Normal Values & Basic Concepts- Tidal volumes regardless of age:8-10 cc/kg
- Total Lung Capacity: 60-80 cc/kg
- Vital capacity: 30-40 cc/kg in infants, 45-55 cc/kg in adults
- Functional residual capacity: ~30 cc/kg
- Compliance= ΔV/ΔP. The more compliant the lungs are, the more change in volume you get for a given change in pressure.This is described by the pressure volume curve (see below):
Figure 1: The Pressure Volume Loop - Closing capacity: volume of gas present in the lung at which small conducting airways begin to collapse. If FRC is below closing capacity (this occurs in infants and young children <6 yrs of age), then small airways and alveoli tend to collapse at end expiration.
- Time constant= Resistance X compliance. One time constant is the amount of time it takes to cause a 63% change in volume (3 time constants causes a 95% change in volume). Hence, the more compliant lungs are and the more resistance exists, the longer it takes to cause a change in volume. Different regions of lung (and actually individual alveoli) can have different time constants, and so respond to a ventilator breath differently
Time Constants, Courtesy of Richard Pierce, MD
- Total pressure required to inflate the lung (Ptp) is the sum of the pressure required to overcome the compliance of the respiratory system (Pcompliance) and the pressure required to overcome airway resistance (Presistance)
- Hence: Ptp = Pcompliance + Presistance
- V= IR or Change in pressure = Flow X resistance; Hence, Presistance = Flow X respiratory resistance
Figure 2: Lung Volumes and Capacities - Inspiration is started by a trigger variable: Pressure and flow triggering require patient effort (ie a pressure support breath is triggered generally by flow). IMV breaths are generally triggered via time.
- Inspiration is terminated via cycling. Cycling can occur based on:
- Time (i.e. i time reached on an IMV breath)
- Pressure
- Volume
- Flow (ie flow reaches 30% of maximal on a pressure support breath)
Respiratory Physiology
Variables in Mechanical Ventilation
| Abbreviation | Term | Definition | | PIP | Peak Inspiratory Pressure | Maximal Airway Pressure. Total pressure delivered (used to overcome both respiratory system compliance as well as airway resistance). Sum of PEEP and IP (in a pressure control mode) | | PEEP | Positive End Expiratory Pressure | Pressure maintained at the airways at the end of expiration. The pressure applied to the lungs never drops below this value. | | IP or ΔP | Inspiratory Pressure or Delta Pressure | Difference between PIP- PEEP | | Vt | Tidal Volume | Volume of gas entering the patient's lung during inspiration | | It | Inspiratory Time | Time spent in inspiration (can be set, i.e. during control breath, or determined by the patient, i.e. during a spontaneous pressure support breath) | | Et | Expiratory time | Time spent in expiration. Generally not directly set but determined by the set rate and inspiratory time. For example, a respiratory rate of 20 breaths/min means each breath is 3 seconds. If Itime is set at 1 second, then Etime is necessarily 2 seconds | | I:E | Inspiratory:Expiratory time ratio | Generally, expiration takes longer than inspiration. A typical ratio is 1:2 (may need to be much greater, i.e. 1:4, with obstructive lung disease or high airway resistance) | | RR | Respiratory Rate | Respiratory rate as set on the ventilator (patient will receive at least these many breaths/min but can spontaneously breathe above this set rate) | | FIO2 | Fraction of Inhaled Oxygen | Varies from 21% (room air) to 100% with >50-60% considered toxic (reactive oxygen species, free radical formation etc). Historically, subambient (ie <21% O2 was sometimes utilized in the cardiac unit to help "balance" circulation (reduce Qp in favor of Qs). | | PS | Pressure Support | Extra pressure given to support the patient's spontaneous breaths above the set rate (ie instead of the IP or set volume, they receive the set pressure support when they take their spontaneous breath) |
- Nomenclature: Generally, when someone states "The patient is on 25 over 5," this refers to a PIP of 25 (rather than an IP) and a PEEP of 5
- STEP 1: Can be divided into the amount of support the ventilator provides for the patient
- Assist Control (AC): More support and control. Every breath, whether mechanical or spontaneous (patient triggered) results in a fully supported breath (still synchronized)
- Synchronized Intermittent Mandatory Ventilation (SIMV): The ventilator delivers the set number of breaths with the preset PIP or Vt, and It. Any spontaneous patient triggered breaths above this set rate receive the set pressure support only. Hence, this is sometimes considered a weaning mode but is routinely used for the majority of patients in the PICU.
- SIMV Pressure Control with a pressure support that is equal to the IP is nearly assist control (all breaths, whether strictly mechanical or patient triggered spontaneous breaths, receive the same pressure). However, the difference is that pressure supported breaths are generally terminated (cycled) when the flow reaches a certain point (ie 30% of maximal) whereas a normal pressure control breath is terminated (cycled) by the set inspiratory time. Patients generally find controlling the duration of their own breaths (ie PS breaths) more comfortable than ventilator controlled duration (ie AC or IMV breaths).
- STEP 2: A second classification can then be made by what variable is controlled or set:
- Pressure Control (PC): You set the pressures (IP and PEEP). Tidal volume becomes the dependent variable and thus depends on the patient's compliance and airway resistance. Hence, if a patient has lungs that are getting worse and stiff (reduced compliance), their tidal volumes will decrease. Conversely, lungs that have improving compliance will have increasingly larger tidal volumes in pressure control mode. The major advantage of PC is that it utilizes a decelerating flow pattern which generally is more comfortable, achieves the same volume at lower peak airway pressures, and is preferred for non compliant lungs.
- Volume Control (VC): You set the tidal volume and the ventilator delivers that volume. Pressure (IP and PIP) becomes the dependent variable and thus depends on the patient's compliance and airway resistance. The more compliant the lung is, the less pressure is required to achieve the set tidal volume. The advantage is that minute ventilation is guaranteed. The disadvantage is that if there is a significant leak around the ETT, it cannot be used. It also utilizes a square flow waveform that may be less comfortable.
- Pressure Regulated Volume Control (PRVC or Volume Guarantee): Hybrid mode where you set a tidal volume and the ventilator looks at the last three breaths delivered to the patient and the pressures/volumes delivered and delivers a breath in a pressure control style (decelerating flow) attempting to achieve your set tidal volume. The ventilator continuously does this. Theoretically gives the advantages of both pressure control and volume control in that you guarantee a minute ventilation like volume control but also deliver the breath with a decelerating waveform like pressure control.
Figure 3: Waveforms (Pressure, Flow and Volume over time) in Pressure and Volume Modes
Ventilation
- Describes CO2 elimination
- Determined by RR X Vt= minute ventilation
- Hence, efforts to reduce CO2 generally involve increasing the respiratory rate (taking into account whether the patient has sufficient time to exhale) or increasing the tidal volume (either directly in volume control or indirectly by increasing IP in pressure control modes)
Oxygenation- Primarily determined by mean airway pressure (Paw) (as opposed to PEEP as commonly noted by early trainees). This is the average pressure in the respiratory system over time (taking into account both inhalation and exhalation)
- Also determined by FiO2- this affects alveolar partial pressure of oxygen (PAO2), as determined by the alveolar gas equation (PAO2= FiO2 (Patm - PH2O) - PaCO2/R ). Nonetheless, FiO2 >50-60% can be associated with oxygen toxicity and thus, increases in FiO2 should be viewed generally as a temporizing measure to allow one to improve oxygenation primarily via changes in mean airway pressure.
- Mean airway pressure can be understood as the area under the curve (the integral) of the pressure vs. time graph (see the figure below). Hence, maneuvers that improve mean airway pressure generally improve oxygenation (ie via alveolar recruitment).
- Mean airway pressure can also be calculated fairly easily. It is a weighted average. The pressure in inspiration multiplied by the relative amount of time spent in inspiration added to the pressure in expiration multiplied by the relative amount of time spent in expiration. ie. PIP X itime/(itime + etime) + PEEP X e time/(itime + etime) or [PIP(itime) + PEEP (etime)]/ (i time+e time)
- Hence, increasing the PEEP (and to a lesser extent PIP since more time is generally spent in expiration than inspiration) increases the mean airway pressure and thus improves oxygenation. Similarly, increasing the i time also leads to increased area under the curve and thus higher mean airway pressure and improved oxygenation.
Figure 4: Mean Airway Pressure is the integral (area under the curve) of Pressure and Time
General Ventilator Settings and Goals
| Setting | General Setting | Special Considerations | | PIP | As needed to deliver desired tidal volume (6-8 cc/kg in healthy lungs, 6 cc/kg in "sick" lungs) | Generally want to keep Plateau pressure (pressure at end of a inspiratory hold which reflects just the pressure needed to overcome respiratory system compliance) <30 cm H20 to avoid barotrauma | | PEEP | 5 cm H2O for healthy lungs, increase as needed to achieve adequate oxygenation in "sick" lungs or if there is bronchomalacia and a need to "stent" the airways | Can sometimes be detrimental if there is significant air trapping. This can be checked via end expiratory hold, giving a measure of AUTOPEEP (intrinsic PEEP). This will be elevated with air trapping or significant airway resistance. Generally set PEEP 1-2 below AutoPEEP | | IP or ΔP | As needed to deliver desired tidal volume (6-8 cc/kg in healthy lungs, 6 cc/kg in "sick" lungs) | See PIP above | | Vt | 6-10 cc/kg in healthy lungs, 4-6 cc/kg in "sick" lungs |
| | It | Based on patient age (ie 0.6 seconds in infants, 1 second in older children/adolescents) | Can be increased to increase mean airway pressure and thus improve oxygenation. However, have to keep in mind the amount of time left for exhalation and your I:E ratio | | I:E | Generally aim for 1:2 | May need to be much higher (ie 1:4+) if obstructive lung disease or high airway resistance (ie status asthmaticus). Occasionally set inverse (i.e. I:E 1:1 or even 2:1) to provide increased mean airway pressure and aid with oxygenation. | | RR | Physiologic for age (ie 30 for small infant, 20 for toddler, 12 for adolescent) | Adjust as needed for minute ventilation, may be supraphysiologic for therapeutic hyperventilation (ie acute intracranial hypertension) | | FIO2 | 30% in healthy lungs, goal to maintain <50-60% to avoid oxygen toxicity but can increase to 100% as needed acutely | Generally left at lower limit of 30% in the PICU although if concerned about ROP or other oxygen toxicity (or historically subambient O2 in the congenital heart population- i.e. HLHS to promote increased Qs vs. Qp) | | PS | 10 in most patients but can be anywhere from 0 (CPAP) to the same as IP (ie 20+) | Can be increased (ie match IP) to provide increased support but allow the patient to regulate their own inspiratory/expiratory time. In addition, can be decreased to force the patient to do more of their own work on the ventilator (ie if there are concerns about deconditioning prior to extubation) |
Healthy Lungs- I.E. intubated for airway protection due to status epilepticus but with healthy lungs
- SIMV generally used
- PC, VC, or PRVC would all be fine but VC or PRVC may be preferable as you guarantee a minute ventilation and aren't worried about delivering high pressures
"Sick" Lungs- I.E. ARDS
- AC or SIMV can be used
- PC or PRVC generally preferred given decelerating flow pattern, generally avoid potential barotrauma although still requires careful monitoring of delivered tidal volumes
From Alviar et al. JACC 2018
- Head to toe approach, focusing on why they were intubated in the first place (i.e. airway protection for mental status, lung disease, metabolic control etc)
- Head: Are they awake and conscious, able to protect their airway? Cough and gag intact, GCS >8. Respiratory drive intact (ie not apneic or hypopneic when set rate removed from ventilator?)
- Oropharynx/upper airway: Significant secretions? Are they a difficult mask or reintubation (doesn't change extubation plan necessarily but may alter threshold for extubation and preparedness for extubation).
- Cuff leak present? Proper way to check is by deflating the ETT cuff, attaching the ambu bag to the ETT, inflating and holding at a pressure of 20-25 cm H2O, and lisy pattening for audible leak. Even with the most airway edema and the most oversized tube, all patients will have presence of an airleak at some pressure, but cuffleak at <20-25 cm H2O is considered an acceptable cuff leak. Presence of a cuff leak is helpful but lack of a cuffleak is less helpful (does not necessarily predict postextubation stridor nor should it delay extubation if the patient meets all other parameters).
- Can treat suspected upper airway obstruction with dexamethasone 0.25 mg/kg q6 hrs x4-8 doses. For prevention of postextubation stridor, giving dexamethasone >12 hours prior to extubation seems to be most efficacious (Iyer N et al, Ann ATS 2022)
- Lungs:
- Compliance ok (generally extubatable settings ~ PIP/PEEP of <23/<6)?
- How did they do on a "sprint," or extubation readiness trial (basically removing the set rate and allowing the patient to breathe on CPAP +/- PS (generally PEEP of 5 and PS 10 in our PICU). Assessment of a sprint is clinical (ie work of breathing, rate of breathing, particularly at the end of a 1-2 hour sprint). A blood gas may not be valuable, especialyl outside of the clinical context. I.e. if you were presented with a blood gas of 7.4/40/90, that could be a patient who did fine or a patient who was breathing 70 times a minute and retracting.
- Diaphragmatic conditioning ok? Patient expected to be able to sustain respirations over prolonged period of time
- Heart:
- Patient cardiac function acceptable if previously had LV systolic dysfunction? Removing positive pressure will increase afterload so if the patient has compromised LV systolic function, this should be a consideration. Conversely, removing positive pressure should improve overall preload.
- Other:
- Any procedures upcoming that may require intubation and/or sedation?
- Discontinue sedation (it may take a while for your patient to wake up- this is one of the advantages of using a propofol washout as you have a generally more predictable time of when your patient will awaken)
- Ensure everything is ready: (MSOAP)- Monitors and reintubation Meds (or meds for postextubation stridor such as racemic epinephrine), suction, oxygen-ie what you are going to extubate to, Airway equipment (ie reintubation supplies, Mapelson mask, airway adjuncts like NP tubes etc), Personnel (RT, RN, MD)
- Can consider preoxygenting the patient with 100% FiO2- similar in concept to preoxygenation for intubation. Recognize that this will make the SpO2 unreliable as a marker of whether your patient is doing well with extubation at least in the acute setting. Instead, you should rely on work of breathing and your clinical exam. It will however, provide a reserve of oxygen and time should your patient require interventions postextubation.
- Generally suction the patient, a few bagged/assisted breaths, remove the ETT. Place planned intervention (ie BiPAP, HFNC, nasal cannula etc) and assess.
- Reassess.
Utility of Daily Chest X-Rays (CXR)- In one study, 45% of routine CXR's in the PICU resulted in 1 or more interventions. 55% of routine CXR's in children <10kg resulted in intervention(s) compared to 40% in those 10-40 kg and 19% in those >40kg
- Routine CXRs are more likely to result in interventions in the smaller, critically ill child with one or more devices and if active cardiopulmonary problems are present
Pulmonary Equations etc.Courtesy of Richard Pierce, MD
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