Question

1a). A 70 kg male is given an injection of 10 mg of morphine the peak plasma concentration of morphine immediately after the injection is 4.5 mcg / dL. What is the volume of distribution of morphine? Express your answer in L/kg.

1b). 70 kg male takes two 325 mg aspirin tablets, given that the bioavailability of aspirin is 68% and volume of distribution of aspirin is 0.15 L/kg, what would the plasma concentration of aspirin 10 minutes after the dose? For the purpose of this calculation, assume that all of the available dose would be absorbed by 10 min and that clearance and metabolism would be negligible by 10 min. Express your answer in mg/L.

Answer 1

1.a) Given,

Weight of the male = 70 kg

Amount of morphine injected = 10 mg

=10×1000 mcg

= 10000 mcg

Peak plasma concentration of morphine = 4.5 mcg/dl

Using unitary method:-

4.5 mcg morphine present in 1 dl

1 mcg morphine present in = 1/4.5 dl

10000 mcg morphine present in = (1/4.5) × 10000 dl

=2222.22 dl

= 222.22 L [ as 1 dl = 0.1 L so, 2222.22 dl = 0.1× 2222.22 L = 222.22 L]

Therefore, volume of distributed morphine (L/kg) = volume of 10 mg morphine/ weight of male

=222.22 L/70 kg

= 3.17 L/kg

1.b) Given,

Weight of the male = 70 kg

Male takes two 325 mg aspirin tablet.

Amount of tablet he took = 325×2 mg = 650 mg

given, Bioavailability of aspirin in tablet = 68%

So, amount of aspirin present in tablet = 68% of 650 mg

=0.68×650 mg

=442 mg

Volume of distribution of aspirin = 0.15 L/kg

For 70 kg male total volume of distribution of aspirin= 0.15 L/Kg × 70 Kg

= 10.5 L

Therefore, plasma concentration of aspirin 10 min after the dose= amount of aspirin present in tablet/ total distributed volume of aspirin

=442 mg/10.5 L

=42.1 mg/L