Anatomy - Assets - Cambridge University Pressassets.cambridge.org/97811076/98680/excerpt/9781107698680_exc… · sents 24% of the neonatal heat loss. Evaporative losses include sensible

Chapter

1Anatomy

Maria Zestos

IntroductionThe anatomical differences between the infant and theadult are numerous and can greatly affect the care ofthe infant and, most notably, the neonate. This chap-ter presents an overview of anatomical differencesbetween the neonate and the adult, including generaldevelopment, with a focus on the airway, body habi-tus, thermoregulation, and vascular cannulation.

General developmentAirwayRespiratory development begins by the fourth week ofgestation with a primitive pharynx, larynx, trachea,and bronchial bud. The trachea and the esophagusdevelop from the foregut, and then separate duringdivision of the endoderm. If this separation fails, atracheoesophageal fistula (TEF) lesion will result.With successive branching of the airways, the respira-tory tree is formed. By 24 weeks gestation, respiratorybronchioles and primitive alveoli are present andsurfactant begins to be produced. Adequate airexchange requires surfactants to maintain alveolarexpansion and adequate lung exchange.[1] Eventhough surfactants can be administered after delivery,adequate air exchange for survival limits the viabilityof premature infants delivered before 23 weeks gesta-tion. The alveoli continue to increase in number andsize until 8 years of age, after which alveoli increaseonly in size.

Many morphological differences exist between theneonatal airway and the adult airway. In neonates, theepiglottis and tongue are relatively large. Other ana-tomical differences in the pediatric airway includelarge head, short neck, narrow nares, redundant softtissues, and a high glottis. All these features can makemask ventilation and direct laryngoscopy a challenge.

Neonates are obligate nasal breathers, with about22% of term infants being unable to breathe if thenares are occluded. Most infants gain the ability tocompensate for nares occlusion with oral breathingby 5 months of age. The relatively large tongue of theneonate can result in upper airway obstruction, ascan be seen in certain syndromes such as Beckwith–Wiedemann and Down syndrome. These anatomicaldifferences along with the high oxygen consumptioncanmake hypoxia more common and alsomore severe.

The anatomy of the airway changes with age. Theglottis functions as an occlusive valve to protect thelower airway from the alimentary tract. The glottis isat the level of C3 in babies, moving caudad to thelevel of C4–5 in adults. The shape of the larynx alsochanges with age. Infants have a cricoid cartilage thatis narrower than that of an adult. This creates a vocalcord aperture that is funnel shaped. As the childgrows, the diameter of the cricoid cartilage alsoincreases resulting in a cylindrical shape of the larynxby the age of 8 years. At the level of the cricoid, thecartilage forms a complete ring to prevent compres-sion. The larynx at the subglottis is the narrowestportion of the respiratory system for all ages.

Delayed development in the neuromuscular toneof the supraglottic muscles can result in laryngoma-lacia with inward collapse of supraglottic structures,namely, the aryepiglottic folds or the anterior collapseof the arytenoid cartilages. As the neuromuscular toneimproves during the first two years of life, symptomsalso improve and often disappear completely.[2]

Body habitusThe degree of difference and variation between neo-nates and adults is striking. When compared to anadult, a newborn infant is 1/21 adult size in weight,1/9 adult size in body surface area, and 1/3 adult size

Essentials of Pediatric Anesthesiology, ed. Alan David Kaye, Charles James Fox and James H. Diaz. Published by CambridgeUniversity Press. © Cambridge University Press 2015.

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in length. It is thus essential to choose carefully thevariable used to compare patients of different sizes.[3]

For medication administration, the actual bodyweight is most commonly used. Drug dosing can alsobe based on ideal body weight or lean body mass.Most obese patients have increased total body weightas well as lean body mass. For these patients, idealbody weight has been shown to be the best measure-ment for dose calculation, but this is not often usedin clinical practice. Calculations for fluids and dosesof medications can be based on body surface area(BSA), which requires both a height and a weightmeasurement. The BSA can be estimated byMosteller’scalculation:[4]

BSA (m2)¼ [ht (cm) ×wt (kg)/3600]½

Body fluid compartment composition also varies withage, with an abrupt fall in total body water (TBW)and extracellular fluid levels (ECF) over the first yearof life, reaching adult levels by 2 years of age.[5]

Water volume and blood compositionTotal body water (TBW) varies inversely with age.While the newborn has 85% TBW, this percentagesteadily decreases to the adult level of 65% TBW by3 years of age. Males also have a higher TBW com-pared to females. The body’s water can be divided intointracellular fluid (ICF), which contains 67% of thewater distribution and extracellular fluid (ECF),which contains 33% of the water distribution. Diffu-sion across the cell membrane results in fluidexchange between the ECF and the ICF. Fluid willmove from an area of low osmolality to an area ofhigh osmolality. The body regulates ECF volume byvarying renal sodium excretion and controls renalosmolality by varying water intake and excretion.

Normal fluid management in the perioperativeperiod includes administration of maintenance, pre-operative deficit and replacement of ongoing losses.Maintenance requirements consist of replacinginsensible losses through the skin and lungs, andurinary volume replacement.[6] Fluid requirementsin infants are greater than adults because of greatersurface-to-weight ratio and higher metabolic rate aswell as reduced renal concentrating ability. In thenewborn, day 1 maintenance fluids are decreasedbecause of immature renal function that slowlyimproves during the first few days of life. For day1 of life, D10W without added salt is administered at arate of 80ml/kg/day. By day 2 of life, sodium

excretion from the kidney has begun and urine outputimproves, resulting in a change of maintenance fluidsto include sodium replacement (2–3mEq/dl NaCl).This is usually given in the form of D10W with 0.2NSat a rate of 100ml/kg/day. By day three of life, potas-sium replacement (1–2mEq/dl KCl) is begun and totalfluids are increased at a rate of 120ml/kg/day. Outsideof the neonatal period, the most commonly used for-mula for calculating hourly maintenance fluid peri-operatively consists of the following calculation:[7]

“4–2–1 rule” for hourly maintenance fluid rate:4ml/kg/hr for the first 1–10 kg body weight plus2ml/kg/hr for each kg from 11–20 kg plus1ml/kg/hr for every kg> 20 kg

Maintenance fluids routinely provide dextrose, usu-ally as 5% dextrose, and sodium supplementation as0.2–0.45 NS. However, in the operating room routinedextrose administration is no longer advised inhealthy children. Moreover, isotonic fluids have beenshown to be significantly safer than hypotonic fluidsfor protection against postoperative hyponatremia inchildren.[8] Thus perioperative maintenance fluids areusually replaced with a balanced salt solution such asRinger’s lactate or 0.9% normal saline solution, exceptin patients at risk for hypoglycemia, such as neonates,children receiving hyperalimentation, and childrenwith endocrinopathies.

Fluid deficits must also be replaced, and may besignificant in the presence of prolonged fasting, fever,vomiting, or diarrhea. Preoperative fasting should beminimized to avoid significant dehydration or hypo-glycemia in infants. Specific instructions should begiven for infants to be encouraged to ingest clearfluids up until two hours before elective surgery.

Third-space losses from surgical trauma, burns orinfection result in isotonic fluid transfer from the ECFto the interstitial compartment with resultant plasmavolume depletion. These losses can be as high as10ml/kg/hr for major intra-abdominal surgery andeven 50ml/kg/hr for a premature infant undergoingsurgery for necrotizing enterocolitis. These losses canbe replaced with Ringer’s lactate solution. Adminis-tration of fluid should be titrated to the clinicalresponse, with maintenance of appropriate hemody-namic variables and a minimum urine output of0.5–1ml/kg/hr.

In addition to fluid replacement, blood loss needsto be closely monitored with prompt replacementas needed. Blood loss is initially replaced with a crys-talloid such as lactated Ringer’s as 3ml per 1ml

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blood lost to an acceptable minimal hematocrit, afterwhich packed red cells should be administered.Factors that determine the maximum allowable bloodloss (MABL) include the patient’s estimated bloodvolume (EBV), body weight and starting hematocrit(HCT). Estimated blood volume decreases with age(see Table 1.1). The MABL can be calculated with thefollowing formula:

MABL¼ EBV× [HCTstart�HCTlow]/HCTstart

ThermoregulationUnder normal conditions, body temperature is one ofthe most accurately maintained physiologic param-eters. The outer skin serves as a shell with the musclecompartment acting as a buffer. Thermoregulatorymechanisms including vasoconstriction can sparebody heat and decrease heat loss up to 50%. In adults,shivering is a main component of this process. Ininfants and neonates, brown fat provides nonshiver-ing thermogenesis. Brown fat can represent 2–6% ofneonatal body weight and is found in the scapulae,axillae, mediastinum, adrenal glands, and the kidneys.Nonshivering thermogenesis from brown fat is themain thermoregulatory response to cold stress in theneonate and can double metabolic heat productionduring cold exposures.[9] This ability persists up totwo years of age.

General anesthesia disrupts thermoregulationby producing vasodilation by two mechanisms. Itreduces the vasoconstriction threshold below core tem-perature, inhibiting centrally mediated thermoregula-tory constriction, and also causes direct peripheralvasodilation.[10] This results in internal redistributionof body heat as the core temperature decreases witha proportional increase in peripheral tissue tempera-ture. The body heat content remains constant. Duringthe first few hours of anesthesia, redistribution contrib-utes 65% of the total decrease in core temperature.[11]

In addition to redistribution of body heat duringanesthesia, total body heat is lost through four mech-anisms: radiation, conduction, convection, and evap-oration. Heat transfer is minimal if the temperature ofthe body surface and the environment are similar butincrease proportionally as the temperature difference

increases.[9] Radiation is the transfer of heat from onesurface to another without direct contact and resultsin 39% of the heat loss in a neonate. Radiation doesnot depend at all on the temperature of the interven-ing air. Radiant heat loss can be reduced by warmingthe room and by covering the patient. Because of thegreater surface area to body mass ratio, infants have agreater radiative heat loss than adults. It is thus essen-tial to increase the temperature of the operating roomto help stabilize the temperature of small infants andespecially neonates.[12] Conduction is the directtransfer of heat from the contact of one object to asecond object, and accounts for about 3% of neonatalheat loss. Conductive losses can be avoided bywarming the IV fluids and the table and by insulatingthe patient. Infants have less subcutaneous fat and agreater surface area to body ratio resulting in largerconductive heat losses than adults. Evaporation is thetransfer of heat by vaporization of water and repre-sents 24% of the neonatal heat loss. Evaporative lossesinclude sensible losses such as sweating and insensiblelosses through the skin or surgical wound. Insensibleloss from the skin in the adult is minimal but issignificant in infants, particularly in the premature,who have less epidermal keratin, resulting in largerevaporative losses. Evaporative losses can be reducedby keeping the skin dry and by warming and humidi-fying gases in the breathing circuit. There is alsosubstantial evaporative loss from within surgicalincisions. Convection is the transfer of heat from anobject to air or liquid and accounts for 34% of heatloss in the neonate. Convection removes heat to theenvironment, and is responsible for the wind-chilleffect. Convection can be reduced by insulating bar-riers such as a plastic drape that prevent movement ofair along the skin.

Anatomy for proceduresAirway managementManagement of the pediatric airway begins with theadequate opening of the airway and effective bag-mask ventilation. The ability to maintain airwaypatency and ventilation can prevent an unexpectedairway problem from becoming an airway emergency.

Table 1.1: Estimated blood volume

Premature Newborn 1 year 3 years 9 years Adult

EBV (ml/kg) 100 90 80 75 70 65

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The neonatal airway can be particularly difficult witha relatively large tongue and redundant tissues. Main-taining a degree of mouth opening while placing atight mask seal can compensate for the large tongueand redundant tissues and make bag-mask ventilationsuccessful. For those who have not mastered the neo-natal airway, insertion of an oral airway can achievethe same effect, if the patient is adequately anesthe-tized to tolerate insertion of an oral airway. Once atight mask fit is achieved with a patent airway, theapplication of constant positive airway pressure(CPAP) can assist in achieving adequate ventilation.

Many airway tools exist to facilitate airway man-agement in children. Many supraglottic airwaydevices exist but the most widely used is the laryngealmask airway (LMA). Anatomical differences in theinfant larynx can affect airway instrumentation witha supraglottic device. The shape of the infant larynxmakes the LMA more difficult to position in infantsless than 10 kg, resulting in more leaks and partialobstruction by the epiglottis, especially duringpositive-pressure ventilation. This occurs becausethe smaller LMA is a scaled-down version of the adultLMA, in which the anatomical differences of thelarynx are not accounted for, making positioningmore difficult.[13] Performing a jaw thrust duringinsertion of the LMA in an infant can minimize thiscomplication. Although leak pressures in an LMA arerarely measured in clinical practice, maintaining theleak pressure at 40 cmH2O can minimize leaks aroundthe cuff as well as the incidence of sore throat.

The small size of infants and children also makesregional anesthesia of the airway more difficult. Mostlaryngeal nerve block techniques normally performedin adults are not commonly used in children. Oneexception is the subcutaneous lateral approach whichallows bilateral subcutaneous administration of localanesthetic just lateral to the hyoid bone.[14]

Intubation can be challenging in the infant, whoselarge occiput and large tongue can hinder the align-ment of the oral, pharyngeal and tracheal axes duringdirect laryngoscopy (DL). Placement of a shoulder rollcan assist in aligning these axes. The infant epiglottis isnarrow and angled away from the axis of the trachea,making a straight blade preferable for tracheal intub-ation in an infant or neonate. Despite these anatomicchallenges, direct laryngoscopy is successful in themajority of infants and children. When laryngoscopyis difficult, the use of muscle relaxant to facilitateintubation in infants and children in conjunction

with sevoflurane anesthetic is associated with feweradverse respiratory events and should be consideredwhile investigating the use of other advanced airwaydevices.[15]

The videolaryngoscope (VL) can provideimproved view of the glottis specifically when theoral, pharyngeal and tracheal axes are not wellaligned. In this misalignment, the epiglottis partiallyobstructs the glottic view or, in the worst case scen-ario, even the epiglottis is unable to be visualizedunder DL. In these patients, the VL has been shownto be a successful rescue technique for unsuccessfulDL. Moreover, utilizing a VL blade smaller than thatbased on weight can further improve the visualiza-tion. Thus, changing to a smaller VL blade can clinic-ally improve successful intubation in the infant orchild with a difficult airway.[16]

Difficult airway: Although data is limited for theincidence of the difficult pediatric airway, it has beencalculated to be anywhere from 0.58% up to 3%, whichis significantly less than the 9–13% often reported inadults.[17] Both younger age and ASA III/IV status isassociated with difficult laryngoscopy. Many studieshave found a significantly higher risk of difficultintubation in infants under one year of age with a rateas high as 5%. Similar to adults, being overweightalone is not a predictor for difficult airway but, unlikeadults, being underweight in children is associatedwith a difficult airway. The Mallampati score is lessuseful in the uncooperative pediatric patient or infant.However, in the cooperative pediatric patient, theMallampati score can be a helpful tool to predictdifficult laryngoscopy.[18] Despite many availablevideolaryngoscopic devices, fiberoptic intubationremains the gold standard for securing the difficultairway for both the adult and pediatric airway.

Should a surgical airway be needed in an emer-gency, needle cricothyroidotomy may be more diffi-cult in the infant because of less room between thechin and the cricothyroid membrane. Caution mustbe given to correct placement as the target is smalland the trachea is compressible making passagethrough the back wall of the trachea and into theesophagus a calculated risk in this procedure.[19]

Vascular cannulationSuccessful cannulation of central veins requires athorough understanding of the anatomy of relevantstructures. Central access in infants can be technically

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more difficult due to the small vein size, and thevariable anatomy. As with many pediatric procedures,once the vessel is identified, the actual cannulationcan be hindered by difficulty threading the guidewireinto the vessel and by kinking or dislodging of small-caliber guidewires when threading the catheter overthe guidewire.

The decision to place a central venous line shouldbe given serious thought before placing the line as thecomplications can be devastating. Due to the smallercaliber of the vessels in infants and children, thrombusformation and vessel occlusion are increased risks.Many other complications exist depending on the sitechosen for central access and will be discussed with theindividual sites. Despite the risk, central venous accessis often required in infants and children for centralvenous pressure monitoring, administration of vaso-active medications, and hyperalimentation. The mostcommonly used sites for central venous cannulation ininfants and children include the umbilical vein, thesubclavian vein, the internal jugular vein, the femoralvein, and percutaneous intravenous access (PICC).Preferred sites depend upon the age of the patient, thetype of surgery and practitioner preference. In neo-nates, undergoing cardiac surgery, central access isrepresented by all sites, including internal jugular/sub-clavian (38.8%), femoral (27.2%) and umbilical/central(32.9%). For infants out of the neonatal period under-going cardiac surgery, internal jugular/subclavian isthe site of choice with 70.5% of infants having accessfrom these sites.[20]

Real-time ultrasound guidance has been defini-tively shown to be beneficial in adults, but pediatricdata is less clear-cut. The use of real-time ultrasoundin adults has been shown to decrease risks of cannula-tion failure, arterial puncture, hematoma, andhemothorax.[21] Although pediatric studies exist, datato evaluate outcomes in pediatric patients remainlimited. Current evidence in the pediatric literaturesupports ultrasound use especially with inexperiencedoperators. For inexperienced operators,[22] ultrasoundguidance versus landmark techniques can improvesuccessful cannulation and decrease the number ofneedle passes needed to cannulate the vessel. In thepediatric intensive care setting, ultrasound guidancehas been shown to decrease the time needed forresidents to cannulate a vessel but does not offer sucha benefit for experienced providers.[23]

Umbilical vein: The umbilical cord contains twoumbilical arteries and one umbilical vein. The

umbilical vein can be used in a neonate up to oneweek of age, after which atresia of the vessel makescannulation unlikely. Use of this vessel has its limita-tions because of significant complications includingnecrotizing enterocolitis, thrombus in the vena cavaor thrombus in the portal vein. Signs of umbilical veinthrombus formation include renal dysfunction andsystemic hypertension. Umbilical vein cannulationcan be life-saving, especially in the resuscitation of anewborn in distress at the time of delivery. Skilledpractitioners can place such an umbilical line withinminutes, providing much needed vascular access.[24]

Femoral vein: Femoral vein cannulation maybe preferred in some patients when hemothorax,pneumothorax or local hematoma is of concern orwhen access is required without interfering with theairway. However, femoral venous access has beenreported to have a higher incidence of thrombosisand infections as long-term complications. Femoralhead necrosis is another risk, especially in the prema-ture infant, and is avoided if possible in the prematureinfant. The femoral vein lies midway between theanterior superior iliac spine and the symphysis pubisand can be accessed just below the inguinal ligamentat the inguinal crease. The femoral vein is medial tothe femoral artery. Appropriate positioning canimprove both cross-sectional area of the femoral veinand minimize overlap with the artery. This includesreverse Trendelenburg, external rotation of the hipand 60o abduction of the leg.[25] Although two-dimensional ultrasound is not required for femoralvein cannulation, ultrasound has been shown toimprove the overall success rate, decrease the inci-dence of arterial puncture, decrease the incidence ofhematoma, and decrease the number of needle passesfor successful cannulation. The use of ultrasoundguidance by trainees for femoral cannulationdecreases the time of insertion, markedly improvesfirst attempt success, and lowers the median numberof passes for success.[22]

Upper body central cannulation: Upper bodycentral lines in the internal jugular veins or the sub-clavian veins can be placed in neonates, infants, andsmall children. Upper body central lines providereliable vascular access and accurate central venouspressure monitoring with a decreased risk of infectionwhen compared to lower body central lines.[26] How-ever, these vessels are sometimes avoided in smallchildren with single-ventricle cardiac physiology dueto a potential risk of stenosis or thrombosis of the

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superior vena cava. Stenosis or thrombosis of vesselsin the upper body can result in “superior vena cavasyndrome,” a post-thrombotic syndrome with markedelevated pressures in the head and neck, facial swell-ing, and headaches. However, studies have shownminimal risk of clinically significant catheter-associated vessel thrombosis or stenosis in patientswith single-ventricle physiology, especially when theright internal jugular access is used.[20]

Subclavian vein: The subclavian vein can beaccessed via an infraclavicular approach using thelandmarks of the lower border of the clavicle justlateral to the intersection of the clavicle with the firstrib. This is also the lowest part of the “bend” of theclavicle. Complications such as pneumothorax andarterial puncture can be avoided with a flat angle ofapproach so the needle stays adjacent to the clavicle toavoid unwanted structures.

Internal jugular vein: The internal jugular anat-omy has easily identified landmarks both anatomic-ally and via ultrasound. Cannulation success rate canbe improved with optimal positioning. This includes

Trendelenburg position with the table tilted down 15o

and passive leg elevation of 50o to increase the cross-sectional area of the IJ vessel.[27] The head should beturned only 45o from midline, as extreme turning ofthe head will cause more overlap of the internal jugu-lar (IJ) and carotid, making carotid puncture morelikely. Ultrasound is recommended for improved can-nulation success. The straighter course of the rightinternal jugular vein towards the heart makes this thepreferred site of access with less difficulty passing theguidewire and catheter into the heart and a lowerthrombotic risk to the patient from catheter place-ment. Figure 1.1 depicts the anatomical view of theleft and right neck anatomy. Figure 1.1a shows theultrasound view of the left neck in a 3-year-old child.Note the depth of the vessels at 2 cm and the well-developed sternocleidomastoid muscle. Figure 1.1bshows the right neck in a newborn. Note the vesselsare shallower at a depth of 1 cm with a less developedsternocleidomastoid muscle. The cross-sectional areasof the vessels in the neonate are smaller than thoseseen in the older child.

References1. P.J. Davis, F.P. Cladis, E.K.

Motoyama. Smith’s Anesthesia forInfants and Children, 8th Edition.Philadelphia PA, Elsevier Mosby,2011, p. 14.

2. B. Bissonnette and B. Dalens.Pediatric Anesthesia: Principlesand Practice. New York NY,McGraw-Hill, 2002, p. 1215.

3. P.J. Davis, F.P. Cladis, E.K.Motoyama. Smith’s Anesthesia for

Infants and Children, 8th Edition,Philadelphia PA, Elsevier Mosby,2011, p. 7.

4. R.D. Mosteller. Simplifiedcalculation of body-surface area.N Engl J Med 1987; 317: 1078.

a b

Figure 1.1 Ultrasound pictures of the internal jugular vessels. Figure 1.1a. shows the left neck vessels in a 3-year-old. Figure 1.1b. showsthe right neck vessels in a newborn. A ¼ carotid artery, V ¼ internal jugular vein, SCM ¼ sternocleidomastoid muscle. The arrow ispointing to the central line within the IJ lumen after cannulation. IJ ¼ internal jugular.

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5. H.I. Hochman, M.A. Grodin, R.K.Crone. Dehydration, diabeticketoacidosis, and shock in thepediatric patient. Pediatr ClinNorth Am 1979; 26: 803.

6. J.D. Crawford, M.E. Terry, G.M.Rourke. Simplification of drugdosage calculation by applicationof the surface area principle.Pediatrics 1950; 5: 783–90.

7. A.G. Bailey, P.P. McNaull, E.Jooste, J.B. Tuchman.Perioperative crystalloid andcolloid fluid management inchildren: where are we and howdid we get here? Anesth Analg2010; 110: 375–90.

8. K. Choong, S. Arora, J. Cheng,et al. Hypotonic versus isotonicmaintenance fluids after surgeryfor children: a randomizedcontrolled trial. Pediatrics 2011;128: 857–66.

9. A. Sarti, D. Recanati, S. Furlan.Thermal regulation andintraoperative hypothermia.Minerva Anestesiol 2005; 71:379–83.

10. D. Sessler. Perioperative heatbalance. Anesthesiology 2000; 92:578–96.

11. D. Galante. Intraoperativehypothermia. Relation betweengeneral and regional anesthesia,upper- and lower-body warming:what strategies in pediatricanesthesia? Pediatr Anesth 2007;17: 821–3.

12. B. Tander, S. Baris, D. Karakaya,et al. Risk factors influencinginadvertent hypothermia ininfants and neonates duringanesthesia. Pediatr Anesth 2005;15: 574–9.

13. C. Park, J.H. Bahk, W.S. Ahn, S.H.Do, K.H. Lee. The laryngeal mask

airway in infants and children.Canadian J Anaesth 2001; 48:413–17.

14. B. Bissonnette and B. Dalens.Pediatric Anesthesia: Principlesand Practice. New York, McGraw-Hill, 2002, p. 571.

15. R.A. Sunder, D.T. Haile, P.T.Farella, A. Sharma. Pediatricairway management: currentpractices and future directions.Review Article Ped Anesth 2012;22: 1008–15.

16. J.H. Lee, Y.H. Park, H.J. Byon,et al. A comparative trial of theglidescope® videolaryngoscope todirect laryngoscope in childrenwith difficult direct laryngoscopyand an evaluation of the effect ofblade size. Anesth Analg 2013;117: 176–81.

17. S. Heinrich, T. Birkholz, H.Ihmsen, et al. Incidence andpredictors of difficultlaryngoscopy in 11, 219 pediatricanesthesia procedures. PediatrAnesth 2012; 22: 729–36.

18. C. Sims, B. von Ungern-Sternber.The normal and the challengingpediatric airway. Pediatr Anesth2012; 22: 521–6.

19. J. Stacey, A.M. Heard, G.Chapman, et al. The ‘Can’tIntubate Can’t Oxygenate’scenario in pediatric anesthesia: acomparison of different devicesfor needle cricothyroidotomy.Pediatr Anesth 2012; 22: 1155–8.

20. J.W. Miller, D.N. Vu, P.J. Chai,et al. Upper body central venouscatheters in pediatric cardiacsurgery. Pediatr Anesth 2013;23: 980–8.

21. S. Wu, Q. Ling, L. Cao, et al. Real-time two-dimensional ultrasoundguidance for central venous

cannulation. A meta-analysis.Anesthesiology 2013; 118: 361–75.

22. M.T. Aouad, G.E. Kanazi, F.W.Abdallah, et al. Femoral veincannulation performed byresidents: a comparison betweenultrasound-guided and landmarktechnique in infants and childrenundergoing cardiac surgery.Anesth Analg 2012; 111: 724–8.

23. C.R. Grebenik, A. Boyce, M.E.Sinclair, et al. NICE guidelines forcentral venous catheterization inchildren: is the evidence basesufficient? Br J Anaesth 2004; 92:827–30.

24. G. Ancora, S. Soffritti, G. Faldella.Diffuse and severe ischemic injuryof the extremities: a complicationof umbilical vein catheterization.Am J Perinatol 2006; 23: 341–4.

25. A.A. Eldabaa, A.S. Elgebaly, A.A.Abd Elhafz, A.S. Bassun.Comparison of ultrasound-guidedversus anatomical landmark-guided cannulation of the femoralvein at the optimum position ininfants. Annals of PediatricSurgery 2012, 8: 65–8.

26. S.T. Verghese, W.A. McGill, R.I.Patel, et al. Ultrasound-guidedcentral venous catheter placementdecreases complications anddecreases placement attemptscompared with landmarktechniques in the patient in thepediatric intensive care unit. CritCare Med 2009; 37: 1090–6.

27. W.H. Kim, J. H. Lee, S.M. Lee,et al. The effect of passive legelevation and/or Trendelenburgposition on the cross-sectionalarea of the internal jugular vein ininfants and young childrenundergoing surgery for congenitalheart disease. Anesth Analg 2013;116: 178–84.

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Chapter

2Anesthesia equipment

Hoa N. Luu

In this chapter, we will review anesthesia equipmentas it relates to respiration. The one major focus wouldbe on the breathing circuits that connect the anesthe-sia machine to the patient. Another focus is ventila-tion devices and ventilation techniques used.

The anesthesia machine is a huge subject that wewill briefly discuss in this chapter. You should beaware that there is no dedicated anesthesia machinefor the pediatric patient but we use the normal adultanesthesia machine. Thus, when providing anesthesiato the pediatric patient it is important for us to knowthe limitations of the anesthesia machine.

When connecting the patient to the anesthesiamachine, we are all familiar with the standard semi-closed circle absorber system used in adults. With thisbreathing system, the use of two unidirectional valvesallows for the anesthetic gases to flow in one directionand prevent rebreathing of carbon dioxide with thecarbon dioxide absorber. This circle system allows forrebreathing of the anesthetic gas mixture, thus, cut-ting the cost by minimizing waste of anesthetic gasesand conserving heat and moisture. The limitations ofthis system are that it is more bulky in size and thereis increased resistance in the circuit if the patient isbreathing spontaneously. In the pediatric patient,there are more options. While these options are rarelyused today, except at some children’s hospital operat-ing rooms, it is important for us to learn aboutbecause they are continued to be used for transport-ing pediatric patients in the hospital.

Before we discuss the Mapleson systems, we willfirst talk about what practical options we have when apediatric patient comes to the operating room. Whilesome argue that the standard adult circuit can still beused, most hospitals do have the modified pediatriccircuit. This modified pediatric circuit would consistof a shorter, stiffer, and smaller-diameter tubing and

smaller rebreathing bag. The advantage of havingthis modified circuit would be having a decreasedcompression volume that means less compliance ofthe circuit. In addition, smaller CO2 canisters can beused to minimize resistance to breathing.

Now we will learn about the Mapleson circuitsthat are semiclosed rebreathing systems. There aresix different types of Mapleson circuits (A to F) thatwe are familiar with (Figure 2.1). Each circuit is dif-ferent from the others by the location of its compon-ents. The five components are fresh gas inflow,expiratory valve, corrugated tubing, reservoir bag,and an adaptor for a face mask or endotracheal tube.

Depending on the locations of the components andmode of ventilation, it affects the amount of rebreathingthat occurs. For simplicity, I like to divide theMaplesoncircuits into three groups so it will help you rememberthe location of the components in each circuit. We willdiscuss the Mapleson systems as three groups as wellbecause it will help us memorize which circuits arebetter for spontaneous or controlled ventilation.

The Mapleson A circuit has the expiratory valveclose to the patient with the corrugated tubing separ-ating away the fresh gas inflow and reservoir bag atthe end. The Mapleson A circuit is the most efficientcircuit for spontaneous ventilation because there is norebreathing when the fresh gas flow is more than 80%of the minute ventilation. However, it is the leastefficient circuit for controlled ventilation so it wouldrequire a much larger fresh gas flow to preventrebreathing. We do not use the Mapleson A circuittoday in the operating room because of the proximallocation of the valve to the patient. It is potentiallyhazardous because the weight of the valve could inad-vertently extubate an endotracheal tube.

Both Mapleson B and C circuits have the expira-tory valve close to the patient with the fresh gas inflow

Essentials of Pediatric Anesthesiology, ed. Alan David Kaye, Charles James Fox and James H. Diaz. Published by CambridgeUniversity Press. © Cambridge University Press 2015.

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nearby as well. The difference is that there is a corru-gated tube separating the reservoir bag in Mapleson Bwhile there is no corrugated tube in Mapleson C. Bothof these circuits are seldom used today because of theproximity of the expiratory valve to the patient andhigh fresh gas flow required to prevent rebreathing.

The Mapleson D, E, and F circuits have the freshgas inflow close to the patient. In the MaplesonD circuit, the expiratory valve is between the corru-gated tube and reservoir bag. While the MaplesonD does require a little more fresh gas flow to preventrebreathing than Mapleson A during spontaneousventilation, it is the most efficient circuit during con-trolled ventilation. Bearing in mind both modes ofventilation, Mapleson D requires the lowest fresh gasflow rate; thus, explains it is the most commonly usedMapleson circuit. Mapleson E is the basically the Ayre

T-piece while the Mapleson F is a modified AyreT-piece with reservoir bag and tubing. They aregrouped together also because they have similarrebreathing characteristics.

To help determine which circuits are better forspontaneous or controlled ventilation, there is a mne-monic that is easy to remember. For spontaneousventilation, it’s “All Dogs Can Bite.” (A>DEF>CB)Thus, there is the least rebreathing in Mapleson A.For controlled ventilation, it’s “Dog Bites Can Ache.”(DEF>BC>A) Thus, there is the least rebreathingin Mapleson D, E, and F. To remember which phrasegoes with which mode of ventilation, I just modify thefirst one to “All Dogs Can Bite Spontanously.”

Now we will discuss ventilation devices. The focuswill be devices we particularly use once the airway issecured. The discussion of airway management will be

SYSTEM FGF

FGF is the fresh gas flow required to avoid rebreathing during spontaneousventilation quoted as multiples of minute volume

MAPLESONCLASSIFICATION

APL valve

AFreshgasflow Face mask

Reservoir bag

0.8–1

1.5–2

1.5–2

2–4

2–4

2–4

B

C

D

E

F

Figure 2.1 Mapleson circuits. APL¼ ??

Chapter 2: Anesthesia equipment

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discussed later in the book. In that section, you willlearn about the devices and techniques for both maskventilation and endotracheal intubation.

As you can see, we have already talked aboutthe most important ventilation device which is ouranesthesia machine and the different circuits toconnect it to the patient. But what happens whenthere is a machine failure or malfunction? The answerbrings us to the last device we will discuss: the self-inflating bag (Figure 2.2).

The self-inflating bag can provide positive-pressureventilation to ventilate the patient’s lungs with roomair or air enriched with oxygen if an oxygen supply isavailable. The Mapleson circuit would require com-pressed gas, thus, you would not be able to ventilate thepatient with the circuit alone. You do not need toworry about rebreathing with the self-inflating bagbecause there is an one-way valve (B) near the patientwhich opens at expiration and closes at inspiration.

When you squeeze the bag, the air within it would flowto the adaptor that connects to the face mask or endo-tracheal tube since the valve (A) would be closed.When the self-inflating bag re-expands, the valve (A)would open allowing the gas in the reservoir tubing tofill the bag. This gas can just be room air or be oxygen-enriched air if it is connected to an oxygen source.Thus, it is very important to have a self-inflating bagand alternate oxygen source prior to the start of everyanesthesia case.

I hope you have a better understanding of theanesthesia equipment regarding breathing circuitsnow so you can address the limitations when caringfor the pediatric patient.

Further reading1. Y.C. Lin. Paediatric anaesthetic breathing systems.

Pediatr Anaesth 1996; 6: 1–5.

2. T. Smith. Fundamentals of Anaesthesia. Cambridge,Cambridge University Press, 2009.

3. J. Cote. A Practice of Anesthesia for Infants andChildren. Philadelphia PA, Elsevier, 2009.

4. P.J. Davis. Smith’s Anesthesia for Infants and Children.Philadelphia PA, Elsevier, 2011.

5. R.D. Miller. Miller’s Anesthesia. Philadelphia PA,Elsevier, 2009.

6. T.Y. Euliano. Essential Anesthesia. Cambridge,Cambridge University Press, 2004.

ReservoirA

O2

B

IV

B

Figure 2.2 Self-inflating bag.

Chapter 2: Anesthesia equipment

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