effects of ppv

• Positive pressure ventilation is an essential life support measure in the intensive care and extended care environments
• The physiologic effects of positive pressure ventilation have complex interactions with the lungs and other organ systems
• Some of the physiologic effects of positive pressure ventilation are beneficial, while others may cause complications
• Spontaneous breathing


  1. Diaphragm and other respiratory muscles create gas flow by lowering the pleural, alveolar, and airway pressures

  2. When alveolar and airway pressures drop below atmospheric pressure, air flows into the lungs
• Negative pressure ventilation


  Uses the principle of creating a negative pressure on the chest wall to decrease pressures in the airways, alveoli, and pleura during inspiration
• Positive pressure ventilation


  Gas flow is delivered to the lungs under a positive pressure gradient (i.e., airway pressure is greater than alveolar pressure)
• Pressure-controlled ventilation


  Mode of ventilation in which a preset peak inspiratory pressure is used to provide ventilation. The delivered volume is affected by changing compliance and resistance.
• Volume-controlled ventilation


  Mode of ventilation in which a preset tidal volume is used to provide ventilation. The airway pressures are affected by changing compliance and resistance.
• During pressure-controlled ventilation, the peak inspiratory pressure (PIP) is preset according to the estimated tidal volume requirement of a patient
• During volume-controlled ventilation, the tidal volume is preset and the pressure used by the ventilator to deliver this preset volume is variable
• In positive pressure ventilation, airway pressures (including PIP and mean airway pressure [mPaw]) are directly related to the tidal volume, airway resistance, and peak inspiratory flow rate and inversely related to compliance
• Compliance


  In lungs with normal compliance, about 50% of the airway pressure is transmitted to the thoracic cavity. In noncompliant or stiff lungs, the pressure transmitted is much less due to the dampening effect of the nonelastic lung tissues.
• High levels of PIP or positive end-expiratory pressure (PEEP) may be required to ventilate and oxygenate patients with low compliance
• The decrease in cardiac output due to excessive PIP or PEEP is less severe than that if the same pressures are applied to lungs with normal or high compliance
• Mean airway pressure (mPaw)

  Average pressure within the airway during one complete respiratory cycle. It is directly related to the inspiratory time, respiratory frequency, peak inspiratory pressure, and positive end-expiratory pressure (PEEP).
• Positive pressure ventilation increases mPaw and decreases cardiac output
• Positive end-expiratory pressure (PEEP)


  PEEP is an airway pressure strategy in ventilation that increases the end-expiratory or baseline airway pressure to a value greater than atmospheric pressure. It is used to treat refractory hypoxemia caused by intrapulmonary shunting.
• Continuous positive airway pressure (CPAP)


  The end-expiratory pressure applied to the airway of a spontaneously breathing patient.
• PEEP exerts a more negative effect on the cardiac output as it raises the mPaw (and PIP) proportionally, compared to CPAP
• Stroke volume


  Blood volume output delivered by one ventricular contraction.
• Oxygen delivery


  Total amount of oxygen carried by blood. It is the product of O2 content and cardiac output.
• A decreased venous return (or filling of ventricles) leads to a reduction in stroke volume and cardiac output, resulting in a decrease in oxygen delivery
• Pulsus paradoxus


  During spontaneous inspiration, a transient decrease of arterial blood pressure. In cardiac tamponade or acute asthma exacerbation, this transient decrease in systolic blood pressure becomes exaggerated (>10 mm Hg decrease).
• Reverse pulsus paradoxus


  During positive pressure ventilation, the arterial blood pressure is slightly higher than that measured during spontaneous breathing.
• A significant reverse pulsus paradoxus (increase of systolic pressure >15 mm Hg) during positive pressure ventilation is a sensitive indicator of hypovolemia
• For patients with cardiopulmonary disease or compromised cardiovascular reserve, positive pressure ventilation and PEEP may further lower the venous return and compromise the cardiovascular functions
• Sure ventilation and PEEP

  May further lower the venous return and compromise the cardiovascular functions
• Stroke volume


  Blood volume output delivered by one ventricular contraction
• Oxygen delivery


  Total amount of oxygen carried by blood. It is the product of O2 content and cardiac output
• During spontaneous inspiration


  A transient decrease of arterial blood pressure is called pulsus paradoxus
• A decreased venous return (or filling of ventricles)

  Leads to a reduction in stroke volume and cardiac output
• A significant reverse pulsus paradoxus (increase of systolic pressure >15 mm Hg) during positive pressure ventilation


  Is a sensitive indicator of hypovolemia
• Comparison of mean airway pressure between CPAP and PEEP

  • The mean airway pressure is higher with PEEP because PEEP (10 cm H2O) is used in addition to positive pressure ventilation
• During positive pressure ventilation

  Intrathoracic pressure changes according to the pressure transmitted across the lung parenchyma, which can affect the pulmonary blood flow entering and leaving the ventricles
• In hypotensive patients, an increase in tidal volume

  Causes a decrease in pulmonary venous return to the left ventricle
• In hypertensive patients, use of large tidal volumes

  Increases venous return to the left ventricle
• Thoracic pump mechanism


  Alternations in pulmonary blood flow caused by changes in intrathoracic pressure during positive pressure ventilation. In hypotensive conditions, positive pressure ventilation decreases the blood flow to the left heart. In hypertensive conditions, this mechanism enhances the outflow of blood from the right ventricle and into the left heart
• Positive pressure ventilation


  Leads to a decrease in O2 delivery
• In the right ventricle, high airway pressures and large tidal volumes used in positive pressure ventilation


  Stretch and compress the pulmonary blood vessels and limit their capacity to hold blood volume. During expiration, the pulmonary vessels are free to fill to their holding capacity with the blood leaving the right ventricle, facilitating the outflow of blood from the right ventricle
• In children with right ventricular dysfunction


  High positive pressure (up to 40 cm H2O) and large tidal volumes (20 to 30 mL/kg) may reduce the workload of the right heart by the action of the thoracic pump mechanism