CARDIOVASCULAR CHANGES :(2,4)
The pregnancy induced changes in maternal cardiovascu
lar system begin in early pregnancy are extensive by the end of the first trimester increasing until term,diminish after delivery until 6 weeks postpartum the mother has returned to her pre pregnant cardiovascular state.
lar system begin in early pregnancy are extensive by the end of the first trimester increasing until term,diminish after delivery until 6 weeks postpartum the mother has returned to her pre pregnant cardiovascular state.
The changes are :
1.Increase in cardiac output(CO).
2.reduced peripheral resistance.
3.increased blood volume.
Ad.1 The CO increases by 40% by the end of the first
trimester is achieved by 30% in stroke volume and
smaller increase in heart rate(HR).
trimester is achieved by 30% in stroke volume and
smaller increase in heart rate(HR).
Ad.2. Peripheral resistance falls early in pregnancy and
is reflected by a lowering of BP in the second
trimesters.
is reflected by a lowering of BP in the second
trimesters.
Ad.3. Circulating blood volume increases in the first
trimester continues to increase until term it is
1250 ml above non pregnant levels is a result of
the expansion of the plasma volume by 45% and
smaller increase the redcell by 20%.
This cause a haemodilution with hemoglobin
concentration around 11g/100 ml and hematocriet
(Ht)of 33% of normal. The blood volume return to
normal about 14 days post partum.
trimester continues to increase until term it is
1250 ml above non pregnant levels is a result of
the expansion of the plasma volume by 45% and
smaller increase the redcell by 20%.
This cause a haemodilution with hemoglobin
concentration around 11g/100 ml and hematocriet
(Ht)of 33% of normal. The blood volume return to
normal about 14 days post partum.
Plasma protein concentration is reduced giving
lower osmotic pressure.The absolute quantity of
plasma albumin is decreased.
The globulin fraction is increased.
lower osmotic pressure.The absolute quantity of
plasma albumin is decreased.
The globulin fraction is increased.
The purpose of the increased plasma volume and CO is three fold :
1.An increase uterine blood flow(UBF) which at term is
600-800 ml/min,the placental flow wholly dependent on
maternal BP.
2.An increase in renal blood flow(RBF) by 400-500ml/min
by the end of the first trimester,which required to
eleminate extra waste products produced by the very
rapid fetal growth.
by the end of the first trimester,which required to
eleminate extra waste products produced by the very
rapid fetal growth.
3.An increase in skin blood flow because peripheral
vasodilatation is necessary to eleminate heat produced
by rapid growing fetus.
vasodilatation is necessary to eleminate heat produced
by rapid growing fetus.
Anesthetic consideration :
The mother has her cardiac stress increased to the extent that output in the first trimeter is half the maximum of 12 litres which she can achieve with vigorous exercise,she
has therefore a lot less cardiac reserve.This is especially important in those with cardiac disease,the older mother
the obese and those with multiple pregnancies.
has therefore a lot less cardiac reserve.This is especially important in those with cardiac disease,the older mother
the obese and those with multiple pregnancies.
Drugs with negative inotropic and chronotropic effects must be used with care and titrated where possible.
Hypotensive side effect must be minimized as the placen
tal flow is pressure dependent.
Drugs withknown cardiovascular effects like ergometrine should where possible be avoided as they increase cardiac work.
The supine hypotensive syndrome can occur from the 16th week of pregnancy or in multiple pregnancy from the 12th week;cause a decrease venour return to the right heart and a reduction in CO. While aortic compression reduced UBF,utero placental perfusion and renal blood flow (RBF).
UBF is not autoregulated,it varies directly with systemic maternalBP and is inversely proportional to uterine vascular resistance.
Maternal SBP < 100 mmHg in the previously normotensive or a 20% drop in the hypertensive for more than 10 minutes can result in fetal asphyxia, bradycardia and acidosis.
Maternal SBP < 100 mmHg in the previously normotensive or a 20% drop in the hypertensive for more than 10 minutes can result in fetal asphyxia, bradycardia and acidosis.
Awareness of this problem should ensure that all pregnant woman from the second trimester onwards must be tilted laterally 20 degrees during surgery or while being transported on a trolley Maternal BP and fetal HR should be continuously monitored.
RESPIRATORY CHANGES:(1,4)
These changes are established by the end of the first trimester they include :
1.Increased tidal volume
2.Fall in PaCO2
3.Increased O2 consumption
Minute ventilation rises 7,5-10,5 litres in the late pregnancy. Functional residul capacity(FRC) is decreased.
PaCO2 of 30 mmHg is present from early in pregnancy until delivery.This reduction of PaCO2 which causes dyspnoe and giddiness in early pregnancy may be progesterone effect.
Oxygen consumption increases from an avarage of 189 ml/min in the non to 234ml/min in late pregnancy
(Fishburne 1979).
Anesthetic consideration :
(Fishburne 1979).
Anesthetic consideration :
It is essential to prevent wide swings in maternal PCO2 for fetal wellbeing.Premedicant and post operative drugs that depress ventilation must be used with caution as the fefus shows greater sensitivity to CO2 than adult.
Excessive hypocarbia with hyperventilation must be avoided may reduce placental perfusion by vasocons
triction in the same way as maternal cerebral perfusion is decreased, PaCO2 should therefore be maintained at about 30 mmHg.
triction in the same way as maternal cerebral perfusion is decreased, PaCO2 should therefore be maintained at about 30 mmHg.
Also it decreases maternal cardiac output and raised pH thus reducing the oxygen releasing capacity of maternal Hb therefore reducing fetal oxygen supply.
Maintaining the slightly raised PO2 must be borne in mind.
Hypoxia during intubation as well as hypotenion which increases ventilation perfusion abnormalities must be avoided. Decreased FRC allows a more rapid rise in partial pressures of inhalational agents and a more rapid uptake of volatile agents.
The nasal mucous membrane is congested throughout pregnancy possibly to humidify the increased tidal volume. Excessive bleeding can be caused when the passage nasal tubes to the stomach or trachea are attempted.
Pharmacokinetic changes:(4)
During pregnancy the absorbtion,distribution,metabolism and excretions of drugs are altered largely due to the increased plasma the fall in absolute quantity of plasma albumin,the possible increase in liver metabolism and greatly increased glomerular filtration rate(GFR).
This result in :
1.a lower plasma concentration because because of the
large dilutional volume.
large dilutional volume.
2.less albumin is available to bind drug to the plasma
protein depot and thus with repeated doses the free
drug concentration rises rapidly once the albumin stores
are taken up.
3.prolonged half life of drugs because of longer time taken
to clear greater volume of plasma.
4.increased liver metabolism may occur and possibly some
alteration in the metabolism.
5.better elimination of metabolized drugs because of
increased GFR.
6.pseudocholine esterase activity is decreased by 30% but
this seems not to prolong the activity of succinylcholine.
Changes in conduction anesthesia(4)
By the end of first trimester some degree of obstruction to venous return even when the mother is not supine;
hence
1.Dilatation of venous collateral in paravertebral venous
plexus which result in a smaller epidural space.
There is an increased risk of epidural venous puncture
with greater hazards of intravenous local anesthetic
solution resulting from a bolus of drug going via the
azygos vein to the heart.
hence
1.Dilatation of venous collateral in paravertebral venous
plexus which result in a smaller epidural space.
There is an increased risk of epidural venous puncture
with greater hazards of intravenous local anesthetic
solution resulting from a bolus of drug going via the
azygos vein to the heart.
2.Increased sympathetic tone of the lower limbs is
common in pregnancy.Usually the pregnant woman
complains of cold feet and has hot vasodilated upper
limbs.When sympathetic block is initiated profound
hypotension occurs.
3.Local anesthetic drugs like buvipacaine are strongly
protein bound to plasma glycoprotein which rises in
pregnancy in maternal leaving less free drugs available
to cause toxic effects.
To be continued
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