PHYSIOLOGICAL CHANGES IN

PREGNANCY AND

UTEROPLACENTAL BLOOD FLOW

SPEAKER : Dr OMAR KAMAL

MATERNAL PHYSIOLOGICAL CHANGES…

result of hormonal alterations,

mechanical effects of the gravid uterus,

increased metabolic and oxygen

requirements,

metabolic demands of the fetoplacental unit,

hemodynamic alterations associated with

the placental circulation

CARDIO VASCULAR

increase in plasma volume as well as in

red cell and white cell volumes The

plasma volume increases by 40% to

50%, whereas the red cell volume goes

up by only 15% to 20%.. Physiological

pregnancy of anaemia..

mother's body compensates for it by

increased cardiac output, increased

PaO2, and a rightward shift in the

oxyhemoglobin dissociation curve.

Two current hypothesis

(1) caused by initial vasodilation, which

stimulates hormones such as renin,

angiotensin, and aldosterone or

(2) characterized by an early increase in

sodium retention (due to an increase in

mineralcorticoids) that retains fluid, causing

an increase in blood volume

Clinical implications :

enlarging uterus ,needs of the fetoplacentalunit,

they become hypercoaguable as the gestation progresses.( 1,7,8 9,10,12 and fibrinogen)

it protects the parturient from the bleeding at the time of delivery

8 weeks post delivery for blood volume to return normal

CARDIO VASCULAR

Cardiac output starts increasing from 5th week,

increases by 30% to 40% , and the max increase

is attained around 24 weeks’ gestation.

Heart rate increases by 20 to 30 %

CO increases further during labor and may show

values 50% higher than prelabor values.

In the immediate postpartum period, CO

increases maximally and can rise 80% above

prelabor values

The increase in stroke volume as well as in heart

rate maintains the increased CO..

Heart rate

Stroke volume

Cariac output

Cvp

Pcwp

Svr

Pvr

Sys blood pressure

Pulmonary art

pressure

20-30 % increase

20-50 increase

30-50 increase

Unchanged

Unchanged

20% decrease

30% decrease

Slight decrease

Slight decrease

CO, HR, and stroke volume decrease to pre-

labor values 24 to 72 hours postpartum and

return to nonpregnant levels within 6 to 8

weeks after delivery

DBP drops by 15 mm Hg, decrease because

of an associated decrease in SVR..

estradiol-17b and progesterone hormones

resonsible , Prostacyclin, nitric oxide for

vascular changes..

ANESTHETIC IMPLICATIONS…

when the parturient lies supine, chance of

aorto caval compression..

Symp : maternal tachycardia, arterial

hypotension, faintness, and pallor

Left uterine displacement maintained

Hyperdynamic state of pregnancy in patients with heart disease and low myocardial reserve, there is increase in myocardial work which may precipitate pulmonary congestion…

adequate pain relief must be given by continuous epidural analgesia…

Healthy gravida wil tolerate upto 1500 ml blood loss as ther is hemodilution and increased volume..

Because of engorgement of epidural veins, accidental intra vascular injection is common..

Cephalad spread of LA drug during regional anesthesia is more in pregnant patients due to

1) Decreased volume of csf

2) Decreased protein concn reduces protein binding

3) Increased neuro sensitivity to LA..

Hence dose requirement reduces upto 25 to 40 %..

Maternal BP should be maintained and should not go less than 20 % of the preoperative values during regional block..

Ephedrine is the preferred drug over peripheral vaso constricors in spinal hypotension

Effect of Pregnancy on Cardiovascular

InvestigationsInvestigation Findings

Chest radiography

Apparent cardiomegaly

Enlarged left atrium (lateral views)

Increased vascular markings

Straightening of left-sided heart border

Postpartum pleural effusion

Electrocardiography

Right-axis deviation

Right bundle branch block

ST-segment depression of 1 mm on left

precordial leads

Q waves in lead III

T-wave inversion in leads III, V2, and V3

Echocardiography

Trivial tricuspid regurgitation (up to 43%-93% at

term)

Pulmonary regurgitation (up to 94% at term)

Increased left atrial size by 12%-14%

Increased left ventricle end-diastolic

dimensions by 6%-10%

Inconsistent increase in left ventricle thickness

Mitral regurgitation (28% at term)

Pericardial effusion (40% postpartum

RESPIRATORY CHANGES…

Starts at 4th week of gestation

Clinical implications :

1. A decreased FRC as well as increased oxygen consumption can cause a rapid development of maternal hypoxemia.

2. Decreased FRC, increased MV, as well as a decreased minimal alveolar concentration (MAC) will make parturients more susceptible to inhalational anesthetics

3. Avoid nasal intubation, and smaller ETT should be used for oral intubation Because of the increased edema, vascularity, and friability of the mucous membrane

TV : + 40 %

RR : + 15 %

MV : + 50 %

Alveolar ventilation : +70 %

Airway resistance : - 36 %

Total pulmonary resistance : - 50 %

Total compliance : - 30 %

Dead space }

Fev1 } no change

all parameters return to normal levels within 6 to 12 weeks post partum

VOLUMES AND CAPACITIES… BLOOD GASES

FRC : - 20 %

RV : - 20 %

ERV : - 20 %

ILC : + 5 %

Diffusing capacity : - 5 %

Vital capacity : no change

Closing volume : no change

Paco2 : -10 to – 20 mmhg

Pao2 : + 10 mmhg

Arterial PH : no change

S. bicarb : - 4 meq/l

O2 consumption : + 20 %

ANESTHETIC IMPLICATIONS

AIRWAY MANAGEMENT :

Laryngoscopy becomes difficult due to breast engorgement and weight gain.. Short handle laryngoscope can be used..

Small et should be used and avoid nasal intubation

RESPONSE TO ANAESTHETICS :

MAC of potent inhalational drugs found to be decreased in pregnancy

Decrease in FRC with increase in MV increases the rapidity of induction with inhalational drugs, hence induction n recovery is rapid

Rapid induction with inhalational agents due to increased MV with decreased FRC.

a decreased FRC , with preexisting alterations in closing volume as a result of smoking, obesity, or scoliosis have early airway closure with advancing pregnancy, leading to hypoxemia and impaired organ perfusion.

The resulting decrease in the FRC/CC ratio causes faster small-airway closure when lung volume is reduced;

thus, parturients can desaturate at a much faster rate as compared with nonpregnant women.

The rapid development of hypoxia as a result of decreased FRC, increased oxygen consumption, and airway closure may be minimized by administration of 100% oxygen for 3 to 5 minutes before the induction of

1ST stage of labour : due to pain, paientshyperventilate leading to maternal alkalosis( pco2 18 mmhg), consequentlly causing fetalacidosis due to :

1. decreased uteroplacental perfusion ( hypocarbia causes vasoconstriction)

2. shifting of the maternal oxygen dissociation curve to the left

Effective epidural analgesia alone can diminish maternal hyperventilation markedly

RENAL SYSTEM

GFR and RPF is increased by 50 % during pregnancy by 4th month of gestation

A rise in the filtration rate decreases plasma blood urea nitrogen (BUN) and creatinineconcentrations by about 40% to 50% ..

Tubular reabsorption of sodium is increased. glucose and amino acids not absorbed efficiently; hence glycosuria and aminoaciduria develop in normal gestation

The renal pelvis and ureters are dilated, and peristalsis is decreased.

ANAESTHETIC IMPLICATIONS

Increase in volume of distribution for drugs

Drugs which are renall excreted have to be

given in higher than normal dosages.

GIT..

Gastrointestinal motility, food absorption, and

lower esophageal sphincter pressure are

decreased due to an increased level of plasma

progesterone

Shift in the position of stomach changes the angle

of GE junction

Gastric emptying time is significantly prolonged

during labor and hence gastric volume is

increased

risk of regurgitation on induction of general

anesthesia depends on the gradient between the

LES and intragastric pressures

HEPATIC CHANGES

Hepatic blood flow is unchanged

Abnormal LFT s do not indicate hepatic disease

Total proteins and albumin level decreased.

The albumin–globulin ratio decreases because of the relatively greater reduction in albumin concentration

Significance : the free fractions of protein-bound drugs can be expected to increase

Serum cholinesterase activity is reduced by 25 t0 30 % , but not associated with prolonged neuromuscular blockade as volume of distribution for s . Cholinesterase is high at term

ANESTHETIC IMPLICATIONS

Antacid prophylaxis should be given before

induction

No solid food should be given during labour

Narcotics delay gastric emptying time and

decreases LES tone..

CHANGES IN THE CENTRAL AND PERIPHERAL

NERVOUS SYSTEMS

The MAC is decreased by 25% to 40% with different inhalational anesthetics due to increased progesterone…

A wider dermatomal spread of sensory anesthesiawas observed in parturients following the use of epidural anesthesia

Reduced epidural space volume caused by an engorged epidural venous plexus because of aortocaval compression ..

Maternal b- endorphin blood level increase during gestation proportional to frequency & duration of uterine contraction. Lumbal epidural analgesia blocks it

OTHER SYSTEMS..

MUSCULO : Hormone relaxin is responsible for both the generalized ligamentous relaxation and softening of collagenous tissues.

Hyperpigmentation of certain parts of the body such as the face, neck, and midline of the abdomen due to MSH

Enlargement of breasts

IOP decrease during pregnancy;

(1) increased progesterone levels,

(2) presence of relaxin,

(3) decreased production of aqueous humor due to increased secretion of HCG

CLINICAL IMPLICATIONS

Relaxation of ligaments and collagen tissue

of the vertebral column leads to lordosis..

Enlarged breasts with short necks make

intubation extremely difficult

Changes in IOP in parturients produce visual

disturbances..

FETAL CIRCULATION

FETAL CIRCULATION..

Fetal circulation is characterized by the presence of 3 main shunts placenta, foramen ovale, and ductus arteriosus

The placenta oxygenates the blood, which courses up through umblical vein(portal sinus and ductus venosus) to IVC then into the right atrium. IVC also receives less oxygenated blood returning from the lower body

The right atrium is divided by a structure called the crista dividends ,so this relatively well-oxygenated blood is shunted from the right atrium through the foramen ovale into the left atrium, thereby bypassing the right ventricle and pulmonary vasculature…

The two separate circulations, well oxygenated and de oxygenated are maintained by the structure of the right atrium,

It effectively directs entering blood to either the left atrium or the right ventricle, depending on its oxygen content, which is facilitated by the pattern of blood flow in the IVC..

The well-oxygenated blood tends to course along the medial aspect of the IVC and the less oxygenated blood stays along the lateral vessel wall

From LA to LV then ascending aorta supplying brain and upper extremeties..

Blood returns from the upper body to the right heart by SVC, where it is directed by the crista dividends into the right ventricle, from which it is then pumped out through pulmonary artery.

The pulmonary vascular bed has a high vascular resistance because the alveoli are relatively closed and filled with fluid, and the blood vessels are compressed

Blood that leaves the right ventricle by the pulmonary artery is shunted (90%) through the ductus arteriosus and down the descending aorta and

10 % to the pulmonry vasculature only enough blood flow to ensure growth and development of the lungs, including surfactant production

Clamping of the umbilical cord and initiation of ventilation produce enormous circulatory changes in the newborn

The transition of the alveoli from a fluid-filled to an airfilled state results in a reduced compression of the pulmonary alveolar capillaries with a reduction in PVR

It is accompanied by constriction of the ductus arteriosus secondary to oxygenation.

This results in an increase in pulmonary blood flow and an increase in left atrial pressure so the foramen ovale functionally closes

UTEROPLACENTAL BLOOD FLOW

Maintenance of uteroplacental blood flow is

the hallmark for fetal well-being..

UBF= UAP-UVP/ UVR(uterine vascular

resist)

At term, 10% of the cardiac output (700

mL/min) supplies the uterus

The placental vasculature remains maximally

dilated, thus placental blood flow will mainly

depend upon perfusion pressure

MEASUREMENT OF UBF

The ratio of the peak systolic waveform and diastolic trough of blood flow velocity (S/D) ,

a high S/D ratio is associated with reduced placental perfusion

Fetal oxygen transfer depends on oxygen affinity and the oxygen-carrying capacity of maternal and fetalblood.

The oxygen carrying capacity will ultimately depend on hemoglobin concentration and the oxyhemoglobindissociation curve shifted to the left in the fetus as compared with the mother

Hb conc mother 12gm/100ml and fetal 15gm

This benefits the fetus by increasing oxygen uptake across the placenta.

CLINICAL IMPLICATIONS

Fetal oxygenation will depend on the uterine vein oxygen content and umbilical vessel blood flow..

fetal oxygen delivery averages 24mL O2/min/kg and that oxygen consumption is 3mL O2/min/kg.

Compensation takes place either by increased oxygen extraction or by redistribution of the fetal circulation

Carbon dioxide (CO2) exchange will depend upon umbilical as well as uterine blood flow

Acute respiratory acidosis can be caused by an accumulation of CO2 because of a decrease in either uterine or umbilical blood flow

Fetal acidosis during maternal hyperventilation due to :

(1) maternal hypocapnia (<25mmHg) will cause uterine and umbilical vessel vasoconstriction..

2) Mechanical hyperventilation will increase

intrathoracic pressure and reduce venous

return as well as cardiac output and thus

reduce uteroplacental blood flow..

3) maternal alkalosis will shift the oxygen-

hemoglobin dissociation curve to the left, and

thus the fetus will have difficulty extracting

oxygen

FACTORS ALTERING UBF

1) Uterine contraction reduces UBF..Contractionsmeasured by observing intrauterin pressure

20 mmhg- no effect

30 mmhg- decrease UBF by 50 %

40 mmhg- completely stop intervillousperfusion..

2) Decreased UBF due to

a) Aortocaval compression by the large gravid uterus in supine position

b) sympathectomy from regional anesthesia and

c) hypovolemia from severe hemorrhage.

3) Pathological conditions :

a)PIH (pre-eclampsia),

b) diabetes, and

c) overdue dates or postmature pregnancy.

4) Pharmacological agents :

Iv induction agents :

thiopental : reduction in placental blood flow of 35% and no drop in mean maternal artery blood pressure

Diazepam,midaz : higher doses reduces UBF by reducing MAP

Etomidate, propofol : Propofol was associated with the greatest drop in MAP, whereas etomidate was seen to be the most cardiostableagent

Halothane, isoflurane : deep level of anaesthesia wil reduce UBF by decreasing maternal arterial pressure

sevoflurane : Sevoflurane and isoflurane at equianesthetic concentrations (0.46 MAC hr) were observed to produce similar drops in blood pressure and heart rate changes during the operation.

Blood loss and uterine tone were similar

When less lidocaine (blood level, 2 to 4mg/mL) is used, during epidural anesthesia, no significant decrease in uterine blood flow was observed even after prolonged infusion ..

Ropivacaine and bupivacaine do not cause vasoconstriction or reduce uteroplacental blood flow in therapeutic doses.

cocaine is associated with a significantly higher degree of uterine vasoconstriction and reduced uteroplacental blood flow

PLACENTAL TRANSFER OF ANAESTHETIC DRUGS

Drugs cross the placenta by three main processes:

simple diffusion, active transport, or pinocytosis.

factors : molecular weight, protein binding, degree of lipid solubility, maternal drug concentration maternal and fetalpH.

The Fick principle governs the rate of transfer of a drug across a membrane:

Q/t = K * A (Cm-CF) / D

Q/t is the rate of diffusion,

K is the diffusion coefficient,

A is the surface area of membrane available for exchange,

Cm - Cf is the concentration gradient between the maternal and fetal circulations, and

D is the thickness of the membrane

Thank you