Assisted Ventilation





But that life may, in a manner of speaking, be restored to the animal, an opening must be attempted in the trunk of the trachea, into which a tube or reed or cane should be put; you will then blow into this so that the lung may rise again and the animal take in air. Indeed, with a single breath in the case of this living animal, the lung will swell to the full extent of the thoracic cavity and the heart become strong and exhibit a wondrous variety of motions…when the lung long flaccid has collapsed, the beat of the heart and arteries appears wavy, creepy, twisting, but when the lung is inflated, it becomes strong again and swift and displays wondrous variations…as I do this, and take care that the lung is inflated at intervals, the motion of the heart and arteries does not stop. Andreas Vesalius, De Humani Corporis Fabrica (1543)


The primary objective of assisted ventilation is to support gas exchange until the patient’s ventilatory efforts are sufficient. Ventilation may be required during immediate care of the depressed or apneic infant, before evaluation and during treatment of an acute respiratory disorder, or for prolonged periods of treatment for respiratory failure. Trained personnel and equipment for emergency ventilation should be available in every delivery room and newborn nursery. Positive pressure ventilation effectively stabilizes most infants who require resuscitation.


This chapter is an introduction to assisted ventilation. Before undertaking assisted ventilation of any form, it must be recognized that the techniques demand time, resources, and experienced personnel. Prolonged ventilation should only be used in units where expert nurses, respiratory therapists, and medical personnel are continuously available.


Respiratory Failure


Hypercapnic respiratory failure is the inability to remove CO 2 by spontaneous respiratory efforts and results in an increasing arterial P co 2 (Pa co 2 ) and a decreasing pH. Assisted ventilation is most commonly needed to treat hypercapnic respiratory failure. Hypoxemia is usually (but not invariably) present; in many instances, arterial oxygenation can be normalized if the inspired oxygen is increased. Infants with hypoxemic respiratory failure have a predominant problem of oxygenation, usually the result of right-to-left shunt or severe ventilation-perfusion mismatch. Respiratory failure can occur because of disease in the lungs, or in other organs and systems ( Figure 11-1 ). Assisted ventilation is usually required when severe respiratory failure ensues ( Box 11-1 ). Depending on many clinical considerations (e.g., extreme prematurity), assisted ventilation may be initiated earlier.




Figure 11-1


Diagram of causes of respiratory distress in neonates. BP, Blood pressure; CVS, cardiovascular system; echo, echocardiogram; HCT, hematocrit.


Box 11-1

Indications for Assisted Ventilation


Respiratory acidosis with pH less than 7.20 to 7.25


Hypoxemia while on 100% oxygen or continuous positive airway pressure with 60% to 100% oxygen


Severe apnea



Clinical Manifestations of Respiratory Failure in the Newborn


The following are findings that should make the clinician suspect respiratory failure:




  • Worsening hypercapnia and/or hypoxemia



  • Increase or decrease in respiratory rate



  • Increase or decrease in respiratory efforts (grunting, flaring, retractions)



  • Periodic breathing with increasing prolongation of respiratory pauses



  • Apnea



  • Decreasing blood pressure with tachycardia associated with pallor, circulatory failure, and ultimately bradycardia



Cardiac Versus Pulmonary Disease


The clinician may frequently need to distinguish between cardiac and pulmonary disease in the sick newborn infant. Cyanotic heart disease may mimic respiratory disease. One possible way to differentiate between the two is to perform a hyperoxia test: place the infant in 100% oxygen for 10 minutes and then obtain an arterial Po 2 (Pa o 2 ). In infants with pulmonary disease, Pa o 2 usually increases to more than 100 mm Hg, whereas infants with cyanotic heart disease show little change in Pa o 2 . The hyperoxia test, although useful diagnostically, may also be misleading. In infants with severe pulmonary hypertension and right-to-left shunt, Pa o 2 may not elevate with 100% oxygen. Alternatively, Pa o 2 may increase more than 100 mm Hg early in life in infants with forms of cyanotic heart disease with high pulmonary blood flow (e.g., total anomalous pulmonary venous return). Echocardiography should be used to distinguish between cardiac and pulmonary disease when hypoxemia is unresponsive to ventilatory support.




Endotracheal Intubation


Most infants should receive positive pressure ventilation before attempting endotracheal intubation. This improves oxygenation and decreases Pa co 2 , thereby decreasing the likelihood of bradycardia during endotracheal intubation. Positive pressure ventilation is impractical for prolonged periods but can be used for the following:




  • Immediate resuscitation



  • Stabilization before and after endotracheal intubation



  • Ventilation in infants whose condition is deteriorating without obvious cause



  • Ventilation during transport to intensive care facilities when mechanical ventilation is unavailable



Mechanical ventilation is a highly invasive therapy and is indicated only when the benefits outweigh the burdens. In situations where there is little reasonable chance of survival, there should be honest discussions with the family regarding of the appropriateness of aggressive measures such as intubation.


Endotracheal Tube Size


It is preferable to use relatively small endotracheal tubes to prevent tracheal damage. The endotracheal tube should fit loosely enough to allow a leak of gas between tube and trachea when 10 cm H 2 O inspired pressure is generated. Tube size can be related to infant size or gestational age. Recommended sizes are as follows:


























Gestational Age (wk) Birth Weight (g) Endotracheal Tube Size (mm internal diameter)
Below 28 Below 1000 2.5
28-34 1000-2000 3.0
35-38 2000-3000 3.5
Above 38 Above 3000 3.5-4.0


Intubation


Insertion of an endotracheal tube should be performed with universal precautions under a radiant heat source to keep the infant warm. Free-flow oxygen should be administered as necessary.


Intubation can be a painful procedure and so premedication with an analgesic agent (morphine, fentanyl, or remifentanil) should be used for all non-emergent intubations in neonates. A muscle relaxant (paralytic agent) should only be used with analgesia. Other agents which may be considered include sedatives (midazolam) and vagolytic agents (atropine). The ideal combination and sequence of premedications in neonates has not yet been established. Each unit should develop protocols and lists of preferred medications to maximize safety.


The infant should receive positive pressure ventilation and oxygen as needed between attempts. The tip of the tube should be placed midway between the carina and the glottis. The length of insertion of the endotracheal tube is shown in Figure 11-2 . The following measurements can be used for endotracheal tube placement:





















Infant Weight (g) Endotracheal Tube Insertion Length (tip to lip, cm)
1000 7
2000 8
3000 9
4000 10



Figure 11-2


Graph for determination of length of insertion of endotracheal tubes. The tip of the endotracheal tube is aimed at the midtrachea.

(From Lough MD, Carlo WA: Clinical care techniques. In Carlo WA, Chatburn RL, editors: Neonatal respiratory care, Chicago, Year Book, 1988, p 122.)


At these lengths, the distal end of the endotracheal tube should be at the midtrachea. It is easy to inadvertently pass the tube into the right mainstem bronchus, but using these insertion length guidelines prevents this complication. Breath sounds should be equal bilaterally. A CO 2 detector should be used to confirm endotracheal placement. The tube should be secured so that movement of the head and neck will not dislodge it. Lightweight plastic connectors can be used to prevent kinking the tube. The endotracheal tube position should be checked radiographically.


Oral Intubation


The advantages of oral intubation are the relative ease of insertion and that a stylet can be used to aid insertion. Oral tubes should always be used in emergencies. The disadvantages of oral intubation are the increased tube mobility if the tube is inadequately secured and the greater difficulty in keeping the tube in position.


A laryngoscope with a Miller number 0 or 1 blade inserted in the vallecula is used to pull upward to visualize the glottis while leaving the head in a neutral position. It is important not to traumatize the gums and tooth buds. The heart rate should be monitored continuously with auditory and visual signals during attempts at intubation. Continuous O 2 saturation or transcutaneous Po 2 monitoring is invaluable because oxygenation can worsen abruptly. It is helpful if the tube has been previously curved. To stiffen the tube for orotracheal intubation, a stylet may be used or it may be cooled.


Nasal Intubation


The advantage of nasal intubation is the improved stability with the reduced likelihood of slippage into the right mainstem bronchus or accidental extubation. The disadvantages are trauma to the nares and nasal septum, greater difficulty in insertion of the tube, possibility of an increased number of gram-negative nasal superinfections, and potential trauma to the developing eustachian tubes and sinuses. Nasotracheal intubation should always be performed as an elective procedure and should not be done in emergencies.


Using a laryngoscope blade, the lubricated endotracheal tube is inserted through the nares until it is visualized in the oropharynx. The McGill forceps are used to guide the tube into the glottis. It is helpful if the endotracheal tube has been previously lubricated with a nontoxic, water-soluble lubricant. A stylet is never used for nasotracheal intubation.


Suctioning


Suctioning can be done if there are copious amounts of secretions or suspicion of endotracheal tube occlusion by secretions, but routine suctioning is not necessary. Strict sterile technique with disposable gloves and suction tubes is necessary. The infant should be allowed to recover between episodes of suctioning by maintaining stable O 2 saturations with increases in inspired oxygen concentrations as needed and by reexpanding the lung with 10% to 20% more pressure than used for routine ventilation. Saline instillation is done to facilitate removal of secretions when secretions are thick.


Although sometimes necessary, suctioning is potentially dangerous; it may cause a hypoxic episode owing to discontinuation of ventilation, extraction of gas from small airways, or atelectasis. It may also produce lesions in the trachea at the site of the suction catheter tip. Use of a special endotracheal tube connector allows mechanical ventilation during suctioning and prevents the catheter tip from going beyond the endotracheal tube.



EDITORIAL COMMENT


Suctioning of the endotracheal tube will decrease oxygenation and pulmonary function. I would advocate being guided by pulse oximetry during the procedure, which may require transiently increasing inspired oxygen concentration by 10% to 15% immediately before and following the period of suctioning. I would also advocate avoiding the practice of “routine” suctioning except as secretions warrant.


John Kattwinkel



Changing an Endotracheal Tube


An endotracheal tube change is required only if the tube becomes dislodged or occluded or if the infant outgrows it. Routine change is not indicated.




Applied Pulmonary Mechanics


The following principles are helpful in understanding mechanical ventilation. A pressure gradient between the airway opening and alveoli must exist to drive the flow of gases during both inspiration and expiration. The pressure gradient required to inflate the lungs is determined largely by the compliance and the resistance of the lungs.


Compliance


Compliance is a property of distensibility (i.e., of the lungs and chest wall) and is calculated from the change in volume per unit change in pressure:


<SPAN role=presentation tabIndex=0 id=MathJax-Element-1-Frame class=MathJax style="POSITION: relative" data-mathml='Compliance=ΔVolumeΔPressure’>Compliance=ΔVolumeΔPressureCompliance=ΔVolumeΔPressure
Compliance = Δ Volume Δ Pressure


The higher the compliance, the larger the delivered volume per unit of pressure. Compliance in babies with normal lungs ranges from 3 to 6 mL/cm H 2 O. Compliance in infants with respiratory distress syndrome (RDS) ranges from 0.1 to 1 mL/cm H 2 O.


Resistance


Resistance is a property of the inherent capacity of the gas-conducting system (i.e., airways, endotracheal tube, and lung tissue) to oppose airflow and is expressed as the change in pressure per unit change in flow:


<SPAN role=presentation tabIndex=0 id=MathJax-Element-2-Frame class=MathJax style="POSITION: relative" data-mathml='Resistance=ΔPressureΔFlow’>Resistance=ΔPressureΔFlowResistance=ΔPressureΔFlow
Resistance = Δ Pressure Δ Flow


Resistance in babies with normal lungs ranges from 25 to 50 cm H 2 O/L/second. Resistance is not dramatically altered in infants with RDS but is increased in intubated infants and ranges from 50 to 150 cm H 2 O/L/second.


Time Constant


Time constant is a measure of the time (expressed in seconds) necessary for 63% of a step change (e.g., airway pressure gradient) toward equilibration. A step change in airway pressure occurs between the beginning and the end of a machine-delivered inspiration (during pressure-limited, time-cycled ventilation). The product of compliance and resistance determines the time constant of the respiratory system:


<SPAN role=presentation tabIndex=0 id=MathJax-Element-3-Frame class=MathJax style="POSITION: relative" data-mathml='Timeconstant=Compliance×Resistance’>Timeconstant=Compliance×ResistanceTimeconstant=Compliance×Resistance
Time constant = Compliance × Resistance


For example, in an infant with normal lungs:


<SPAN role=presentation tabIndex=0 id=MathJax-Element-4-Frame class=MathJax style="POSITION: relative" data-mathml='Onetimeconstant=0.005L/cmH2O×25cmH2O/L/second=0.125second’>Onetimeconstant=0.005L/cmH2O×25cmH2O/L/second=0.125secondOnetimeconstant=0.005L/cmH2O×25cmH2O/L/second=0.125second
One time constant = 0.005 L/ cm H 2 O × 25 cm H 2 O/ L/ second = 0 .125 second

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Sep 29, 2019 | Posted by in PEDIATRICS | Comments Off on Assisted Ventilation
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