PRESSURE CONTROL VENTILATION Michael A. Venditto D.O., FACOI, FCCP

Traditional Ventilation Volume ventilation (CMV) is designed to deliver mandatory breaths at a preset volume and peak flow Adjustments in the three variables (tidal volume, respiratory rate and peak flow) results in a specific inspiratory time and generated a variable pressure dependent on the patient”s airway resistance and lung compliance

Traditional Ventilation Inspiration is never longer than expiration and in fact rarely exceeded 1/3 of expiratory time Flow rates were adjusted within a narrow range to keep the I:E within the limits and with the criteria that delivering the present tidal volume was most important

Volume Ventilation--Disadvantage Ventilator air tends to take the path of least resistance In disease, some lung units have longer time constants because of  airway resistance or  local compliance This leads to over distention of normal alveolar units and under-ventilation of diseased unit 

PCV - History Uses in neonatal ventilation since the late 70’s Technology allowed for a higher inspiratory flow and decreased expiratory resistance When used with inverse ratio ventilation, was found to decrease Vd, increase paO2 and decrease shunt

Pressure Control Ventilation Delivers a mandatory number of breaths but at a preset pressure limit and inspiratory time Inspiratory time is set by establishing the I:E ratio For example, if the respiratory rate is 20/min, each respiratory cycle is 3 seconds. If the I:E is 2:1, the inspiration is 2 seconds and expiration is 1 second This results in variable volume and flow patterns dependent on the patients airway resistance and lung compliance

Pressure Control Ventilation When the ventilator cycles into inspiration, airway pressure rises rapidly to the preset value When the inspiratory time is elapsed, the airway pressure fall to baseline or the an established PEEP level Tidal volume is variable

Pressure Control Ventilation Airway pressure is at its highest from the beginning of inspiration This longer airway exposure to higher pressure allows those lung units with long pulmonary time constants the driving force and time to expand The “bad” lung units can catch up to the “good” ones Creates a more equitable distribution of gas

Pressure Curves Comparison

Inverse Ratio Ventilation Much more effective with pressure control than with volume ventilation With IRV it is essential that Critical alveolar opening pressure is overcome rapidly Airway pressure are maintained for a sufficient time No significant time should pass during expiration that would allow alveolar closure

PCV—What to set Inspiratory Pressure (above PEEP) in cm H2O Respiratory Rate FiO2 PEEP (if so desired) I:E ratio It is suggested that you start at 1:2 It could then be altered to 1:1, 2:1, 3:1, or even 4:1

PCV—Settings An Example Inspiratory Pressure = 25 cm H2O Rate = 10/min Fio2 = .40 I:E = 1:2 This implies that there will be 6 seconds allotted for each breath of which 2 seconds will be for inspiration and 4 seconds will be for expiration

PCV - Monitoring Since pressure is constant, tidal volumes are variable Changes in lung compliance or airway resistance will change to tidal volume ET tube kinking/obstruction, water in tubing—will decrease tidal volume Intrinsic PEEP must be assessed since if it increases, the tidal volume will decrease

PCV - Adjustments The initial inspiratory pressure should be ½ to 2/3 of original CMV pressure Manipulate PIP and RR to influence paCO2 Manipulate I:E and RR to adjust intrinsic PEEP and thus influence paO2

Adjustments Volume Mode PCV Mode To increase paO2 Increase FiO2 Increase PEEP To decrease paO2 Decrease FiO2 Decrease PEEP To increase paO2 Increase FiO2 Increase PEEP Decrease expiratory time To decrease paO2 Decrease FiO2 Decrease PEEP Increase expiratory time

Adjustments Volume Mode PCV Mode To decrease paCO2 Increase tidal volume Increase respiratory rate To increase paCO2 Decrease tidal volume Decrease respiratory rate To decrease paCO2 Increase pressure Increase respiratory rate To increase paCO2 Decrease pressure Decrease respiratory rate

Transition Back to Volume Mode ABG should be approaching normal and underlying pathophysiology should be resolving Attempt to lower PIP to below 50 cm Then lower FiO2 Once FiO2 is less than .50, intrinsic PEEP should be reduced by increasing expiratory time When I:E is 1:1, can convert to volume ventilation

Airway Pressure Release Ventilation An application of CPAP Patient cycles between to levels of CPAP—the higher one call P High (P1), the lower P Low (P2) The patient can breath spontaneously at either level Maintains an optimal FRC Occasional pressure releases augments CO2 removal

APRV After a preset time (T high) at the higher CPAP setting (P High), a very low resistance valve opens and drops the airway pressure to the lower level (P Low). With the release of pressure, the patient exhales a volume determined by the difference between P High and P Low and the respiratory compliance.

APRV After a brief period of deflation, P High is reapplied---expanding the lung The respiratory rate is determined by adding T High and T Low and dividing that number into 60 seconds. Example: T High = 5.0 sec and T Low = 1.0 sec; therefore total time is 6.0 sec and the respiratory rate is 60/6 = 10 per minute

APRV

APRV - Indications Hypoxia in a spontaneously breathing patient Risk of barotrauma – peak airway pressure is usually just above what mean airway pressure would be in CMV Risk of cardiovascular compromise that can occur with higher pressures

APRV - Advantages Improves pulmonary mechanics and paO2 and decreased dead space ventilation Negates use of paralysis – don’t “buck” the vent Decreases need for sedation Improved ability to mobilize patient Preserves active cough Improves ability to identify complication that can be masked by sedation

APRV - Contraindications Patients with increased airway resistance (bronchospasm) –they need to empty their lungs in a relatively short period, usually 2 seconds Patients with audible wheezes on known anatomical narrowing of their airway would be poor candidates Lack of spontaneous breaths

APRV Pressure settings Time Settings FiO2 P High –desired mean airway pressure + 3 cmH2O (range: 20 to 30 cm) P Low – usually set at 0 cm H2O (range: 0 to 5 cm) Time Settings T High – usually set at 5.0 seconds (range: 4.5-6.0 sec) T Low – usually set at 0.6 seconds (range: 0.5-1.5 sec) FiO2

APRV Monitoring Release tidal volume at least 5 ml/kg Arterial oxygen saturation Hemodynamics ABG 20 minutes after initial setup

APRV Monitoring If the patient shows inspiratory activity, they are struggling to achieve good lung volumes---raise the P High If the patient is actively exhaling, they are struggling to get down to FRC---either decrease P High or increase the T Low

APRV Adjustments To increase paO2: Increase FiO2 Increase P High by 2 cm H2O increments or increase T High by 0.2 seconds

APRV Adjustments To decrease paCO2: Increase the T High in O.5 second increments Allows for more CO2 accumulation in the airways before releasing If the CO2 increases with this, there is inadequate lung volumes and P High should be increased Increase T Low to achieve a larger tidal volume Never let the expiratory flow fall to less than 25% of the peak expiratory flow

APRV Adjustments Never change: P High by more than 2 cm H2O T High by more than 0.2 seconds T Low by more than 0.1 seconds

APRV Weaning Decrease P High in increments of 1-2 cm H2O while increasing the T High by 0.5 seconds per cm H2O drop in P High When the P High reaches an acceptable CPAP level, the patient should be considered for extubation