PHYSIOLOGICAL CHANGES IN PREGNANCY MODERATOR: Prof. Dr. Surinder Singh

PRESENTED BY: Dr.Chittra

CAUSESHormonal factors Mechanical effects of the gravid

uterusIncreased metabolic and oxygen

requirementsDemands of the fetoplacental unitHemodynamics of placental

circulation.

CARDIOVASCULAR SYSTEMPhysiologic changes in CVS begin in

the first trimesterCARDIAC OUTPUTIncreases from 5th week of pregnancy

reaches its maximum levels at around 32 weeks, after which there is a slight increase til labor, delivery, and the postpartum period

↑ in stroke volume and heart rateStroke volume ↑ 20% to 50% at term Heart rate ↑ 20% by 4th week

BLOOD PRESSURESystemic arterial pressure is never increased during

normal gestationMid pregnancy, slight decrease in diastolic pressure Pulmonary arterial pressure maintains a constant

level. Vascular tone is more dependent upon

sympathetic control than in the nonpregnant state, hypotension develops more readily and more markedly consequent to sympathetic blockade following spinal or extradural anaesthesia.

Central venous and brachial venous pressures remain unchanged during pregnancy, but femoral venous pressure is progressively increased due to mechanical factors

Supine hypotension syndrome At term decrease in cardiac output

due to obstruction of the inferior vena cava by the gravid uterus, which did not occur when women are placed in the lateral position.

Despite the increase in blood volume and cardiac output, at term chances of hypotension, especially when in the supine position.

10% pregnant patients at term show signs of severe hypotension when assuming the supine position

Aortocaval Compression. From mid-pregnancy, the enlarged

uterus compresses both the inferior vena cava and the lower aorta when the patient lies supine.

Obstruction of the inferior vena cava reduces venous return to the heart leading to a fall in cardiac output by 24% towards term.

In the unanaesthetised state, most women are capable of compensating for the resultant decrease in stroke volume by increasing SVR and HR

During anesthesia these compensatory mechanisms are reduced or abolished so that significant hypotension may rapidly develop

Obstruction of the lower aorta and its branches causes diminished blood flow to kidneys, uteroplacental unit and lower extremities.

During the last trimester, maternal kidney function is markedly lower in the supine than in the lateral position.

Parameter Change Amount (%)Heart rate Increased 20-30Stroke volume Increased 20-50Cardiac output Increased 30-50Contractility Variable ±10Central venous pressure Unchanged  

Pulmonary capillary wedge pressure Unchanged  

Systemic vascular resistance Decreased 20

Systemic blood pressure Slight decrease

Midtrimester 10-15 mm Hg, then rises

Pulmonary vascular resistance Decreased 30

Investigation Findings

Chest radiography

Apparent cardiomegaly

Enlarged left atrium

Increased vascular markings

Straightening of left-sided heart border

Postpartum pleural effusion

Electrocardiography

Right-axis deviationRight bundle branch block

ST-segment depression of 1 mm on left precordial leads

Q waves in lead IIIT-wave inversion in leads III, V2, and V3

Small decrease in PR and QT interval

Rotation ± 15 degrees (QRS axis)

HEMATOLOGICAL SYSTEMBlood volume begins to increase early in

pregnancy as a result of changes in osmoregulation and the renin-angiotensin system

By term blood volume increases by up to 45% red cell volume increases by only 30%.This differential increase leads to the physiologic anemia of pregnancy with an average hemoglobin and hematocrit of 11.6 g/dL and 35.5% respectively.

Oxygen transport is not impaired mother's body compensates for it by increased cardiac output, increased PaO2, and a rightward shift in the oxyhemoglobin dissociation curve.

Factor Change II- Unchanged VII- Increased +++ VIII, IX, X, XII- Increased XI- Reduced Fibrinogen- Increased +++ Platelets- Stable

Hypercoagulability exists in pregnancy, with increased levels of most coagulation factors

Fibrinogen and factor VII are markedly increased other factors increase to a lesser extent.

Protective adaptation to lessen the risks associated with the acute hemorrhage that occurs at delivery.

The platelet count remains unchanged throughout most of pregnancy

The platelet count increases in the postpartum period because of activation of hemostasis at the time of delivery

The incidence of low platelet counts in normal pregnancy is approximately 8%.

Previously the cutoff for initiation of neuraxial blocks was considered 100,000 × 109/L but now in regional technique with platelet counts it is above 75,000 × 109/L and with counts between 50,000 and 75,000 if the level is stable and clinical laboratory abnormalities or signs of a coagulopathic state are absent.

RESPIRATORY SYSTEMRespiratory Tract. Hormonal changes to the mucosal vasculature of

the respiratory tract lead to capillary engorgement and swelling of the lining in the nose, oropharynx, larynx, and trachea

These symptoms can be exacerbated by fluid overload or oedema associated with pregnancy-induced hypertension or pre-eclampsia

In such cases manipulation of the airway can result in profuse bleeding from the nose or oropharynx endotracheal intubation can be difficult only a smaller than usual endotracheal tube may fit through the larynx.

Airway resistance is reduced due to the progesterone-mediated relaxation of the bronchial musculature

 Lung Volumes. Upward displacement by the gravid uterus

causes a 4 cm elevation of the diaphragm, but TLC decreases only slightly because of compensatory increases in the transverse and antero-posterior diameters of the chest, and flaring of the ribs

Diaphragm moves with greater excursions during breathing in the pregnant than in the non-pregnant state,breathing is more diaphragmatic than thoracic during gestation an advantage during supine positioning and high regional blockade.

From middle of the second trimester, expiratory reserve volume, residual volume and functional residual volume are progressively decreased, by approximately 20% at term.

Lung compliance is relatively unaffected, but chest wall compliance is reduced, especially in the lithotomy position.

Ventilation and Respiratory Gases.Increase in minute ventilation starts soon

after conception and peaks at 50% above normal levels around the second trimester.

It is effected by a 40% rise in tidal volume and a 15% rise in respiratory rate

Alveolar ventilation is about 70% higher at the end of gestation

Arterial and alveolar carbon dioxide tensions are decreased by the increased ventilation

An average PaCO2 of 32 mmHg and arterial oxygen tension of 105 mmHg persist during most of gestation

During labour, ventilation may be further accentuated, either voluntarily or involuntarily in response to pain and anxiety.

Such excessive hyperventilation results in marked hypocarbia and severe alkalosis, which can lead to cerebral and uteroplacental vasoconstricton and a left shift of the oxygen dissociation curve

The latter reduces the release of oxygen from haemoglobin with consequent decreased maternal tissue oxygenation as well as reduced oxygen transfer to the fetus.

Increased oxygen consumption and the decreased reserve due to the reduced functional residual capacity, may result in rapid falls in arterial oxygen tension despite careful maternal positioning and preoxygenation

Even with short periods of apnea, whether from obstruction of the airway or inhalation of a hypoxic mixture of gas, little defense against the development of hypoxia

The increased minute ventilation combined with decreased functional residual capacity hastens inhalation induction or changes in depth of anaesthesia when breathing spontaneously

GASTROINTESTINAL SYSTEMMechanical Changes. Enlarging uterus causes a gradual

cephalad displacement of stomach and intestines.

At term the stomach has attained a vertical position rather than its normal horizontal one

These mechanical forces lead to increased intragastric pressures as well as a change in the angle of the gastroesophageal junction, which in turn tends toward greater oesophageal reflux

Progesterone relaxes smooth muscle consequently, it impairs esophageal and intestinal motility during pregnancy

Whether gastric emptying is delayed during pregnancy is controversial

Risk of pulmonary aspiration of gastric contents is there especially when undergoing an emergency cesarean delivery under general anesthesia.

Established labor and the administration of parenteral opioids delay gastric emptying

Epidural analgesia using local anesthetics without opioids does not affect gastric emptying

The pain of labor may delay gastric emptying and promote emesis

These changes may be caused by the effects of placentally derived gastrin

RENAL SYSTEMRenal vasodilatation increases renal

blood flow early during pregnancy but autoregulation is preserved

Increased renin and aldosterone levels promote sodium retention

Renal plasma flow and the GFR ↑ 50% during the first trimester

Serum creatinine and blood urea nitrogen might decrease

A decreased renal tubular threshold for glucose and amino acids is common

The increase in GFR generally precedes the expansion of blood volume and is considered to be a marker of pregnancy-induced vasodilation

Parameter Pregnant Nonpregnant

Creatinine clearance 140-160 mL/min 90-110 mL/min

Urea 2.0-4.5 mmol/L 6-7 mmol/L

Creatinine 25-75 µmol/L 100 µmol/L

Uric acid 0.2 mmol/L 0.35 mmol/L

pH 7.44 7.40

Bicarbonate 18-22 mmol/L 23-26 mmol/L

DRUGS Increased sensitivity to both

regional and general anesthetics Pregnant women require less

local anesthetic than nonpregnant women to reach a given dermatomal sensory level

MAC of halothane and isoflurane are reduced by 25% and 40% during pregnancy

Obstruction of the inferior vena cava by the enlarging uterus distends the epidural venous plexus and increases epidural blood volume

(1) decreased spinal cerebrospinal fluid volume (2) decreased potential volume of the epidural

space (3) increased epidural space pressureThis enhance the cephalad spread of local

anesthetic solutions during spinal and epidural anesthesia

Higher incidence of dural puncture with epidural anesthesia

Bearing down during labor further accentuates all these effects

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