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Physiological Changes in Pregnancy and Its Anaesthetic Implications.

Physiological Changes in Pregnancy and Its Anaesthetic Implications.

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Physiological Changes in Pregnancy and Its Anaesthetic Implications.

  1. 1. PHYSIOLOGICAL CHANGES IN PREGNANCY AND ITS ANAESTHETIC IMPLICATIONS DR. MOHTASIB MADAOO DEPARTMENT OF ANAESTHESIOLOGY, SAIFEE HOSPITAL.
  2. 2. 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.
  3. 3. 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
  4. 4. 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.
  5. 5. SUPINE HYPOTENSION SYNDROME 8 to 15% of pregnant women have Overt Caval Compression (supine hypotension syndrome) • Hypotension • Sweating • Bradycardia • Pallor • Nausea • Vomiting Prevention of SHS: Uterus should be displaced by placing a rigid wedge under the right hip and tilting the table left side down. Regional anaethesia – Profound Hypotension The patient can be turned to full left lateral position.
  6. 6. 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%
  7. 7. 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.
  8. 8. 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.
  9. 9. 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.
  10. 10. 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.
  11. 11. 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.
  12. 12. CNS CHANGES & ITS IMPLICATIONS Decrease in minimum alveolar concentrations secondary to increased levels of progesterone and β- endorphin levels. • Rapid induction with inhalation agents – The increased minute ventilation combined with decreased FRC and decreased MAC.
  13. 13. CNS CHANGES & ITS IMPLICATIONS The amount of local anaesthetic drug required in a pregnant woman is less compared to the non pregnant state. (Approx two-thirds of the normal dose is adequate) • Exaggerated lumbar lordosis contribute to cephalad spread of the local anaesthetic. • Engorged epidural plexus of veins will decrease the volume of the epidural and subarachnoid space. • The CSF pressure is increased due to compression from the epidural veins in the epidural space. • Increased sensitivity to opiods, sedatives, and local anaesthetics when used for neuraxial anaesthesia.
  14. 14. 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.
  15. 15. 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).
  16. 16. 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.
  17. 17. 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
  18. 18. 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.
  19. 19. 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.
  20. 20. Thank You.

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