A 4 month old, 6 kg girl is admitted to the PICU for respiratory failure. She is cyanotic and retracting. She is intubated due to worsening tachypnea, increasing work of breathing, and fatigue. Her oxygen saturations have been falling and her pCO2 is 75 on an arterial blood gas. With intubation, her oxygen saturations briefly improve as she is hand ventilated, but her oxygen saturation falls into the high 80s when placed on mechanical ventilation, SIMV (synchronized intermittent mandatory ventilation) mode with a tidal volume of 80, IT=0.7 seconds, FiO2=100%, PEEP=5, and rate of 25. Her saturations again rise to 98-100% with hand bagging. After numerous attempts to ventilate her with volume ventilation she is changed to SIMV pressure control/pressure support mode, PIP=30, PEEP=6, FiO2=60%, RR=25, IT=0.7 and her saturations remain 98-100% on mechanical ventilation. CXR shows her ET tube to be in good position, but there are bilateral, patchy infiltrates and right lower lobe consolidation. She is placed on chest physiotherapy and IV antibiotics. Over the subsequent week, her ventilator rate, FiO2 and PEEP are gradually reduced. She is successfully extubated one week after admission to the PICU.
Mechanical ventilation is an art that remains in flux. While there are some basic tenants, each child and disease process have different characteristics. Therefore, the mode of ventilation chosen must be evaluated to be sure it is optimal for the child and their illness.
The two commonly used ventilation modes are pressure and volume, with many variations depending on the ventilator. The modes are based upon what variables cause the ventilator to cycle from inspiration to exhalation. These variables include time, flow rate, pressure, and volume. Air/oxygen is delivered to the patient under positive pressure until a certain volume is delivered, a certain pressure is achieved, or time/flow criteria are met. The ventilator then diverts flow and allows the patient to passively exhale. The positive end expiratory pressure (PEEP; which is the pressure which is maintained using flow resistance, during exhalation), is also set, depending on the child's underlying illness. The ventilator stops diversion of flow when this pressure is achieved and maintains the end expiratory pressure until the next positive pressure breath is initiated.
There are pros and cons to each type of ventilation and advocates for each. It is known that mechanical ventilation may cause lung damage either due to "volutrauma" (trauma due to rapid, repetitive changes in lung volume) and/or "barotrauma" (trauma due to rapid, repetitive changes in lung pressure). The repetitive expansion and collapse of the lung can cause parenchymal injury and may alter lung water and mucociliary clearance. Which mode of ventilation is superior (if there is a "best" mode) depends upon the patient and their disease process. The basic difference between the ventilator methods, is the parameter used to end the inspiration cycle (pressure or volume). In pressure controlled ventilation, a PIP (peak inspiratory pressure) and an inspiratory time (IT) duration determines the inspiratory cycle. The advantage of a pressure ventilator is that it should help protect the lungs from excessive pressures. However, tidal volume (TV) may then be compromised. TV and pressure are related by the equation:
Lung tissue compliance = change in volume / change in pressure
So, as a child's lung compliance worsens (decreases), the TV delivered will decrease for a given PIP. Similarly, if volume ventilation is chosen, the peak pressure will change based upon changes in lung compliance.
There are other characteristics of ventilators, such as the "mode", which should also be considered. These include:
Assist control modes: The ventilator delivers a set TV or PIP at a preset interval and with each patient's spontaneous respiratory effort (i.e., the patient's initial breathing effort, will trigger the ventilator to deliver another breath in synchrony). This mode is not commonly used in pediatrics.
IMV (intermittent mandatory ventilation): The ventilator delivers mandatory positive pressure breaths at a set rate. The patient may have unassisted spontaneous breaths between ventilator breaths, but the ventilator breaths are not synchronized with the patient's breaths.
SIMV (synchronized intermittent mandatory ventilation) pressure support: This method synchronizes the ventilator breaths with the patient's inspiratory efforts, thereby preventing the stacking of a ventilator breath on top of a spontaneous breath. Pressure support is the provision of a specified amount of positive pressure to assist the patient's own respiratory effort.
With some understanding of the modes of ventilation, the variables to be set on mechanical ventilators will be reviewed. These generally include respiratory rate (breath per minute), FiO2 (fraction of inspired oxygen), inspiratory time, and TV or PIP (depending on mode chosen). The starting respiratory rate (RR) is in part age determined, commonly: 30-50 in neonates, 25-30 in infants, 20 in children, 10-15 in teenagers. The rate is also dependent on the disease process. For example, patients who have air trapping/hyperinflation disorders (such as asthma) need a longer expiratory phase and therefore, a slower rate. You may have noticed that the set rate on the ventilator is often lower than that of a spontaneously breathing child of the same age/size. This is because the ventilator gives larger than normal tidal volumes "sigh breaths"). Spontaneous breaths are usually about 6-7cc/kg, whereas set tidal volumes are 10-l5 cc/kg. Recall that minute ventilation = TV x RR. So, if large tidal volumes are given, the RR needed to maintain the same minute ventilation will be lower.
In choosing a TV or PIP, the most important tenant to remember is, in general, to use a volume or pressure that causes good visible chest rise and air entry on auscultation. For TV ventilation, the starting range is usually about 10-l5 cc/kg. For pressure ventilation the pressure needed to move the chest will depend on lung compliance. A good way to judge this is to hand ventilate the child using an anesthesia bag with a manometer, to determine what pressure is required to move the chest. A typical range for a PIP in a lung with good compliance might be 16 to 20 mmHg vs up to 30 or 40 in a poorly compliant lung (20 being used for a mildly stiff lung such as a neonate with RDS).
The inspiratory time (IT or I-time) is also age and rate dependent and will also need to be altered depending on the child's disease. A guideline is 0.4-0.7 seconds for infants and 0.5-1 seconds for children and adults. Longer I-times increase mean airway pressure (by prolonging the inspiratory cycle) and therefore usually improve oxygenation.
In nonventilated patients, the glottis opens and closes during spontaneous respirations. Partial closure of the glottis provides a physiologic "PEEP" of 3-4mmHg by preventing complete emptying of the airway (i.e., we normally exhale against some resistance to maintain positive pressure in the alveoli during exhalation). In patients with good oxygenation and little pulmonary disease, a PEEP of 3-4 is adequate. Higher PEEPs are necessary for the patient with pulmonary edema, pneumonia, or atelectasis. High PEEP may also be useful for the post operative heart patient with surgical bleeding. Be aware that increasing PEEP increases mean airway pressure. Patients with high mean airway pressures may require volume infusions to maintain venous return and cardiac output. Inotropic support may also be needed in patients requiring very high PEEP (>10).
FiO2 is generally 100% during intubation but should be rapidly reduced, if possible, once mechanical ventilation is initiated. Weaning FiO2 can be done by monitoring pulse oximetry or ABGs. Oxygen is thought to be non toxic if the FiO2 is maintained at 40% or less. Since patients who are intubated are at high risk for ET tube plugging or displacement and resulting hypoxemia, we rarely reduce the FiO2 below 30%. Exceptions to this rule include children less than 34 weeks gestation (who are at risk for retinopathy of prematurity), and those with left to right shunts where the pulmonary vasodilation due to hyperoxygenation may result in excessive pulmonary blood flow. In general, saturations 95-100% are acceptable, although for children with severe ARDS (adult respiratory distress syndrome), BPD (bronchopulmonary dysplasia), or cyanotic heart disease, lower sats are expected/accepted.
In managing a ventilator, the settings of the ventilator should be adjusted to optimize the ventilatory support required by the patient. Too much oxygen or mechanical force may result in lung injury. Insufficient oxygen or mechanical force will result in hypoxia and hypoventilation. Assume that a normal blood gas is: pH 7.40, pCO2 40, pO2 100, BE 0, oxygen saturation 99% (refer to the chapter on interpreting blood gasses). In ICU patients, a higher pCO2 is sometimes tolerated (pCO2 45) to minimize ventilator trauma to the lung, and a lower pO2 is tolerated to minimize oxygen toxicity (oxygen saturation 95%). In premature infants, who are usually maintained with higher hemoglobins, lower pO2 values may be tolerated to minimize the risk of retinopathy of prematurity.
Adjusting the FiO2 will only affect the pO2 and oxygen saturation. Increasing the ventilator rate, will increase the minute ventilation so this decreases the pCO2 (and hence increases the pH). These are the two most basic changes that occur in ventilator management. One could also increase the minute ventilation (which would decrease the pCO2) by increasing the tidal volume (on a volume ventilator) or the PIP (on a pressure ventilator). Also realize that any parameter change which increases the mean airway pressure (MAP) will also increase the pO2. One could increase the mean airway pressure by increasing the PEEP, the inspiratory time, or the PIP. Increasing the tidal volume (TV) on a volume ventilator, in essence, increases the PIP so this also increases the MAP. Refer to the table below which describes the most commonly expected changes in pCO2, pO2 and MAP which occur with increases in the ventilator parameters in the column on the left:
For example, a patient with an ABG: pH 7.28, pCO2 50, pO2 70, BE -3. One could improve oxygenation by increasing the FiO2, PEEP, IT, or/and PIP/TV. One could decrease the pCO2 and improve the pH by increasing the rate or/and PIP/TV. The best adjustment would be based on assessment of chest wall movement, aeration, expansion on chest x ray, the patient's pulmonary problems, and the current ventilator settings. For example, if the FiO2 is already at 95%, then it would be better to increase the PIP or IT rather than increase the FiO2.
Consider another ABG which may be encountered when the patient is improving: pH 7.45, pCO2 35, pO2 130, BE +0. The pCO2 is too low indicating that the minute ventilation is too high. The minute ventilation could be reduced by decreasing the rate and/or the PIP/TV. The pO2 could be lowered by decreasing the FiO2, PEEP, IT, and/or PIP/TV.
The rate and methods of weaning are quite variable, depending on the child's condition. It is therefore difficult to make general rules regarding this process. Some generalizations may be made:
. . . . . The more acute the process, the faster weaning may take place.
. . . . . ETCO2 monitoring or TCM monitoring may facilitate weaning and reduce the need for blood gases.
. . . . . The child's baseline CO2 and O2 should be considered as weaning takes place. A"well" child with BPD may not have a normal CO2 of 40.
. . . . . The child's clinical status should be considered as the weaning process takes place. An acceptable PCO2 is not evidence of "tolerating weaning" if the child is clinically in distress.
Prerequisites to extubation include:
. . . . . 1) A good cough/gag (to allow the child to protect their airway).
. . . . . 2) NPO about 4 hours prior to extubation (in case the trial of extubation fails and reintubation is required).
. . . . . 3) Minimize sedation.
. . . . . 4) Adequate oxygenation on 40% FiO2 with CPAP (or PEEP) = 4.
. . . . . 5) The availability of someone who can reintubate the patient, if necessary.
. . . . . 6) Equipment available to reintubate the patient, if necessary.
High frequency ventilation and negative pressure ventilation are specialized modes, which do not follow many of the "rules" of conventional ventilation. This is beyond the scope of this chapter but this is described well in a review article by Krishnan (4). High frequency ventilation is usually reserved for patients with very non-compliant lungs or those with air leak. Negative pressure ventilation (the old "iron lung" was a type of negative pressure ventilator) is infrequently used and is generally only useful for patients with neuromuscular disorders requiring long term ventilation at night.
1. SIMV stands for:
. . . . . a. synchronized intermittent mandatory ventilation
. . . . . b. simplified intermittent mechanical ventilation
. . . . . c. synchronized interspersed mechanical ventilation
2. Name 2 prerequisites for extubation.
3. True/False: The ventilator FiO2 should never be reduced below 40%.
4. True/False: There are very specific, pediatric evidence based protocols that will guide you, step by step, on ventilation management.
5. Minute ventilation = respiratory rate x _______
6. Physiologic PEEP is (in mmHg):
. . . . . a. 3-4
. . . . . b. 1-2
. . . . . c. 5-6
7. A good indicator of adequate tidal volume is:
. . . . . a. good chest rise
. . . . . b. adequate breath sounds
. . . . . c. oxygen saturation = 100%
. . . . . d. a and b
8. As compliance worsens in a child receiving pressure controlled mechanical ventilation, the TV delivered to the patient will:
. . . . . a. increase
. . . . . b. decrease
9. If the patient has the ABG: pH 7.28, pCO2 50, pO2 120, BE -3, which of the following ventilator changes would NOT be a good idea:
. . . . . a. decrease the FiO2
. . . . . b. decrease the I-time
. . . . . c. decrease the PEEP
. . . . . d. decrease the rate
1. Hanson JH. Chapter 2 - Ventilation. In: My Way, A Resident Handbook for the Pediatric Intensive Care Unit. 1990, Children's Hospital Oakland, pp. 5-10.
2. Perkin RM, Habib DM. Chapter 128 - Continuous Distending Pressure and Assisted Ventilation. In: Levin DL, Morriss FC (eds). Essentials of Pediatric Intensive Care. 1990, St. Louis: Quality Medical Publishing, Inc., pp. 897-910.
3. Venkataraman ST, Orr RA. Chapter 48-Mechanical Ventilation and Respiratory Care. In: Fuhrman BP, Zimmerman JJ (eds). Pediatric Critical Care. 1992, St. Louis: Mosby Year Book, pp. 519-543.
4. Krishnan JA, Brower RG. High-Frequency Ventilation for Acute Lung Injury and ARDS. Chest 2000;118(3):795-807.
Answers to questions
2. Coughing or gag intact. NPO. Minimized sedation. Adequate oxygenation on 40% FiO2 with CPAP less than or equal to 4. Availability of personnel to reintubate if necessary. Availability of equipment to reintubate if necessary.
5. tidal volume