Changes in Respiratory System in Pregnancy

Changes in Respiratory System in Pregnancy

Dr Muhammed Aslam N K

INTRODUCTION

• 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 changes.

• 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 NORMALPREGNANCY

Upper Airways• Hyperemia, friability, mucosal edema, and

hypersecretion of the airway mucosa -- most pronounced in the upper airways, especially during the third trimester

• 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 advised

Lower airways

• 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.

PHYSIOLOGICAL CHANGES

• 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 trimester

• Residual volume to total lung capacity ratio is low in the third trimester.

• 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 late pregnancy

Decrease in transpulmonary pressure

Peripheral airway collapse

• Tidal volume increases 30 to 35 ( increased ventilatory drive )

Increase in minute ventilation

• Respiratory rate either does not change appreciably or increases slightly.

• 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 weeks

• The increase in minute ventilation results in a respiratory alkalosis with compensatory renal excretion of bicarbonate

• 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 hypoventilating

During parturition

• Respiratory responses during parturition are greatly affected by stage of labor and the response to pain and anxiety.

• 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 dyspnea.’’

• The increase in minute ventilation and the load imposed by the enlarging uterus cause an increase in the work of breathing.

• Other factors contribute to the sensation of dyspnea include increased pulmonary blood volume, anemia, and nasal congestion

• 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 echocardiography

• Abrupt or paroxysmal episodes of dyspnea suggest an abnormal condition

SUMMARY

Chest Wall/Lung Mechanics

• Chest wall compliance Decreased• Thoracic diameter Increased• Diaphragm Elevated• Lung compliance Unchanged

Lung Volumes

• Total Lung Capacity -Unchanged or slightly decreased• Vital capacity -Unchanged or slightly increased• Inspiratory capacity -Slightly increased• Functional residual capacity- Decreased• Residual volume -Slightly decreased• Expiratory reserve volume- Decreased

Spirometry• FEV1 Unchanged• FVC Unchanged• FEV1/FVC Unchanged

Gas Exchange• DCO Unchanged or slightly decreased

Ventilation• Minute ventilation Increased• Tidal volume Increased• Respiratory rate Unchanged

Blood gas

• pH 7.40 to 7.45• PaO2 Slightly elevated (100–105 mmHg)• PaCO2 decreased to 28 to 32 mmHg• Bicarbonate Slightly decreased (18 to 21 mEq/L)

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