Atlas of Procedures in Neonatology, 4th Edition

Physiologic Monitoring

7

Cardiac/Respiratory Monitoring

Rebecca J. Eick

The monitoring of vital signs in neonates provides an important indicator of overall well-being. Progress in computer technology has facilitated the development of bedside monitors that can integrate multiple monitoring parameters into a single system. This chapter covers the fundamentals of cardiac and respiratory monitoring.

Cardiac Monitoring

  1. Purpose
  2. To provide reliable and accurate monitoring of neonatal cardiac activity
  3. Provide trends of heart rate over time
  4. Monitor beat-to-beat heart rate variability (1,2)
  5. To allow assessment and surveillance of critically ill neonates
  6. To provide early warning of potentially significant changes in heart rate by identification of heart rates above or below preset alarm limits
  7. To identify bradycardia (with or without associated apnea) in at-risk infants
  8. Background
  9. Electrical activity of the heart is detected using impedance technology through skin surface electrodes (3).
  10. The low-level electrical signal is amplified and filtered to eliminate interference and artifacts.
  11. The electrical signal, defined in millivolts, is displayed as an electrocardiogram (ECG) tracing.
  12. R-wave detection from the QRS complex is used to calculate heart rate.
  13. The typical three-lead configuration (i.e., leads I, II, III) provides alternative vectors for ECG analysis.
  14. Contraindications

None

  1. Limitations
  2. The three-lead ECG is most useful for long-term continuous cardiac monitoring; more detailed cardiac evaluation (i.e., assessment of hypertrophy or axis) or the identification of abnormal cardiac rhythms may require complete 12-lead ECG with rhythm strip.
  3. Close proximity of electrodes in extremely small infants may cause difficulty with signal detection.
  4. Equipment

Hardware—Specifications

  1. The monitoring system should have the appropriate frequency response and sensitivity to track the fast and narrow QRS complex of the neonate accurately.
  2. Heart rate is processed on a beat-to-beat basis with a short updating interval.
  3. Default heart rate alarm limits should be tailored to the neonatal population.
  4. Low heart rate (bradycardia) limit of 100 beats/min (Note: Some term infants may have resting heart rates of 80 to 100 beats/min, requiring lower bradycardia alarm settings.)
  5. High heart rate (tachycardia) limit of 175 to 200 beats/min
  6. Monitor displays
  7. Cathode-ray tube (CRT)
  8. Has highest resolution and best definition
  9. Display can be either color or monochrome
  10. Liquid-crystal display (LCD)
  11. Flat, thin display monitor
  12. Resolution may be suboptimal for fast and narrow QRS complex of neonate
  13. Back-lighting is necessary for viewing in low-light environments
  14. Unlike with CRT, viewing angle is critical
  15. Heart rate displayed as alphanumeric part of waveform display or in a separate numerical display window

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  1. Recorder (optional)

 .   Electronic memory

  1. Real-time ECG
  2. Delayed ECG—stored retrospective display used primarily for review of short time interval prior to the occurrence of an alarm
  3. Printed record of ECG trend information
  4. Typically used to document selected segments of ECG tracings such as periods associated with alarms or abnormal rhythms
  5. Monitors may have dedicated printers (often integrated into monitor cases)
  6. Central monitoring stations can provide remote access to information from all networked monitor units with printing capabilities.
  7. Units available for both bedside and transport monitoring (Figs. 7.1 and 7.2)

 .   Transport monitors typically smaller and battery-powered

  1. Similar capabilities regarding parameter availability, but monitor-specific
 

FIG. 7.1. Typical multiparameter neonatal bedside monitor. (Courtesy of Philips Medical Systems.)

Consumables—Specifications

  1. Disposable neonatal ECG electrodes
  2. Silver–silver chloride electrodes are available in a variety of forms designed specifically for the neonatal population.
  3. Patient contact surfaces of electrodes are coated in adhesive electrolyte gel, which acts as conductive medium between the patient and the metal lead while preventing direct patient contact with the metal.
  4. Typical commercially available neonatal leads incorporate silver–silver chloride electrodes directly onto paper, foam, or fabric bodies with integrated lead wires.
  5. Less commonly, adhesive electrode pads are separate from lead wires, which connect to the electrodes via clips.
  6. ECG limb plate electrodes may be used rarely, when the application of chest leads would interfere with resuscitation or the performance of other procedures. Use of electrode gel as a conductor at the skin interface (rather than alcohol pads) is imperative in such cases.
  7. Characteristics to consider in electrode selection:
  8. Adherence to skin of an active infant
  9. Quality of signal attained
  10. Minimal skin irritation
  11. Ease of removal using water or adhesive remover without damage to or removal of skin
  12. Performance in the warm, moist environment of an infant incubator
  13. Adhesive–skin interaction under overhead infant warmers
 

FIG. 7.2. Typical multiparameter neonatal transport monitor with integrated printer. (Courtesy of Philips Medical Systems.)

  1. Lead wires and patient cable
  2. All cables should be clean and the insulation should be free of nicks or cuts.

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  1. Lead wires should lock or snap into the patient cable, preventing easy disconnections.
  2. If using electrodes that attach via clips, use infant/pediatric lead wires with small electrode clips—standard adult-size clips will place too much torsion on the infant electrode, tugging on the skin and possibly peeling off the electrode.
  3. Precautions
  4. Do not leave alcohol wipes under electrodes as conductors.
  5. Do not apply electrodes to broken or bruised skin.
  6. Do not apply electrodes to clear film plastic dressings—dressing will act as an insulator between the skin and the electrode.
  7. To avoid skin damage, do not use fingernails to remove electrodes.
  8. Secure the patient cable to the patient's environment to prevent excessive traction.
  9. Use only monitors that have been checked for safety and performance—usually indicated by a dated sticker on the monitor.
  10. Do not use monitors with defects such as exposed wires, broken or dented casing, broken knobs or controls, or cracked display.
  11. Monitor alarms should prompt immediate patient assessment.
  12. Note alarm indication (i.e., tachycardia or bradycardia).
  13. Treat patient condition as necessary or correct the source of any false alarm.
  14. If alarm is silenced or deactivated during the course of patient evaluation, it should be reactivated prior to leaving the patient's bedside.
 

FIG. 7.3. Basic electrode placement and lead vectors for optimal electrocardiogram signal detection. Right arm/left arm positions also provide maximal signal for impedance pneumography.

  1. Techniques
  2. Familiarize yourself with the monitor prior to beginning.
  3. Electrode and lead wire placement: Although you should refer to the monitor manufacturer's placement instructions, general electrode placement guidelines follow.
  4. Skin preparation:Skin should be clean and dry to provide the best electrode-to-skin interface.
  5. Wipe skin with an alcohol pad and allow to dry thoroughly.
  6. Avoid the use of tape to secure electrodes—for optimal performance and proper electrical interface, electrodes must adhere directly to skin.
  7. Basic three-lead configuration for electrode placement (for electrodes with integrated lead wires) (Fig. 7.3)
  8. Right arm (white): right lateral chest at level of the nipple line
  9. Left arm (black): left lateral chest at level of the nipple line
  10. Left leg (red or green): left lower rib cage
  11. Although this configuration allows the use of the same electrodes to monitor both ECG and respiration, optimal ECG signal may be obtained when the right arm lead is at the right mid-clavicle and the left leg lead is at the xiphoid (4).
  12. If not using electrodes with integrated wires, place electrode pads in basic three-lead configuration as above, then connect lead wires via electrode clips.
  13. White lead (right arm) to right chest electrode
  14. Black lead (left arm) to left chest electrode
  15. Red or green lead (left leg) to left lower rib cage electrode
  16. Turn monitor on—most monitors will conduct an automatic self-test.
  17. Connect the patient cable to the monitor.

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  1. Select the lead that provides the best signal and QRS size (lead II is usual default) (Fig 7.4).
  2. Ensure that heart rate correlates to QRS complexes seen on display—make sure that the QRS detector is not counting high or peaked T or P waves.
  3. Verify that low and high heart rate alarms are set appropriately.
 

FIG. 7.4. Typical electrocardiogram tracings: lead I (top), lead II (middle), and lead III (bottom).

  1. Complications
  2. Skin lesions (rare)
  3. Irritation from alcohol—may occur with even short-term application to immature skin
  4. Trauma caused by rubbing with excessive vigor during skin preparation
  5. Irritation from incorrectly formulated electrode gel
  6. Secondary effects of skin breakdown
  7. Cellulitis or abscess formation
  8. Increased transepidermal water losses
  9. Hypo- or hyperpigmented marks at sites of prior irritation or inflammation (Fig. 7.5)
 

FIG. 7.5. Residual hyperpigmented marks on the extremities present more than 1 year after application of electrocardiogram leads for cardiorespiratory monitoring.

TABLE 7.1 Steps to Minimize Artifact Interference

Problem

Treatment

Poor electrode contact/connection

5. Gently clean skin with alcohol wipe and allow to dry prior to electrode reapplication

6. Check electrode/cable connections

Dried electrode

Replace

Equipment interference

7. Systematically turn off one piece of adjacent equipment at a time while observing monitor for improvement in signal quality

8. Once source for interference identified, increase distance between that equipment and patient while rerouting power cords and cables as necessary

9. If above maneuver unsuccessful, replace equipment

60-hertz interference

10.     Follow procedure for poor electrode contact

11.     Replace patient cable

12.     If 1 and 2 unsuccessful, try alternate monitor

  1. Erroneous readings caused by artifacts (5) (Table 7.1)

 .   Electrical interference

  1. Sixty-cycle electrical interference (frequency of typical power lines)
  2. Interference from other equipment used in the patient's immediate environment

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  1. Electrical spike may be generated when certain types of polyvinyl chloride tubing are mechanically deformed by infusion pump devices—spikes appear as ectopic beats on the monitor (rare) (6).
  2. Decreased signal amplitude with motion artifact
  3. Poor electrode contact or dried electrode gel
  4. Incorrect vectors because of inaccurate lead placement (Fig. 7.6)
  5. Inappropriate sensitivity settings
  6. Monitor or cable failure

 .   Hardware or software failure

  1. Cable disconnection
  2. Alarm failure

 .   False alarms (either tachycardia or bradycardia) resulting from inaccurate interpretation of heart rate

  1. Inappropriate alarm parameters for patient
 

FIG. 7.6. Normal P-, QRS-, and T-wave detection. Top: Lead II tracing with electrodes properly placed. Note normal P-, QRS-, and T-wave detection. Middle: Lead II tracing with electrodes close together on anterior chest wall. Note altered QRS- and decreased T-wave amplitude. Bottom: Lead II tracing with electrodes placed lateral on the abdomen. Note decreased wave amplitude and flattened P wave.

Respiratory Monitoring

  1. Purpose
  2. Reliable and accurate monitoring of neonatal respiratory activity
  3. Trending of respiratory activity over time
  4. Detection of apnea
  5. Assessment and surveillance of critically ill neonates
  6. To provide early warning of potentially significant changes in respiratory rate by identifying respiratory rates above or below preset alarm limits
  7. Background
  8. Measurement of transthoracic impedance is the most commonly used method for determining respiratory rate (7).
  9. A low-level, high-frequency signal is passed through the patient's chest via surface electrodes.
  10. Typically utilizes the same electrodes as are used for cardiac monitoring
  11. Signal path usually from right arm (white) to left arm (black) electrodes, although some monitors may use right arm (white) to left leg (red or green) (Fig. 7.7)
  12. Impedance to the high-frequency signal is measured.
  13. Impedance is the electrical resistance to the signal.
  14. Changes in lung inflation cause an alteration in the density of the chest cavity, which is detected as a change in impedance.
  15. Changes in impedance modulate a proportional change in the amplitude of the high-frequency signal.
  16. The change in impedance, as seen by the modulation of the high-frequency signal, is detected and quantified by the monitor and recorded as breaths per minute.
  17. The monitor has an impedance threshold limit below which changes in impedance are not counted as valid respiratory activity—cardiac pumping with associated changes in pulmonary blood flow will also cause changes in thoracic impedance (usually much smaller changes than those associated with respiration).

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FIG. 7.7. Transthoracic impedance pneumography: diagrammatic representation of the path of the high-frequency signal between chest wall electrodes. Most monitors transmit signal right arm (white) → left arm (black), less commonly right arm (white) → left leg (red).

  1. Contraindications

None

  1. Equipment

Hardware—Specifications

  1. Equipment is the same as that for cardiac monitoring; multiparameter monitors incorporate both cardiac and respiratory monitoring into single units.
  2. Respiratory monitoring parameters
  3. Low-level threshold (for impedance) for breath validation should not be below 0.2 to minimize cardiogenic artifact.
  4. Coincidence alarm with rejection applies when respiratory rate being detected is equal to the heart rate activity being detected by the cardiac portion of the system.
  5. Default limits should be tailored to the neonatal population.
  6. Adjustable apnea time-delay setting (length of apnea in seconds before alarming)
  7. Typical apnea time delay is 15 to 20 seconds.

Consumables—Specifications

  • Same as for cardiac monitor
  1. Precautions
  2. Include previously discussed precautions for cardiac monitoring
  3. Muscular activity may be interpreted as respiration, resulting in failure to alarm during an apneic episode (see G3a following).
  4. Technique
  5. Same as for cardiac monitor
  6. Ensure that the respiratory waveform correlates to the true initiation of inspiration.
  7. Move right and left arm electrodes up toward the axillary area if detection of respiration is poor due to shallow breathing.
  8. Set desired low and high respiratory rate and apnea delay alarm limits.
  9. Complications
  10. Skin lesions (see H1 under Cardiac Monitoring)
  11. Monitor or cable failure
  12. Hardware or software failure
  13. Cable disconnection
  14. Alarm failure
  15. False-positive “respiratory” signal in the absence of effective ventilation
  16. Chest wall movement with airway obstruction (obstructive apnea)
  17. Nonrespiratory muscular action (i.e., stretching, seizure, or hiccups) producing motion artifact (Fig 7.8)
  18. False apnea alarm despite normal respiratory activity
  19. Improper sensitivity not detecting present respiratory activity
  20. Incorrect electrode placement
  21. Loose electrodes
  22. Inappropriate alarm parameters for patient
  23. Accurate assessment of respiratory rate not practical when using high-frequency ventilatory modes

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FIG. 7.8. Tracings of artifacts affecting electrocardiogram/ respiratory tracings. Top: Loose electrode affected by motion. Bottom: Motion artifact caused by patient's moving arm coming in contact with chest electrodes (note change in respiratory frequency signal).

Cardiorespirograph Monitoring

  1. Definition
  2. Graphical representation of heart rate and respiratory rate over time
  3. Purpose
  4. Monitoring of infants for identification and quantification of heart rate and respiratory activity, with detection of apnea, periodic breathing, and bradycardia
  5. Identification of chronologic relationships between bradycardia and apnea
  6. Many systems also provide continuous SaO2information to allow correlation with desaturation events.
  7. Background
  8. Heart rate is plotted graphically as beats per minute (Y axis) versus time (X axis).
  9. Respiratory waveform is compressed to allow display of time range.
  10. Short-term trending allows constant updating as the oldest information is displaced (typically based on a 2-minute window of time).
  11. Time relationship between heart rate and respiratory activity is maintained.
  12. Allows for visualization of entire apneic episodes and identification of precipitating factors (e.g., a drop in respiratory rate may precede bradycardia)
  13. Inclusion of SaO2allows identification of temporal relationship for desaturation events (SaO2is plotted in the same fashion as the heart rate on a second Y axis)
  14. Contraindications

None

  1. Equipment

Standard features of most neonatal monitors

References

  1. Cabal LA, Siassi B, Zanini B, et al. Factors affecting heart rate variability in preterm infants. Pediatrics.1980;65:50.
  2. Valimaki IA, Rautaharju PM, Roy SB, et al. Heart rate patterns in healthy term and premature infants and in respiratory distress syndrome. Eur J Cardiol.1974;1:411.
  3. Di Fiore JM.Neonatal cardiorespiratory monitoring techniques. Semin Neonatol. 2004;9:195.
  4. Baird TM, Goydos JM, Neuman MR.Optimal electrode location for monitoring the ECG and breathing in neonates. Pediatr Pulmonol.1992;12:247.
  5. Jacobs MK.Sources of measurement error in noninvasive electronic instrumentation. Nurs Clin North Am. 1978;13:573.
  6. Sahn DJ, Vaucher YE.Electrical current leakage transmitted to an infant via an IV controller: an unusual ECG artifact. J Pediatr.1976;89:301.
  7. Hintz SR, Wong RJ, Stevenson DK.Biomedical engineering aspects of neonatal monitoring. In: Martin RJ, Fanaroff AA, Walsh MC, eds. Fanaroff and Martin's Neonatal-Perinatal Medicine: Diseases of the Fetus and Infant. 8th ed. Philadelphia: Mosby; 2006:609.