Pediatric Anesthesiology:
The Basics

Objectives:

1. List the differences between adult and pediatric airway anatomy and understand their implications for intubation

2. List standard required NPO times for infants and children for clear liquids, breast milk, formula and solids

3. Calculate fluid requirements and estimated blood loss in uncomplicated pediatric cases

4. Define a rapid sequence induction and list indications

5. List normal vital sign ranges for children

6. Discuss the implications of upper respiratory tract infections for anesthesia

7. List physiologic differences between adults and children by organ system

8. Discuss methods of induction of anesthesia for pediatric patients

9. Discuss anxiety in children preparing for surgery, along with sedative premedication and other methods to reduce anxiety.

The anesthetic care of infants and children is challenging, rewarding, and often fun. We will cover some basic information regarding pediatric cases in this handout. A pediatric case is also provided in another section of the web page.

I. Preoperative evaluation

In small children, it is important to ascertain a short pregnancy and birth history. Your history should include the number of weeks gestation of the pregnancy. Less than 37 weeks is considered preterm. We use a construct called post conceptual age to stratify risk in infants.

Post conceptual age (PCA) = Weeks gestation + weeks of life.

Infants are at increased risk of apnea and must stay overnight in the hospital for monitoring up to 55 weeks PCA. Events during pregnancy such as maternal hypertension or diabetes may also impact the child's health and management.

Briefly, other history to be obtained includes the condition requiring surgery, ancy coexisting medical conditions, medications, allergies, and recent illnesses. Upper respiratory tract infections are very common in children, and can significantly increase the risk of postoperative respiratory complications.

Upper respiratory tract infections are a dilemma which faces the pediatric anesthesiologist on a daily basis. They increase the incidence of laryngospasm, bronchospasm and desaturation. It is important to ask about the duration and severity of the URI, as well as its course. Children with URI symptoms are at increased risk for four to six weeks after the onset of symptoms. Reasons to cancel a case might include fever, concurrent wheezing or lower respiratory tract infection, and productive cough.

In addition to taking a history, you must also listen to the patient, and perform an airway examination. You may be the first person to detect a wheeze or a murmur.

Preoperative laboratory evaluations are NOT necessary in otherwise healthy infants. Formerly premature infants may require a hemoglobin to test for anemia.

II. Preparation for surgery

Young children are often quite anxious when confronted with a strange hospital environment and the necessity for a surgical procedure. Separation anxiety starts at approximately nine monthe of age. There are a number of interventions that can be used to decrease postoperative anxiety. Hospital tours and preparatory videotapes have been shown to help significantly. Parental presence during induction is a controversial topic. Parents who are not anxious may help some children.

Another approach is sedative premedication. The most commonly used drug is midazolam, a benzodiazapene which can be administered orally., intranasally, or intravenously. It is also a good amnestic. Children have been shown to have fewer nightmares and negative behaviors after surgery if midazolam has been given. It is contraindicated in children with previous adverse reactions or obstructive sleep apnea.

NPO status:

Children, like adults, are kept with nothing to eat or drink to minimize their stomach contents and decrease the risk for aspiration. Although not all aspiration is clinically significant, it can lead to bronchospasm and pneumonia. Most npo guidelines have been based on tradition rather than science, however, recent work has looked at stomach volume in children. The stomach content volume appears to reach a minimum 2 hours after clear liquid infestion. Children therefore follow these guidelines:

No clear liquids for 2 hour prior to surgery
No breast milk for 4 hours prior to surgery
No solids or cow's milk for 6 hours prior to surgery.

III.Physiologic differences

A. Cardiovascular.

Children have a higher cardiac output and oxygen consumption per kilogram than adults. They support this higher output with a higher baseline heart rate. Infants are heart rate dependent for their cardiac output. In other words, they have a fixed stroke volume, and must increase their heart rate to increase cardiac output. They may respond to stress, such as hypoxia, by becoming bradycardic, and therefore decreasing CO. This can make resuscitation quite difficult. Normal vital signs for children include higher heart rates and lower blood pressures than adults.

Age	Heart Rate	Systolic Pressure	Diastolic Pressure
Preterm 1000g	130-150	45	25
Newborn	110-150	60-75	27
6 Months	80-150	95	45
2 years	85-125	95	50
4 Years	75-115	98	57
8 Years	60-110	112	60

B. Respiratory.

Minute volume is increased, corresponding with the increased cardiac output. This is supported by an increased respiratory rate. Tidal volume and dead space are equivalent to adults on a per kilogram basis. Children desaturate rapidly because of increased utilization of oxygen per kilogram , and because their FRC is decreased under anesthesia. FRC decreases under anesthesia because of small alveoli, and a very compliant chest wall. (No rigid box effect like adults have. ) Both of these things lead to increased atelectasis.

Variable	infant	adult
Respiratory frequency	30-50	12-16
Tidal Volume ml/kg	6-8	7
Dead space ml/kg	2-2.5	2.2
Alveolar vent.	100-150	60
FRC	27-30	30
Oxygen consumption	6-8	3

C. Temperature regulation

Pediatric patients lose heat to the environment more readily than adults. This is due to an increased surface area per kilogram. This is compounded by cold intravenous fluids, dry anesthetic gases, and wound exposure. Hypothermia is a serious problem which can result in delayed awakening, cardiac irritability and respiratory depression. Infants cannot shiver, but must metabolize brown fat to maintain temperature. It is important to PREVENT heat loss with a warm operating room environment.


D. Perioperative fluid requirements

This area requires the use of your calculator !!!!

1. Maintenance fluid
   For the first 10 kg of weight give 4cc/kg/hr
   For the second 10 kg, ADD 2cc/kg/hr
   For the remaining kg, add 1cc/kg /hr

Example: A 35 kg child requires 40+20+15 =75 cc/kg/hr
                A 5 kg child requires only 5x4=20cc/kg/hr

2. Preoperative deficit.
This is simply maintence fluid x hours NPO. Half is replaced in the first hour of surgery, one quarter in the second hour, and the remaining quarter in the third hour

3. Third space losses:
   Superficial procedures 2-4 cc/kg/hr
   Moderate procedures 4-6 cc/kg/hr
   Major procedures 6-8cc/kg /hr

This is given in addition to maintenance and deficit fluids to account for loss of fluid to the environment from open wounds

4. Blood replacement
Replace each cc of blood lost with 3 cc of crystalloid or 1 cc of colloid or PRBC

Estimated allowable blood loss is calculated as follows
ABL = EBV x HB (current)- HB (acceptable)/ HB (mean)
EBV is estimated blood volume

Age	EBV
Premature	90-100cc/kg
Newborn	80-90 cc/kg
3mo-1yr	70-80 cc/kg
>1yr	70 cc/kg
Adult	55-60 cc/kg

IV. Anatomic differences

A. Airway anatomy.

There are five key differences between the adult and pediatric airway .

1. Proportionately larger head and tongue
2. More anterior and cephalad larynx
3. Long, sometimes floppy epiglottis
4. Short trachea and neck.
5. THE NARROWEST POINT IN THE PEDIATRIC AIRWAY IS THE CRICOID CARTILAGE

Children require a gentle mask fit, taking care not to obstruct the airway by compressing the soft tissues under the chin. A straight blade is generally more appropriate for intubating children because of the shape of the epiglottis.

Because the airway is cone shaped, with the narrowest point at the cricoid cartilage, an uncuffed tube is adequate to seal the trachea. Using an uncuffed tube allows us to maximize the inner diameter of the tube, decreasing airway resistance and turbulence. (Remember Poseuille's Law-resistance is proportaional to 1/radius to the fourth power)

After placement of the endotracheal tube, we look for a leak of air around the tube at 15 to 25 cm of water pressure. If there is no leak at high pressures, the tube is too tight and may exert pressure on the tracheal mucosa, causing edema and postoperative croup.


B. Cardiac anatomy.

Infants are born with an anatomically patent foramen ovale and ductus arteriosus. The ductus closes in the first day of life. The foramen ovale may remain probe patent for life, but physiologically closes in the first day of life. This can be important, because bubbles in IV fluid can cross the PFO and go directly to the brain.

V. Pharmacologic Differences

Pediatric drugs are generally dosed on a per kilogram basis

Neonates are more sensitive to opiate analgesics during the first four weeks of life, leading to an increased risk of apnea.

Inhalational anesthetics reach the brain faster in children , allowing us to perform inhalation inductions more easily.

The volume of distribution for most drugs, including muscle relaxants, is increased in children, so a standard dose leads to a lower plasma level than in adults. However, children are more sensitive to the effects of muscle relaxants , so a lower plasma level leads to the same effective dose.

We will not discuss the pharmacology of specific drugs in this page.

VI. Induction of Anesthesia

A. Inhalation induction.

This is generally the preferred method in children under the age of ten. The patient may be premedicated first. On arrival in the operating room, noninvasive monitors are placed. (Minimum of pulse oximeter and precordial stethoscope) The patient then breathes a mixture of nitrous oxide and oxygen, then a potent inhalational anesthetic. The patient will go through stage II - the excitement phase of anesthesia. During this stage, it is important to avoid stimulating the patient in order to minimize the risk of laryngospasm. Inhalatian induction is contraindicated in children with full stomachs, because the airway is unprotected for and extended period of time.

B. Intravenous induction.

This method is used in some older children who may prefer an IV to the anesthetic mask., and in children who have full stomachs. Children may accept iv placement more readily when premedicated. EMLA cream may also be placed over veins to numb the skin. It takes about an hour to work. Once the IV is in place, induction proceeds as it would for and adult.

VII. Complications

We will briefly review some of the more common complications seen in pediatric anesthesia. All patients, especially those who are intubated, may complain of a sore throat. Usually this can be treated with acetominophen alone. Nausea and vomiting are also common, especially after the age of 2. We may choose to give prophylactic antiemetic drugs.

Postintubation croup is the result of an endotracheal tube that is too tight, or is moved a great deal. It is a result of swelling of the tracheal mucosa, and may be treated with racemic epinephrine and humidified mist

Laryngospasm is another fairly common complication. It is a forceful closure of the vocal cords withch prevents respiration. It occurs during stage II of anesthesia when the airway is unprotected. This is the reason a child should not be manipulated during stage II. If it occurs, it must be recognized and treated promptly. The initial treatment is constant positive pressure in order to reestablish ventilation. If this is not successful, succinylcholine is administered, along with atropine to prevent bradycardia.