Anesthetic implications for the physiological changes in pregnancy basic fhr monitoring
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Anesthetic Implications for the Physiological Changes in Pregnancy & Basic FHR Monitoring. J.E. Pellegrini, CRNA, PhD. Changes during the Puerperium. Changes to anatomy & physiology Most changes to physiology occur during the 1st trimester

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Anesthetic Implications for the Physiological Changes in Pregnancy & Basic FHR Monitoring

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Anesthetic Implications for the Physiological Changes in Pregnancy & Basic FHR Monitoring

J.E. Pellegrini, CRNA, PhD


Changes during the Puerperium

  • Changes to anatomy & physiology

    • Most changes to physiology occur during the 1st trimester

    • Most changes to anatomy occur during the 2nd and 3rd trimester

    • Many of the changes are beneficial

      • As an anesthetist you must have a good understanding of these changes and so that you can determine if they will have an impact on your your anesthetic management


Physiological Changes of Pregnancy

Primarily we’ll discuss:

  • Respiratory Changes

  • Cardiovascular Changes

  • GI/Hepatic/Renal Changes

  • Changes in Neural network (metabolism)


Factors influencing the Respiratory Systemand endotracheal intubation

  • Weight gain

  • Breast enlargement

  • Vascularity of the respiratory tract mucosa

  • Possible edema of the oropharynx, nasopharyx, and vocal cords (**most prevalent in preeclampsia)

  • Progesterone-beneficial


Respiratory System Changes


Respiratory Changes with Pregnancy


Compensatory Respiratory SystemChanges

  • Chest Expansion-expands anteroposterior

  • FRC - decreased

    • FRC & CC differences  underventilated aveoli

  • Airway closure - (-a DO2)occurs in 50% of all parturients but hypoxemia extremely rare secondary to increased vent & CO

  •  Residual Volume and ERV   tolerance for apnea

  • ABG Changes- reflect chronic hyperventilation

    • PACO2 32-34 mm Hg by 12 weeks gestation

    • Respiratory Alkalosis(7.44) HCO3, BE and buffer base 

    • More prone to metabolic acidosis during prolonged labor secondary to pyruvate & lactic acid accumulation


Compensatory Respiratory SystemChanges

  • Ventilation (8-10 wks gestation)

    • MV  50% at term ( 40% TV and 15% RR)

      • Helps decrease dead space component

      •  PaCo2 levels (respiratory alkalosis - 7.44)

  • Hypoxia & Hypercarbia -develop rapidly with obstruction, prolonged apnea or hypoxic gas mixture

    • PO2 can  80 mm Hg/min faster than non-pregnant 

      • Due to  O2 consumption,  FRC,  C.O. &  tissue extraction of Oxygen

  •  Airway Resistance

    • Effects of Progesterone

    • Chest wall but not lung compliance decreases


  • Compensatory Respiratory SystemChanges

    • Oxygen Consumption  20%

      • demands during labor where it is estimated that the avg. labor  jogging 12 miles

    • Oxyhemoglobin dissociation curve to the right

      • (P50 Values  from 26 to 28 mm Hg)


    Clinical Implications of these Respiratory System Changes

    • Effects on Inhalation Anesthetics

      • Faster induction rate ( RR and C.O.)

      • MAC decreased by 30-40%

      •  MAC noted as early as the 8th week gestation

    • Effects of Maternal Hyperventilation

      • Constriction of umbilical and uterine vessels

      •  incidence of fetal acidosis

  • Can attenuate most responses with adequate analgesia

    • Studies indicate that adequate pain relief (i.e. CLE can normalize oxygenation &  MV & O2 consumption)


  • Cardiovascular System

    • Blood Volume

      •  35% (plasma volume  50% & red cell mass  15%)

      • Blood loss usually well tolerated at delivery

      • See fall in Hct in Postpartum by approximately 5% secondary to diuresis

      • Normally only have to consider blood after 1500 ml EBL

    • Cardiac Output

      •  30-40% in 1st trimester and 40-45% during labor and 50-60% in immediate postpartum period

      • Prone to Aortocaval Compression


    Changes in Cardiovascular System


    Aorto-Caval Syndrome

    • Hypotension

      • 20 weeks gestation

      • Gravid Uterus Weight

      • Can Decrease C.O. 30%

      • Management Plan

        • Pre-induction hydration

        • Left Uterine Displacement (or RUD)

        • Ephedrine/Phenylephrine

    • Venal Caval Compression

      • Distention of epidural venous plexus

      • Decrease LA dose 1/3 (>14 wks)


    Cardiovascular Changes


    Anesthetic Significance of Cardiovascular Changes

    • Venodilation- increases accidental epidural vein puncture

    • Oxytocin with free H20  volume overload

    •  Hgb levels > 14 indicates low volume status, HTN or diuresis

    • C.O. high in 4 hrs postpartum

    • B/P < 90 to 95 torr   uterine blood flow

    • Hypotension occurs 75% with T4 level


    Gastrointestinal Changes

    • Stomach displaced upward and 45 to the right & displaces the intra-abdominal segment of the esophagus into the thorax decreased tone of the lower esophagus  incidence of pyrosis

    • Delayed gastric emptying   incidence of full stomach


    Gastrointestinal Changes

    • Obesity - associated 2-20 fold  in mortality (PIH, IDDM)

    • Progesterone

      •  Gastrointestinal motility & esophageal sphincter tone

    • Parturients beyond 18th week of gestation more prone to vomiting and regurgitation

      • Treat as full stomach at 12th week

        *put it all together and this spells trouble


    Other Compensatory Changes

    • Renal System - GFR  60% at term

      •  in aldosterone and  plasma osmolarity (ADH resetting)

      •  RBF   Creatinine clearance & a  BUN & Uric Acid levels (½ to 2/3 that of normal)

    • Hepatic System

      • Usually no significant changes except slight in level enzymes and 2-4 fold  in alkaline phosphatase & cholesterol (from growing placenta)

      • Slight  in plasma cholinesterase & serum albumin

      • Can see spider angiomata & palmar erythema (from  estrogen levels)


    Neuromuscular Changes

    • Endorphins

    • MAC  by 40%

    • Sedative Effect from Progesterone

    • Changes in SNS

      • See down-regulation

      • Altered Response to Catecholamines


    Altered Responses to Anesthesia

    •  sensitivity of neural network

      • Probably secondary to  levels of circulating progesterone

        • Possible influence from circulating endorphins

      • Applicable for both neuraxial and peripheral blockades

      • Applicable for parturients beyond 24th week gestation

        • Decrease local anesthetic dose by as much as 1/3


    Sensitivity of Nerve Fibers with Pregnancy


    Summary

    • Multiple physiological changes in pregnancy have profound impact on your anesthetic management

    • The conservative approach is the best approach when dealing with the OB patient

    • Your principle patient is the parturient


    Fetal Monitoring

    • No ideal way to assess fetal well-being

    • FHR one of the better methods

      • FHR influenced by Para and sympathetic outflow

      • FHR responds to Baro & Chemo receptors


    Maternal & Fetal Monitoring


    Fetal Heart Rate

    • Normal Baseline between 110-160/min

      • Small square = 10 seconds

      • Large square = 1 minute

    • Baseline rate determined by rate between contractions


    Three Primary Mechanisms that Uterine Contractions cause FHR Abnormalities


    FHR Accelerations

    • The FHR will normally remain steady or accelerate with uterine contractions

      • Typically viewed as a reassuring phenomenon


    Early Decelerations

    • Begins with onset of contraction & ends at the conclusion of contraction (with return to baseline)

    • Typically caused from Head Compression & routinely not viewed as a sign of fetal distress


    Late Decelerations

    • Transitory Decreases in FHR caused by Utero-Placental deficiency (hypoxia) indicating the fetus is not able to withstand the uterine contractions

    • Persistent Late Decelerations are considered an ominous sign especially when associated with loss of short term variability


    Nonreassuring Patterns

    • Nonreassuring, or "warning," patterns suggest decreasing fetal capacity to cope with the stress of labor.

    • Nonreassuring Patterns (Warning Signs)

    • ·Decrease in baseline variability

    • ·Progressive tachycardia (>160bpm)

    • ·Decrease in baseline FHR

    • ·Intermittent late decelerations with good variability

    • Ominouspatterns suggest possible fetal compromise.


    Ominous Patterns

    • ·Persistent late decelerations, especially with

    • decreasing variability

    • ·Variable decelerations with loss of variability,

    • tachycardia, or late return to baseline

    • ·Absence of variability

    • ·Severe Bradycardia


    Treatment for FHR Abnormalities


    So – In summary

    • If an ominous pattern appears to be present:

      • Have the mother lie on her left side or in a knee chest position immediately followed by:

        • Increase IV fluid.

        • Give her oxygen @ 10-12L to breathe by mask.

        • Discontinue or decrease any CLE infusion

        • Notify the obstetrical nursing staff & Obstetrician


    Pellegrini@son.umaryland.edu


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