PHYSIOLOGICAL CHANGES IN

PREGNANCY AND ITS

ANAESTHETIC IMPLICATIONS

DR. MOHTASIB MADAOO

DEPARTMENT OF ANAESTHESIOLOGY,

SAIFEE HOSPITAL.

PHYSIOLOGICAL CHANGES IN

PREGNANCY

• Pregnancy produces profound physiological changes that

alter the usual responses to Anesthesia .

• Unique challenges - two patients are cared for

simultaneously .

• Failure to take care can be disastrous for one or both of

them.

CARDIOVASCULAR CHANGES

Parameter Change

Blood Volume Increases by 30%

Plasma Volume Increases by 45%

Cardiac Output Increases by 30-50%

Stroke Volume Increases by 25%

Heart Rate Increases by 15-25%

Peripheral Vascular Resistance Decreases by15-20%

CVP Unchanged

ANAESTHETIC IMPLICATIONS

AortoCaval Compression

• Enlarged uterus compresses IVC and Lower Aorta when the

patient lies supine Obstruction of IVC Decreases Venous

Return leads to Decrease in Cardiac Output

• When awake most women are capable of compensating for the

decrease in stroke volume by increasing Sytemic Vascular

Resistance and Heart rate. There are also alternative venous

pathways : the paravertebral and azygos systems.

• During Anaesthesia compensatory mechanisms are reduced or

abolished.

• Obstruction of lower aorta causes reduced blood flow to

kidneys, uteroplacental unit and lower extremities.

SUPINE HYPOTENSION SYNDROME

8 to 15% of pregnant women have Overt CavalCompression (supine hypotension syndrome)

• Hypotension

• Sweating

• Bradycardia

• Pallor

• Nausea

• Vomiting

Prevention of SHS: Uterus should be displaced by placing arigid wedge under the right hip and tilting the table left sidedown.

Regional anaethesia – Profound Hypotension

The patient can be turned to full left lateral position.

RESPIRATORY CHANGES

Parameter Change

Oxygen consumption Increases by 20 to 50%

Minute ventilation Increases by 50%

Tidal volume Increases by 40%

Respiratory rate Unchanged/Slightly Increases

PaO2 Increases by 10%

PaCO2 Decreases by 15%

HCO3 Decreases by 15%

FRC Decreases by 20%

ANAESTHETIC IMPLICATIONS

• Decreased FRC and Increased oxygen consumption promotes rapid oxygen desaturation during periods of apnea. This is more marked in obese patients.

• The reduced FRC causes airway closure in 50% of parturients at term in the supine position making pre-oxygenation less effective.

• Regional block further diminishes FRC which leads to rapid development of Hypoxemia.

• Preoxygenation prior to induction of general anesthesia is therefore mandatory to avoid hypoxemia in pregnant patients.

Factors affecting Smooth Intubation

• There is capillary engorgement and edema of the upper

airway down to the pharynx, false cords, glottis and

arytenoids.

• The increase in chest diameter and enlarged breasts can

make laryngoscopy difficult.

Failure to intubate the trachea is 7 times more common in the

term parturient compared to non pregnant patients.

A smaller diameter endotracheal tube may be required for

intubation especially in cases of pre eclampsia.

Blood flow to the nasal mucosa is increased so

Oropharyngeal intubation is preferred over Nasal intubation.

COAGULATION CHANGES

Parameter Changes

Fibrinogen Increased from 2.5g/l to 5g/l

Factor 2 Slightly Increased

Factor 5 Slightly Increased

Factor 7 Increased 10 folds

Factor 8 Increased 2 folds

Factor 9 and 10 Increased

Factor 11 Decreased by 70%

Factor 12 Increased by 40%

Factor 13 Decreased by 40%

Bleeding time, PT, PTT is unchanged.

Pregnancy is a hypercoagulable state.

There is increased risk of thromboembolic episode.

GIT CHANGES

The parturient should be considered a full stomach patient

during most of gestation because

• Upward & anterior displacement of the stomach by the uterus

leads to increase in intragastric pressure and decrease in

gastroesophageal angle.

• Reduction of lower esophageal sphincter pressure due to

increased progesterone levels.

Risk of Regurgitation and aspiration of gastric contents.

Increased placental gastrin secretion which worsens gastric

acidity.

ANAESTHETIC IMPLICATIONS

• Prophylaxis in the form of H2 blocking drug and Prokinetic

drugs to all pregnant patients for surgery from 2nd

trimester onwards is a must.

• During GA airway protection by means of cuffed ETT is

mandatory; So is rapid sequence induction from 2nd

trimester of pregnancy till 48hrs post partum.

• Extubation should be done when the patient is awake and

on their side to reduce the risk of aspiration.

• The danger of aspiration is almost eliminated when

regional anaethesia is used.

CNS CHANGES & ITS IMPLICATIONS

Decrease in minimum alveolar concentrations secondary toincreased levels of progesterone and β- endorphin levels.

• Rapid induction with inhalation agents – The increasedminute ventilation combined with decreased FRC anddecreased MAC.

The amount of local anaesthetic drug required in a pregnantwoman is less compared to the non pregnant state. (Approxtwo-thirds of the normal dose is adequate)

• Exaggerated lumbar lordosis contribute to cephaladspread of the local anaesthetic.

• Engorged epidural plexus of veins will decrease thevolume of the epidural and subarachnoid space.

• The CSF pressure is increased due to compression fromthe epidural veins in the epidural space.

• Increased sensitivity to opiods, sedatives, and localanaesthetics when used for neuraxial anaesthesia.

CNS CHANGES & ITS IMPLICATIONS

RENAL CHANGES

• Renal vasodilatation increases renal blood flow early

during pregnancy.

• Increased Cardiac output leads to Increased GFR &

Increased renal plasma flow by 50% which increases

clearance of urea, uric acid and Creatinine.

• Increased Renin & Aldosterone level promotes Na+

retention leading to volume overload.

• Decreased Renal tubular threshold for glucose & amino

acids → mild glycosuria & proteinuria (< 300mg/d).

• Progesterone mediated ueretetic smooth muscle

relaxation can lead to urinary stasis making pregnant

women prone to urinary tract infections.

There is increase in the volume of distribution for drugs and

may have to be given in higher than normal dosages.

HEPATIC CHANGES

• Hepatic function and blood flow are unchanged.

• A mild decrease in serum albumin is due to an expanded

plasma volume. Thus, the free fraction of albumin-bound

medications is increased.

• A 25—30% decrease in serum pseudocholinesterase

activity is also present at term,but it rarely produces

significant prolongation of SCh action.

• Increased cholesterol gall stone formation(progesterone).

PLACENTAL TRANSPORT

MECHANISMS

• Simple diffusion – 02 and CO2 transport occurs due to the

difference between partial pressures on both sides. Ffatty

acids are also transported by means of simple diffusion.

• Secondary active transport – amino acids are transferred

mostly as linked carriers.

• Pinocytosis – Placenta is Impermeable to proteins, only

IgG is transported.

• Bulk transport – Water and electrolytes moves across bulk

flow.

FACTORS AFFECTING PLACENTAL

TRANSFER OF DRUGS

• Lipid solubility – The placental membrane is freely

permeable to lipid soluble substances, which undergo

flow dependent transfer. Higher the lipid solubility , higher

the transfer of drugs.

• Molecular weight – Drugs with smaller molecular weight

diffuse easily (<600da)

• Degree of ionization – Ionized form will not cross the

barrier easily. The degree of ionization of acidic drugs is

greater on the maternal side and lower on the fetal side.

• Protein binding – protein bound drugs will not diffuse

easily, only free drug would cross the placental barrier

easily. Acidosis reduces the protein binding of local

anaesthetic. Reduced albumin concentration increases

the proportion of unbound drug

ANAESTHETIC DRUGS

Opioids – All opioids cross the placenta in significant

amounts. They are weak bases, bound to α-glycoprotein.

Pethidine – Longer half life is due to its active metabolite

norpethidine, which may lead to respiratory depression in the

neonate.

Morphine – It is poorly lipid soluble but readily crosses the

placenta due to low protein binding.

Fentanyl – It is highly lipid soluble and albumin bound, so

crosses the placental barrier easily.

IV Induction agents – Sodium thiopentone is highly lipid

soluble, weakly acidic, 75% protein bound and less than 50%

ionized at physiological pH. It crosses the placenta easily.

Propofol – It is highly protein bound and lipophilic.

Inhalational Agents – These agents are highly soluble with low

molecular weights.

Muscle relaxants – These are quaternary ammonium compounds

and fully ionized. These drugs are fully ionized as well as have

low lipid solubility, hence they do not cross the placenta.

Local Anaesthetics – These drugs have low molecular weights

and also are lipid soluble. Different drugs have different protein

binding.

Thank

You.