ساعت ۸:٥٢ ‎ق.ظ روز ۱۳٩٠/٤/۳٠   کلمات کلیدی: respiration ،vital signs
RESPIRATION

 

Respiratory frequency reveals only part of the clinical picture. The pattern, effort, and volume of respiration may be more indicative of altered respiratory physiology. An abnormality in respiration may be a primary complaint or a manifestation of other systemic disease.

Physiology

 

Breathing is initiated and primarily controlled in the medullary respiratory center in the brainstem. The respiratory center is modulated by the pneumotaxic and apneustic centers in the pons. The pneumotaxic center limits the length of the inspiratory signal and therefore can greatly increase or decrease RR.

In addition to being modified by other areas of the brainstem, the medullary respiratory center is modified by voluntary centers in the cerebral cortex; pulmonary stretch receptors of the airways; type J or juxtapulmonary capillary receptors of the pulmonary capillaries; arterial baroreceptors of the carotid sinus; and receptors found in skeletal muscle, tendons, and joints. Central and peripheral chemoreceptors also influence RR.[24]

Indications and Contraindications

 

Generally, ED patients should have their RR documented during their evaluation. Repeated assessment and documentation of the patient's respiratory status are indicated in patients who present with an abnormal RR or a complaint referable to the airway or breathing.

The only contraindications to a careful measurement of RR are the scenarios of respiratory distress, apnea, or upper airway obstruction that require immediate therapeutic intervention. A measurement of RR and effort should be performed as soon as patient care demands allow it in these circumstances.

Observation and palpation of chest movement are the techniques most frequently used to monitor respiratory frequency and amplitude noninvasively. Discussion of inductive plethysmography and other noninvasive monitors are beyond the scope of this chapter. Periodic manual measurements as described later generally suffice for ED patients.

Respiratory status in both adults and children plays a crucial role in determining the overall assessment of illness. Although it is a sensitive yet nonspecific indicator of respiratory dysfunction, the RR can also predict nonpulmonary morbidity. Several prehospital- and hospital-based illness or injury severity scores feature RR as a cardinal value. An prehospital RR <10 or >29 is associated with a major injury in 73% of children.[25] Other studies have linked abnormal RRs to inhospital mortality and level of care required in the ED.[26][27] Using tachypnea alone as a predictor for pulmonary pathology, infants with a RR of >60 are found to be hypoxic 80% of the time.[28]

Procedure

 

RR is the number of inspirations per minute. Generally, it is best measured with the patient unaware that breathing is being observed because awareness makes the patient conscious of the breathing pattern, which may alter the rate. Commonly, examiners count respirations while appearing to count the pulse. The RR is most accurately determined by counting for a full minute. Because the frequency is much less than the pulse, and breathing is less regular, an inaccurate measurement is more likely to occur if a 15-second interval is used.

Infants, in addition to being principally nasal breathers, are predominantly diaphragmatic breathers, and an infant's RR is easily determined by observing or palpating excursion of the chest or the abdominal wall.[29]

Complications

 

There are no inherent complications from measuring respiration by observation. Problems related to the measurement of RR are generally due to failure to recognize a patient in obvious respiratory distress or failure to monitor RR in a patient who may be at risk for respiratory depression (e.g., in the case of sedative-hypnotic or narcotic overdose).

Interpretation

 

Respiratory Rate

 

A limited number of studies have examined RRs. Hutchinson evaluated RRs in 1897 healthy males at rest and found that 91% had RRs between 16 and 24 breaths/min. He also noted that 30% had exactly 20 breaths/min.[30] Hooker and colleagues note that current texts vary considerably in their definitions of a normal RR and cite published values that range from 8 to 20 breaths/min.[31]

Hooker and associates, in a study that specifically investigated normal RRs in an ED, measured RRs in 110 afebrile ambulatory patients without respiratory complaints (53 females and 57 males).[31] They report a mean rate of 20.1 breaths/min. For patients whose RR was measured again before release from the ED, no significant difference was noted between initial and subsequent RRs. When analyzed by gender, females had a mean RR of 20.9 breaths/min and males had a mean RR of 19.4 breaths/min, a statistically significant difference. The researchers concluded that a normal RR in the adult patient population was 16 to 24 breaths/min.[31] This study also suggested a significant variability in the accurate measurement of RR by different examiners. Rates obtained by nurses versus medical students varied significantly, as did those obtained by medical students versus residents versus attending clinicians.[1]

Other studies have provided additional information on normal resting and sleep state RRs in children younger than 7 years.[14][15][16][17][18] RRs obtained with a stethoscope were higher than those obtained by observation (mean difference, 2.6 breaths/min in awake and 1.8 breaths/min in asleep children). Smoothed percentile curves demonstrated a larger dispersion at birth (5th percentile, 34 breaths/min; 95th percentile, 68 breaths/min), while at 36 months of age (5th percentile, 18 breaths/min; 95th percentile, 30 breaths/min) dispersion was less.

RR will generally increase in the presence of fever. It is often difficult to determine if tachypnea is a primary finding or simply associated with hyperpyrexia. Taylor et al.[32] studied 572 children younger than 2 years of age, 42 (7%) of whom were subsequently diagnosed with pneumonia and found that age-appropriate limits for resting tachypnea in the presence of fever could be defined. A sensitivity and specificity of 74% and 77% for pneumonia was achieved when children 6 months of age had a RR >59/min, those aged 6–11 months had a RR >52/min and those 1–2 years had a RR >42/min. Therefore, even in the face of physiologic compensation for fever, an interpretation of RR alone can help predict the presence of pulmonary disease.

Respiratory Pattern and Amplitude

 

Abnormal respiratory patterns may be characteristic of metabolic or central nervous system pathologic conditions. Hyperventilation and hypoventilation may result from an extensive differential diagnosis including primary pulmonary disorders, such as pneumonia or chest wall pain. Respiratory disturbances also occur secondary to other disease processes. For example, Kussmaul respiration describes the hyperventilation pattern seen in diabetics with ketoacidosis. Abnormal respiratory patterns in adults can be used in differential diagnosis or in determining the location of central nervous system lesions.

The recognition of subtle tachypnea can be difficult in the emergency setting, although this can be the solitary harbinger of disease. Measurement of an accurate RR in this patient population is crucial. Another instance of pathology that can confuse the routine measurement of RR is diaphragmatic breathing or retractions. The variability in counting respiratory effort versus effective respirations is generally not appreciated in a single recorded value.

Respiratory patterns in children must be observed carefully. In infants, periodic breathing, which may be normal, must be distinguished from apnea. By definition, periodic breathing consists of three or more respiratory pauses >3 seconds in duration, with <20 seconds between pauses. There is no associated bradycardia or cyanosis. This contrasts with apnea, which is a particular problem in preterm infants. Apnea is defined as a respiratory pause of >20 seconds. It may be associated with bradycardia and hypoxia.[29] Periodic breathing and apnea are believed to be disorders on a continuum, both stemming from abnormal physiologic control of respiration. However, periodic breathing is considered a benign disorder, whereas infants with symptomatic apneic episodes resulting in an apparent life-threatening event (ALTE) are thought to be at increased risk for sudden infant death syndrome.[33]