3. Physiological Changes of Late Pregnancy Relevant to General Anaesthesia Flashcards
Cardiovascular system
12 kg gain
Half of this is accounted for by an increase in plasma volume and interstitial fluid.
Plasma volume increases by up to 40% and total body water by around 7–8 litres
mild cardiac dilatation, and so heart murmurs (for
example, that of mitral regurgitation) are common.
Cardiac output increases by 40–45% to near maximal at 32 weeks’ gestation.
The resting heart rate increases by 15%, and
tachyarrhythmias are more common.
The ECG shows left axis deviation caused by
mechanical displacement by the gravid uterus, and minor T wave and ST segment
changes may be seen.
Blood pressure falls, with the diastolic drop of 10–15 mmHg making a bigger contribution than the systolic, and there is a decrease in systemic vascular resistance
There is reduced sensitivity to circulating vasopressors, although it
appears that the uterine circulation may be more sensitive to these than the systemic.
Aortocaval compression
Aortocaval compression (supine hypotension syndrome): this is of potential
importance because it occurs to some degree in all women, although modern
methods of assessment using, for example, femoral vein ultrasound, suggest that it
is significant in around only 30%.
The remainder are able to compensate by an
increase in sympathetic tone and diversion of venous return through the effective
collateral circulation of the azygos veins and the vertebral venous plexus.
Compression by the gravid uterus of the great vessels affects mainly
venous return, but it can also compromise aortic and uterine blood flow. Turning
from the lateral to the supine position at term in some women may decrease
cardiac output by up to 30%.
Anaesthetic implications CVS changes
Anaesthetic implications: because it is not possible to predict those mothers in
whom aortocaval compression will reduce cardiac output, there should still be
appropriate positioning to avoid the possibility. Cardiac output and systemic
blood pressure must be maintained to ensure continued perfusion of the uteroplacental
unit, but equally the anaesthetist must be aware of the consequences of
fluid-loading a mother who is in effect already waterlogged.
Respiratory system:
- There is an increase in minute volume by 40% at term,
but this is initiated early in pregnancy
when progesterone-induced hyperventilation reduces PaCO2 by around 1 kPa - This would shift the oxygen– haemoglobin dissociation curve downwards and to the left were it not for an increase in maternal 2,3-DPG, which offsets this effect
- Increased metabolic demand for oxygen increases by around 50%, along with an increase in the work of breathing and a decrease in both chest wall and lung compliance
- The increased demand for oxygen is more than compensated
by the increase in cardiac output, and so there is a
small rise in PaO2 of about 1 kPa - There are anatomical changes which influence the upper airway;
-general fluid retention and oedema of pregnancy may complicate
laryngoscopy and intubation. - With regard to pulmonary volumes, the most important
change is the 20% decrease in FRC, which, by the third trimester,
may fall in the supine position to half its predicted value
Anaesthetic implications of Respiratory changes
the FRC must be filled with oxygen prior to induction
to minimize risk of desaturation. This can be achieved either by preoxygenating the
mother for 3 minutes with 100% O2, or by asking her to take three vital capacity
breaths. Slight head-up or ramped positioning will reduce encroachment of the
closing volume on the FRC. The reduced FRC means that the onset of the effect of
volatile anaesthetic agents will be more rapid, as will maternal desaturation.
Relative hyperventilation and low-normal PaCO2 should be maintained, although
it is not until the PaCO2 falls below about 2.7–3.3 kPa (20–25 mmHg) that uterine
blood flow is compromised.
The congested and more oedematous upper airway may be traumatized during
instrumentation. A smaller tracheal tube (7.0) may be required.
Gastrointestinal system:
by the third trimester some 70% of mothers have symptoms
of gastro-oesophageal reflux and heartburn.
Oesophageal barrier pressure decreases
with the loss of lower oesophageal sphincter tone,
and there is also a fall in intestinal transit time
and some duodenal gastric reflux.
Gastric emptying itself, however, is not delayed in late pregnancy.
Gastric residual volumes are increased, as
is placental gastrin secretion.
Whether this translates into maternal gastric hyperacidity
remains disputed.
Anaesthetic implications GI
the airway must be protected against the risk of pulmonary aspiration
of gastric contents by antacid prophylaxis
(H2 antagonists, proton pump inhibitors and sodium citrate).
Effective cricoid pressure applied
during a rapid sequence induction is also considered essential.
CNS
under the influence of progesterone and endogenous β-endorphins,
the MAC of anaesthetic agents decreases by about one-third,
and there is an increased sensitivity to all drugs which act centrally.
(Requirements for local anaesthetics also decrease,
which may be related to an increased availability of free drug and to
hormonally enhanced neural sensitivity.)
— Anaesthetic implications: cns
reduction in the doses of anaesthetic agents, sedatives
and analgesics may be possible.
Interpatient variability, however, is so great that it
would be unwise to assume that anaesthetic awareness or severe postoperative
pain are less likely.
Musculoskeletal system
: pregnancy increases ligamentous laxity owing to the rises
in the hormones progesterone and relaxin. There is also an increased lumbar lordosis
which helps to accommodate the enlarging uterus.
There is also an increased lumbar lordosis
which helps to accommodate the enlarging uterus.
Anaesthetic implication MSK
Anaesthetic implications: scrupulous positioning of the patient with appropriate
supports and protection may minimize the risk of postoperative backache or other
joint problems.
Haematological
: pregnancy is a hypercoagulable state.
There is an increase in all clotting factors, except for Factor XI,
and fibrinolysis is impaired by a plasminogen
inhibitor that is derived from the placenta.
Haematological — Anaesthetic implication
— Anaesthetic implications:
the risk of venous thromboembolism is increased fivefold
and routine preventative measures should be used
Should a mother have additional risk factors,
then pharmacological intervention may be necessary,
although in many units low molecular weight heparin
is given routinely after caesarean section or other surgery.
Metabolic: + — Anaesthetic implications:
there is a 30% fall in the levels of plasma cholinesterase.
— Anaesthetic implications:
this fall has the greatest implications for those patients with atypical cholinesterases.
It is often claimed that this decrease does not
produce a clinically important increase in the duration of suxamethonium.
Clinical experience would suggest, however, that the actions of suxamethonium
are prolonged in many pregnant patients and that rapid offset with the resumption
of spontaneous respiration is by no means guaranteed.
Drug handling
Drug handling: increased renal blood flow and glomerular filtration enhances the
clearance of drugs excreted renally. The reduction in maternal albumin may increase
the amount of free drug present in plasma, which may enhance its effects.
