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1Formulas and methods
Maternity & obstetric nursing
Method
Place the woman in time: weeks of pregnancy, stage of labour or day after birth, and decide what is normal at that point.
Look for the patient in danger: the mother, the fetus or the newborn, or more than one of them.
Link the finding to its cause: VEAL CHOP for decelerations, the four Ts for bleeding, BUBBLE-HE after birth.
Choose the action that improves blood flow and oxygen first (side-lying, stopping oxytocin, fundal massage), then escalate.
Dosage calculations
Dose from strength
volume = dose / strength
Weight-based dose
dose = mg/kg × body weight
Tablets
number = dose / tablet strength
Units
1 g = 1000 mg, 1 mg = 1000 mcg
Method
Convert the ordered dose and the stock strength to the same unit.
Use desired over have times quantity: (D/H) × Q.
For weight-based doses, multiply by the weight in kg first, then divide the daily dose by the number of doses.
Estimate first and check that the answer is reasonable.
IV infusion rates
Pump rate
mL/h = volume (mL)/time (h)
Drip rate
drops/min = volume (mL) × drop factor / time (min)
Infusion time
time (h) = volume (mL)/rate (mL/h)
Method
Pump rate in mL/h: volume divided by time in hours.
Drip rate in drops/min: volume × drop factor divided by time in minutes.
For time remaining: volume left divided by the rate.
Round as the question or the equipment requires.
2Practice problems
1.Maternity & obstetric nursingMedium
At 30 weeks, which fundal height is consistent with expected growth?
A29 cm
B18 cm
C36 cm
D22 cm
2.Maternity & obstetric nursingHard
Four days after birth, a mother says she hears voices telling her that her baby is evil. What is the priority?
AReassure her that this is the baby blues and will pass
BAsk the partner to take the baby home overnight and leave her to rest alone in a quiet room
CMake sure mother and baby are not left alone and arrange urgent psychiatric assessment
DAdvise her to get more sleep and review in a week
3.Dosage calculationsMedium
A patient weighs 80 kg. The prescribed dose is 10 mg/kg per day, given in 3 equal doses. How many milligrams D are given per dose?
80 kg10 mg/kg/day3× per day
D = mg
4.Dosage calculationsHard
The prescription is 330 micrograms. The ampoule contains 1 mg/mL. Draw up the dose in the 1 mL syringe: what volume V do you give?
330 mcg1 mg/mL
V = mL
5.IV infusion ratesMedium
A drip runs at 35 gtt/min through a set of 20 gtt/mL. How many millilitres V does the patient receive in 6 hours?
35 gtt/min20 gtt/mL6 h
V = mL
6.IV infusion ratesHard
A drip runs at 30 gtt/min through a set of 20 gtt/mL. There are 200 mL left in the bag. How many hours t until it is empty? Round to one decimal place.
30 gtt/min20 gtt/mL200 mL
t = h
3Solutions
1.Maternity & obstetric nursing
Answer: A · 29 cm
Why this is the answer
The expected fundal height at 30 weeks is about 28–32 cm. A difference of more than 2–3 cm from the gestational age should be followed up, usually with an ultrasound.
Answer
A · 29 cm
2.Maternity & obstetric nursing
Answer: C · Make sure mother and baby are not left alone and arrange urgent psychiatric assessment
Why this is the answer
Hallucinations soon after birth suggest postpartum psychosis, which carries a risk to both mother and baby. Ensure constant safe supervision and urgent specialist assessment.
Answer
C · Make sure mother and baby are not left alone and arrange urgent psychiatric assessment
3.Dosage calculations
Answer: D = 266.7 mg
Dose calculation: the amount to give equals what is prescribed divided by what you have, times the quantity it comes in: amount = (desired dose/stock strength) × stock volume. Always keep the units the same (convert g to mg, mg to mcg first) and check that the answer is sensible.
Total per day
10·80 = 800 mg
Per dose
D = 800/3 = 266.7 mg
Answer
D = 266.7 mg
4.Dosage calculations
Answer: V = 0.33 mL
Drawing up a dose: volume = (prescribed dose/stock strength) × stock volume. Convert to the same units first, then check that the volume fits the syringe and lands on one of its graduations.
Same units
1 mg/mL = 1000 mcg/mL
Volume
V = 330/1000 = 0.33 mL
Answer
V = 0.33 mL
5.IV infusion rates
Answer: V = 630 mL
Infusion rates: a pump is set in mL/h = volume/time in hours. A gravity drip is counted in drops per minute: gtt/min = volume (mL) × drop factor (gtt/mL)/time (min). The drop factor is printed on the tubing package: macrodrip sets deliver 10, 15 or 20 gtt/mL (blood sets often 10 gtt/mL in the US, 15 in the UK) and microdrip sets 60 gtt/mL. Always use the drop factor the question or package gives.
Rate in mL/h
35·60/20 = 105 mL/h
Volume
V = 105·6 = 630 mL
Answer
V = 630 mL
6.IV infusion rates
Answer: t = 2.2 h
Infusion rates: a pump is set in mL/h = volume/time in hours. A gravity drip is counted in drops per minute: gtt/min = volume (mL) × drop factor (gtt/mL)/time (min). The drop factor is printed on the tubing package: macrodrip sets deliver 10, 15 or 20 gtt/mL (blood sets often 10 gtt/mL in the US, 15 in the UK) and microdrip sets 60 gtt/mL. Always use the drop factor the question or package gives.