Changes in Respiratory System in Pregnancy
Dr Muhammed Aslam N K
• The anatomic and physiological changes of pregnancy
have major pulmonary and cardiovascular
consequences throughout the gravid period.
• Physiological values and requirements, as well as
normal laboratory assessment parameters, dynamically
• An appreciation of these changes is essential to
understanding the clinical cardiopulmonary
manifestations of both pre existing diseases during
pregnancy and cardiopulmonary diseases that may be
unique to pregnancy
ANATOMIC CHANGES OF NORMAL
• Hyperemia, friability, mucosal edema, and
hypersecretion of the airway mucosa -- most
pronounced in the upper airways, especially during the
• Nasal obstruction, epistaxis, sneezing episodes, and
vocal changes may occur, and these may worsen when
the individual lies down.
• Nasal and sinusoidal polyposis is often seen and tends
to recur in women with each pregnancy
• Recurrent or chronic “head colds,”
• Nasal obstruction may contribute to upper airway
obstruction during sleep, leading to snoring and even
obstructive sleep apnea.
Clinical consequences of the anatomic
changes of the upper airway
• Preferential mouth breathing and intolerance of nasal
cannula delivery of oxygen.
• Nasopharyngeal obstruction may make the pregnant
individual poorly tolerant of the introduction of
nasogastric tubes, nasal airways, or nasotracheal tubes
• Small endotracheal tubes, 6.0 mm or less, may be
• Mucosal changes that affect the upper airwaysmay also
occur in the central portion of the airway, such as the
larynx and trachea.
• Nonspecific complaints of airway irritation, such as
irritant cough or sputum production
• The physiological causes of nasal mucosal changes
appear to be predominantly mediated by estrogens.
• Estrogens increase tissue hydration and edema. They
also cause capillary congestion and hyperplastic and
hypersecretory mucous glands.
RespiratoryMuscles and the Thoracic Cage
• The enlarging uterus produces upward displacement of
the diaphragm → increase in the anteroposterior and
transverse diameters of the thoracic cage
• Diaphragm may be elevated up to 4 cm cephalad, but
diaphragmatic function is not impaired
• Thoracic cage increase by 5-7 cm in circumferance
• Diaphragmatic excursion during breathing may be
greater in pregnancy than during the puerperium
,suggesting that breathing may be more diaphragmatic
than costal during pregnancy
• Progressive relaxation of the ligamentous attachments of
the ribs broadens the subcostal angle by approximately
50 percent (from68 to 103 degrees).Consequently, there
is a 5- to 7-cm increase in chest circumference.
• The shortening and widening of the thoracic cavity
results in upward and lateral displacement of the cardiac
apex on chest radiography.
• Enlarging uterus cause serial changes in lung volumes
• Expiratory reserve volume decreases by 8 to 40 percent
• Residual volume decreases by 7 to 22 percent
• 10 to 25 percent decrease in functional residual capacity
after the fifth or sixth month of pregnancy (more
pronounced in the supine position)
• Inspiratory capacity increases (due to the
counterbalancing effects of widening of the lower rib
cage, attenuation of the abdominal musculature, and
unimpaired diaphragmatic movement)
• Vital capacity preserved
• Total lung capacity minimally decrease in the third
• Residual volume to total lung capacity ratio is low in the
• In late pregnancy, airway closure may occur at a lung
volume close to or greater than functional residual
capacity (more significant in the supine position)
Increased gastric and esophageal pressure occurring in
Decrease in transpulmonary pressure
Peripheral airway collapse
• Tidal volume increases 30 to 35 ( increased ventilatory
Increase in minute ventilation
• Respiratory rate either does not change appreciably or
• Maximum voluntary ventilation does not change greatly
• FEV1 -- not significantly different.
• Progressive increases of airway conductance occur
between 6 months of pregnancy and term with a
decrease in airway resistance.
• Total pulmonary resistance is reduced by 50 percent.
• Lung compliance does not change significantly.
• Compliance of the thoracic cage decreases
Lung volume changes associated with pregnancy
Although total lung capacity, residual volume, and expiratory reserve volume
diminish, vital capacity is preserved in values similar to nonpregnant women
• In early pregnancy Diffusing capacity is either
unchanged or slightly increased
• Rest of pregnancy, the diffusing capacity decreases.
• Carbon dioxide production and oxygen consumption
increase (increase in basal metabolic rate, coupled with
growth in the mass of fetal and maternal tissue and a
small increase in cardiac and respiratory work)
• Since the increase in minute ventilation is approximately
two times greater than the increase in oxygen
consumption, without significant change in respiratory
exchange ratio, the increased respiratory drive of
pregnancy results in alveolar hyperventilation.
• Progesterone levels increase gradually during
pregnancy from 25 ng/ml at 6 weeks to 150 ng/ml at 37
• The increase in minute ventilation results in a respiratory
alkalosis with compensatory renal excretion of
• PCO2 falls to levels of 28 to 32 mmHg.
• Arterial pH is maintained in the range of 7.40 to 7.45
• Bicarbonate decreases to 18 to 21 mEq/L
• The increase in ventilatory drive and the decrease in
functional residual capacity accelerate induction and
recovery from inhalational anesthesia.
• The decrease in functional residual capacity, the
increase in closing volumes, and the increase in oxygen
consumption lead to a more precipitous decline in
arterial PO2 in pregnant patients who are apneic or
• Respiratory responses during parturition are greatly
affected by stage of labor and the response to pain and
• During labor, tidal volumes ranges from 350 to 2250 ml
and minute ventilations from 7 to 90 L/min
Physiologic Dyspnea of Pregnancy
• The increase in minute ventilation that accompanies
pregnancy is often perceived as shortness of breath.
• Shortness of breath at rest or with mild exertion is so
common that it is often referred to as „„physiologic
• The increase in minute ventilation and the load imposed
by the enlarging uterus cause an increase in the work of
• Other factors contribute to the sensation of dyspnea
include increased pulmonary blood volume, anemia, and
• Differentiate the normal dyspnea of pregnancy from that
due to disease pathology.
• Pathologic dyspnea : increased respiratory rate greater
than 20 breaths per minute, arterial PCO2 less than 30
or greater than 35, hypoxemia or abnormal measures on
forced expiratory spirometry, or cardiac
• Abrupt or paroxysmal episodes of dyspnea suggest an