and Goyan 1950). However, to minimise irritation and discomfort, ophthalmic solutions should ideally be isotonic with the tears, equivalent to 0.9 percent w/v solution of sodium chloride. 

Blood plasma has an osmolarity equivalent to a 0.30 M glucose solution or a 0.15 M NaCl solution. This latter is true because in solution NaCl dissociates into Na+ and Cl and thus contributes twice the number of solute particles as a molecule that does not ionize. If red blood cells, which have an osmolarity equal to blood plasma, are placed in a 0.30 M glucose solution, no net osmosis will occur because the osmolarity and water concentration inside the red blood cell are equal to those of the 0.30 M glucose solution. The solutions inside and outside the red blood cell are said to be isotonic iso means "same," and tonic means "strength") solutions. Because the osmolarity is the same inside and outside, the red blood cell will remain the same size (Figure 18.20b). 

Epilepsy Phenytoin is effective against partial and tonic-clonic but not absence seizures. Phenytoin preparations differ significantly in bioavailabdity and rate of absorption. In general, patients should consistently be treated with the same drug from a single manufacturer. However, if it becomes necessary to temporarily switch between products, care should be taken to select a therapeutically equivalent product and patients should be monitored for loss of seizure control or onset of new toxicides. 

THERAPEUTIC USE Topiramate is equivalent to valproic acid and carbamazepine in children and adults with newly diagnosed partial and primary generalized epilepsy the drug is also effective as monotherapy for refractory partial epilepsy and refractory generalized tonic-clonic seizures. Topiramate also is more effective than placebo against both drop attacks and tonic-clonic seizures in patients with Lennox-Gastaut syndrome. 

---