Physiological Changes During Pregnancy And Anaesthetic Implications

 CNS

i] MAC:
progressive decrease during pregnancy
decrease by 40% at term
causes: increase progesterone level which is sedatind even given in normal doses
increase beta endorphin levels during labour and delivery plays major role

ii] Local anaesthetic sensitivity:
Increase in pregnancy
recent data doesn’t show increased sensitivity
dose requirement reduced by 30%

Gravid uterus ➔IVC obstruction ➔ epidural vein engorgement ➔ decrease CSF volume ➔ decrease epidural space ➔ increase cephaloid spread of local anaesthetic
Increase epidural pressure ➔ increase chances of dural puncture while locating epidural space

Respiratory Effects

Increase in O2 consumption➔ at term 20-50%
Increase in minute ventilation ➔ due to increase tidal volume
Decreasae in PaO2 to 28-32 mmHg
Hyperventilation ➔ increase in PaCo2
Increase in 2,3,DPG offsets effect of increase in PaO2 on Hb affinity for O2
P50 increase from 27 to 30mmHg

III rd trimester ➔ elevation of diaphragm of diaphragm and increase AP diameter of chest ➔ no restriction of thoracic breathing hence thoracic breathing is favored over abdominal .
VC and CC is not affected
FRC is decreased due to larger tidal volume upto 20% at term ,returns to normal at 48 hours after delivery.

• Airway resistenace is decreased
• Physiological dead space is decreased
• Increased intra pulmonary shunting

Spiromertric changes:

• Increase in TV
• Increase in RR
• Increase in MV
• Increase in FRC due to high tidal volumes decrease upto 20% term
• VC and CC are minimally affected
• Flow volume loops are unaffected
• CXR- Prominent vascular markings

ABG changes

Decrease in PaCO2 to 28-32mmHg
Respiratory alkalosis
Increase in PaO2 , decrease in HCO3

Clinical implications:

1. Due to capillary engorgement ➔ increased risk of trauma , bleeding , and URI , nasal intubation causes epistaxis . Gentle laryngoscopy and use of small ET tubes is required
2. P50 is increased from 27 to 30mmHg causing increase in O2 unloading to tissues
3. Closing capacity(CC) exceeds FRC, decrease FRC to 20% at term and increase O2 consumption ➔ rapid desaturation during apnea

 

CVS Effects

• Increase in maternal and fetal metabolic demands
• Increase in cardiac output(CO) and blood volume and increase in plasma volume
• Blood volume increases by 35%
• Plasma volume increases by 45%
• Cardiac ouput increases by 40%
• Stroke volume increases by 30%
• Heart rate increases by 20%
• SBP – 5%
• DBP – 15%
• Peripheral resistance – 15%
• Pulmonary resistance – 30%

Plasma volume increases in excess to RBC mass ➔ dilutional anemia
Tissue O2 delivery maintained by ➔ increase in CO rt shift of O2 dissociation curve
Decrease SBP, DBP, PVR – Blunting response to adrenergic agents and vasoconstrictors

Total blood volume increases by 1- 1.5 lit during pregnancy i.e; 90ml/kg allowing them to easily tolerate blood loss associated with delivery.
CVP, PCWP , PAP remains same
Increased pulmonary blood volume but pulmonary vasodilatation decreases PVR

Supine Hypotension Syndrome(SHS)

Upto 20% at term develop SHS.
Charecterised by hypotension associated with pallor, sweating , nausea or vomiting
Cause: appears to be complete or near comlete occlusion of IVC by gravid uterus in supine bposition
Gravid uterus compresses aorta and decreases utero placental perfusion.
Uterine contraction relieves caval compresiion but exacerbates aortic compression produces fetal hypoxia

Prevention:
Parturients >28 weeks should not lie supine without left uterine displacement
Place wedging 15 degrees under right hip
Elevation of diaphragm ➔ shifting of herarts position in chest ➔ appearance of enlarged heart on plain chest film ➔ left axis deviation and T wave inversions

Physical Examination:

• Grade I or II systolic ejection flow murmur
• Exaggerated splitting of 1st Heart sound
• S3 may be audible
• Few patients develop small asymptomatic pericardial effusion

 

Renal Effects

Increased RBF, RPF and GFR ➔ kidneys can enlarge ➔ increase reninand aldosterone ➔ increase Na+retention ➔ decrease in serum creatinine by times, decrease in BUN by 8 – 9 mg , decrease in nrenal tubular threshold to glucose, and amino acids ➔ glycosuiria and proteinuria

 

GI Effects

Gravid uterus ➔ increase progesterone levels ➔ decrease gastro esophageal sphincter ➔GERD ➔upward and anterior displacement of stomach ➔incompetence of LES ➔increase placental gastrin secretion -➔hyperacidity➔esophagitis
Both these factors increase chances of regurgitation and pulmonary aspiration
Nearly all parturients have gastric pH < 2.5 and gastric volume > 25ml hence hish risk for aspirtaion pneumonitis

Anaesthesia implications:

Opioids and antocholinergics decrease LES tone ➔ GERD and delays gastric emptying
Recent food ingestion and gastric emptying prior to labour predispose to nausea and vomiting

Hepatic effects:

• Overall hepatic functions and blood flow unchanged
• Mild increase in serum aminotransferrases
• Increase in serum alkaline phosphatases due to placental secretion
• Increase in serum albumin due to increase in plasma volume
• Increase in psuedocholinestesrase activity which can lead to prolonged succinylcholine
• apnea but it does not cause significant prolongation
• High progesterone level ➔ decrease cholecystokinin release ➔ increase risk for gall stones formation

 

Haemotological effects

• Pregnancy is a hyper coagulable state
• Benuificail in limiting blood loss at delivery
• Factors which incarese during pregnancy fibrinogen , factor 7,8,9,10 and 12
• Factor decrease in pregnancy id factor XI
• Accelerated fibrinolyisis in late 3rd Trimester
• Dilutuional anemia
• 10% decrease in platelet count
• Leucocytosis upto 21000
• Iron deficiency , folate deficiency readily depressed
• Cell mediated immunity is depressed due to susceptibility to infections

 

Metabolic Effects

• Complex metabolic and hormonal changes
• Resembles starvation
• Blood glucose .amino acids levels are low
• Frrefatty acids , ketones and triglyceride levels are high.

 

Endocrine Effects

• Insulin resistance is seen
• Human placental lactogen is produced by placenta resembles growth harmone and may be responsible for insulin resisatnce
• Thyroid gland – undergoes hypertrophy , increased T3 and T4 production but thyroxine binding globulin is also increased
• Level of parathyroid harmone level falls leading to decrease in Ca level but ionized ca levels remains normal

Musculoskeletal system

• Placenta produces relaxin which causes relaxation of ligaments resulting in widening and increased mobility of pubis and sacro iliac joints to allow passage of fetus through sixth canal
• Lumbar lordosis – sub arachanoid block is challenging
• Back ache is common complaint.