Respiratory: PICU Handbook

Normal values:
pH = 7.35-7.45
PCO2 = 35-45
HCO2 = 22-26
Anion gap < 16

Caveats

Metabolic acid base problem exists if

• pH is abnormal and pH and PCO2 change in same direction (both up or down)
• Respiratory compensation is intact if PCO2 resembles last 2 digits of pH

Respiratory acid base problem exisits if:

• PCO2 is abnormal
• pH and PCO2 change in opposite directions

Mixed acid base problem exists if:

• PCO2 is abnormal and pH has not changed as expected or normal
• pH is abnormal and PCO2 has not changed as expected or is normal

Phases of injury

1. Exudative – Characterized by acute development of decreased pulm compliance and arterial hypoxemia
2. Fibroproliferative – Characterized by increased alveolar dead space fraction, chronic inflammation, and scarring of alveolar-capillary unit
3. Recovery – Characterized by restoration of the alveolar epithelial barrier, gradual improvement in pulm compliance, resolution of arterial hypoxemia, and eventual return of pulm function

Clinical presentation

First signs are tachypnea, dyspnea, agitation and hypoxemia. May occur over hrs to 1-5 days. As lung becomes edematous and consolidated, tachypnea and hypoxemia are caused by progressive restrictive lung dz and muscle fatigue.

CXR w/ diffuse opacities +/- small effusion.
CT w/ dense regions in dependent areas reflecting collapse of edematous lung/ w secondary atelectasis. Aerated regions prevail in non-dependent areas. PFTs show decreased FRC. Total lung compliance is ↓’d but some regions may be normal.

Hypoxemia results from intrapulmonary shunting and V/Q mismatch. In fibroproliferative phase, lung compliance is reduced by progressive lung fibrosis. PEEP effects on oxygenation are less impressive. CO2 retention is common.
Significant bronchoreactivity can be seen post-recovery. Muscle wasting and weakness are most prominent extrapulmonary complications.

Figures

• Pediatric acute respiratory distress syndrome definition
• At risk of pediatric respiratory distress syndrome definition

Oxygenation/ventilation strategies

Lung protective strategies

PEEP – High PEEP, used to maximize alveolar recruitment, improve oxygenation insufficiency, and minimize need for O2. Can cause overdistension and hemodynamic compromise if too high.

TIDAL VOLUME – Low TV, 6-8cc/kg.

PERMISSIVE HYPERCAPNIA – Well tolerated at pCO2 levels of 65-85mmHg, pH < 7.25.

PRONE POSITIONING – Recruits atelectatic dependent zones.

Adjunct therapies

HFOV – Extreme form of low tidal volume.

iNO – Relaxes pulmonary vascular smooth muscles.

Surfactant replacement – May improve compliance and reduce intrapulmonary shunting

Corticosteroids – Used in most severe forms of ARDS.

ECMO – Will provide gas exchange and circulatory support in life-threatening cases, rescue until ARDS resolves.

APRV applies continuous positive airway pressure (CPAP) with an intermittent release phase.

First described 30 years ago.

The application of CPAP (P high) for a prolonged time (T high) maintains adequate lung volume and alveolar recruitment.

There is a time-cycled release phase to a lower set of pressure (P low) for a short period of time (T low or release time) where most of ventilation and CO2 removal occurs.

If patient has spontaneous respiratory effort, spontaneous breathing can happen at any time regardless of the ventilator cycle. If the patient has no spontaneous respiratory effort, APRV becomes typical of inverse ratio (inspiration >> expiration) pressure limited ventilation.

Applying constant high pressure (P high) for an approximately 80-90% of cycle time (T high) results in persistent application of elevated mean airway pressure (MAP). This elevated MAP allows almost allows almost constant lung recruitment (open-lung approach), in contrast to repetitive inflation and deflation of the lung using conventional ventilatory methods.

Spontaneous breathing plays an important role as it is believed to improve patient comfort and patient-ventilatory synchrony with reduction in sedation necessary.

Indications:

Safe, effective mode in cases of acute lung injury, acute respiratory distress, and profound atelectasis

Initial settings:

• P high: 20-30 cm H₂O
• P low: 0-5 cm H₂O initially
• T high: 4-6 s
• T low: 0.2-0.8 s

Maneuvers to correct poor oxygenation:

1. Minimize air leak
2. Increase either P high T high, or both to increase MAP

Maneuvers to correct poor ventilation:

1. Increase P high and decrease T high  simultaneously to increase minute ventilation while keeping stable MAP (preferred), 
2. Increase T low by 0.05-0.1 s increments,
3. Decrease sedation to increase pt’s effort

References:

1. Downs JB, Stock MC. Airway pressure release ventilation: a new concept in ventilatory support. Crit Care Med. 1987;15(5):459-461.
2. Daoud EG. Airway pressure release ventilation. Ann Thorac Med. 2007;2(4):176-179.
3. Habashi NM. Other approaches to open-lung ventilation: airway pressure release ventilation. Crit Care Med. 2005;33(3 Suppl):S228-240.
4. Hotchkiss JR, Jr., Blanch L, Murias G, et al. Effects of decreased respiratory frequency on ventilator-induced lung injury. Am J Respir Crit Care Med. 2000;161(2 Pt 1):463-468.
5. Sydow M, Burchardi H, Ephraim E, Zielmann S, Crozier TA. Long-term effects of two different ventilatory modes on oxygenation in acute lung injury. Comparison of airway pressure release ventilation and volume-controlled inverse ration ventilation. Am J Respir Crit Care Med. 1994;149(6): 1550-1556.

Basic principles

• Oxygenation: determined by MAP and FiO2; MAP mostly derived from PEEP
• Minute Ventilation: determined by RR and TV; use RR as primary tool for CO2 manipulation
• MAP = mean airway pressure
• PEEP = positive end expiratory pressure
• TV = tidal volume
• RR = respiratory rate
• IT = inspiratory time
• PS = pressure support
• FiO2 = fraction of inspired oxygen
• PIP = peak inspiratory pressure
• Plateau Pressure = airway pressure measured at end inspiration with inspiratory hold maneuver (helps to differentiate cause of high PIP) (PIP – pause pressure)
• Auto PEEP = airway pressure measured after expiratory hold maneuver (detects air trapping)
• SIMV = Synchronized intermittent mandatory ventilation

Pay attention to minute ventilation at all times and monitor exhaled TV. Exhaled TV less than inhaled TV suggest air leak in the respiratory system.

SIMV PRVC with PS is the PICU preferred mode. Pressure Regulated Volume Control (PRVC) is where breaths are pressure generated with a decelerating flow pattern.

Set

1. TV = typically 6-8 cc/kg; adjust for chest wall excursion, breath sounds and PIP
2. RR = usually 10-30 breaths/min; adjust for age, ETCO2, pCO2 (younger patients need faster rates)
3. PEEP = 5-12 cm H20; adjust for FiO2, CXR (higher PEEP needed for sick lungs)
4. IT = 0.4-1; adjust for age (lower iT for younger patients)
5. PS = usually 10 cm H20, common range 6-14; adjust for desired work of breathing, lower PS makes pt work harder
6. FiO2 = 21-100%; adjust for saturations (FiO > 60% toxic)

Variable PIP - must follow trends of PIP, typically values in the 20s, concerning if > 30 cm H20

Plateau Pressure should be < 28 always

Important differences between modes of ventilation

SIMV PC w/ PS
Pressure Control Ventilation
Peak Inspiratory Pressure is set.
Tidal Volume is variable.

• Must follow trends
• Goal to be returning > 6-8 cc/kg

Better in the presence of an air leak in the respiratory system
Normal PIP < 25 cm H2O

Flow in this mode of ventilation is decelerating.

SIMV VS w/ PS
Volume Control Ventilation
Tidal Volume is set.
PIP is variable.

• Must follow trends
• Concerning if PIP is high (> 30 cm H2O)

Flow in this mode of ventilation is constant.

Pressure Support Mode
Spontaneous Mode
Pt triggers ALL breaths
Target TV is > 5-6 cc/kg
PS assists pt to overcome resistance of vent circuit/ETT tube
Often used to help determine if pt is ready for extubation

***There is no guarantee that a patient will extubate successfully even when following these suggestions. Always be prepared to re-intubate if needed. Know ahead of time if it was a difficult airway where ENT or anesthesia should be present at the time of extubation!

Ventilator considerations

• Minimal PEEP (usually 5-6)
• Low rates (5-10 unless pt was intubated for short term)
• Low oxygen requirement
• Tolerating pressure support trials (especially if intubated for a longer period of time)

Other considerations

• Sedation - is the pt awake enough, minimal pain/sedation gtts or prns given prior to extubation
• Secretions - no recent changes to suggest pneumonia, not requiring frequent suctioning
• Neuro exam - is it appropriate for age
• Leak around ETT tube - does pt need steroids prior to extubation
• CXR - did you review last film (no effusions or pneumo)
• Supplies and drugs - know what drugs you would use for reintubation and have intubation supplies ready if needed
• Any upcoming procedures or imaging where sedation may be needed, which may delay extubation

Immediately prior to extubation

• Pre-oxygenate pt - FiO2 to 100% unless limiting oxygen
• Suction - suction ETT prior to extubation and have supplies ready to suction mouth immediately after extubation
• Oxygen/CPAP/BiPAP - have supplies ready in the room if you anticipate a transition to anything but room air
• Head of bed elevated
• Aerosols - have racemic epinephrine ready if any concern about leak or swelling. Consider albuterol if hx of asthma/airway hyperreactivity
• Supplies and drugs - always have nearby and know what you will use if reintubation is needed

General principles

1. Uses extremely low tidal volumes (1-3 cc/kg) at high respiratory rates (frequencies of 5to 15 Hz)
2. Effectively ventilates with intrapulmonary pressures and volume changes that are less than with conventional ventilation - prevents barotrauma and volutrauma

Indications

Failure of conventional mechanical ventilation, ARDS/ALI, air leaks (pneumothorax, PIE), persistent pulmonary hypertension, pulmonary hemorrhage, congenital diaphragmatic hernia

Complications

Hypotension secondary to decreased venous return
Pneumothorax as evidenced by sudden hypotension/desaturations, decreased chest wall movement
ETT Obstruction causing or secondary to suboptimal pulmonary toilet and mucus clearance

HFOV settings

MAP (cmH2O): level of pressure held in the lung

• Supports oxygenation
• Initial MAP 2-5 cm higher than on CMV, range variable
• Monitor degree of lung expansion by CXR

Frequency (Hertz): number of breaths per second

• Infants started on 10, lowered to increase ventilation (lower Hz = increased tidal volume)

Power/Amplitude (cmH2O): Volume of gas generated by each high frequency wave

• Inversely proportional to PaCO2
• Initial Power 4-5, adjusted to obtain adequate chest wall vibration

Inspiratory Time (%): set at 33%, 1:2 ratio

Management principles

Hypercarbia

• Suction Pt using inline suction
• Increase POWER to increase ventilation
• *A change of 0.3 units in power changes the CO2 by ~3 mm Hg
• Decrease Frequency to increase tidal volume
• Take cuff down in ETT
• Increase Bias Flow to 30-40 L/min
• Ensure ETT patent

Hypoxemia

• Increase MAP or FiO2
• *Follow CXR closely to avoid hyperinflation!

Never disconnect to suction unless emergent (de-recruitment occurs quickly)

Wean slowly when ready (MAP change of 1-2 every 8-12 hours)

Patient with Spontaneous Respirations

• Monitor clinical status closely (WOB, anxiety, single breath count, airflow, mental status, pulse pressure, pulsus paradoxus)
• Do NOT intubate based on blood gas
• Ominous Signs
  • Silent chest - no air flow or pneumothorax
  • Poor air movement - may be sign of impending respiratory failure
  • Normal or high pCO2 in a tachypneic patient - may be sign of early muscle fatigue and impending respiratory failure

Intubated Patient

• Allow permissive hypercarbia (pH >7.25)
• Use low respiratory rates and short inspiratory times to minimize hyperinflation and air trapping
• Ensure each breath returns to baseline
• Maintain adequate oxygenation
• PEEP settings controversial - Consider low PEEP (0-5) b/c patient is already hyper expanded versus matching patient's auto peep on the ventilator

General management principles

1. Hydration - Patient will need fluid resuscitation because has been hyperventilating and has increased insensibles
2. Continuous albuterol - Use 0.25-0.5mg/kg/hr (usually 10 -20 mg/hr). May need to do back to back nebs while waiting for pharmacy and RT to set up.
3. Oxygen - Use as carrier for nebulizer, will not suppress respiratory drive.
4. Steroids - Give 2 mg/kg methylprednisolone as a bolus, then start 0.5mg/kg Q 6 hrs
5. Mg bolus - Give 25-40 mg/kg bolus (over 30 mins), may repeat if needed. Thought to help relax smooth muscle

Other considerations

• Sub-Q Epinephrine - β agonist, often used if no IV access. Dose is 0.01 mg/kg of the 1:1000 solution.
• Terbutaline (β-agonist) - Continuous infusion, typical dosing range 0.5-2 mcg/kg/min
• Ipratropium bromide (anti-cholinergic) - Usually Atrovent 2 puffs Q 4-6 hrs
• Theophylline (methylxanthine) - Give loading dose, followed by continuous infusion. Monitor levels (goal 10-20)
• Heliox - Used to reduce air flow resistance in small airways, limited by degree of hypoxemia in pt. 70/30 or 80/20
• Ketamine - If sedation is needed, preferred drug due to bronchodilating properties, typical dosing range 0.1-2 mg/kg/hr
• Zithromax - Most common infectious trigger for asthma is Mycoplasma or Chlamydia pneumoniae
• Anesthesia (inhaled anesthetics) and ECMO (VV) (for refractory cases)