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Positive Pressure Ventilation in the CICU
Study Questions:
What are some of the fundamental concepts of respiratory mechanics and positive pressure ventilation (PPV), and how do they impact cardiopulmonary stability and management of specific conditions commonly encountered among patients in the cardiac intensive care unit (CICU)?
Methods:
Review of Basic Concepts
During spontaneous breathing, negative inspiratory pleural pressure enhances right ventricular (RV) filling, minimizes RV afterload by maintaining low pulmonary vascular resistance (PVR), and maintains relatively higher left ventricular (LV) afterload. These physiologic conditions are beneficial in RV failure, pulmonary hypertension, or other preload-dependent conditions.
During PPV, increased pleural pressure decreases RV preload and LV afterload, while increased transpulmonary pressure raises PVR and RV afterload. Both PPV and positive end-expiratory pressure (PEEP) decrease LV diameter and increase transmural LV pressure, and LV afterload decreases due to baroreceptor reflex response to aortic compression. These mechanisms augment LV stroke volume, benefiting patients with left heart failure ± severe mitral regurgitation. However, benefit depends on a number of coexisting factors including preload, RV function, pulmonary compliance, and interventricular dependence.
Modes and Applications of Oxygenation and Ventilatory Support
High flow nasal cannula oxygen can deliver 10-fold higher flow rate than conventional nasal cannula, exceeding inspiratory flow rates to produce consistently high FiO2 plus the equivalent of 7 cm H2O of PEEP.
Noninvasive PPV (NI-PPV) includes continuous positive airway pressure (CPAP) and bilevel PAP (BiPAP), where inspiratory and expiratory pressure components may be titrated separately. CPAP and BiPAP both can improve oxygenation and lung compliance, whereas BiPAP also augments tidal volume, decreases work of breathing, and can mitigate need for intubation.
Tracheal intubation and use of invasive mechanical PPV (IM-PPV) become mandatory for impending respiratory failure, hypoxia, cardiac arrest, and altered mental status ± factors otherwise limiting the patient’s ability to maintain airway protection.
Accumulating evidence indicates patients with acute lung injury have reduced mortality with lung-protective ventilation, involving limiting tidal volume to 6-8 ml/kg ideal body weight, and plateau pressure to ≤30 cm H2O. Whether lung-protective ventilation affords similar outcome advantage in the CICU population warrants further investigation.
Results:
Applications of PPV in Conditions Commonly Seen in the CICU
NI-PPV may provide acute benefit in patients with LV failure who develop cardiogenic pulmonary edema. Respiratory fatigue or hypercapnia favor selection of BiPAP over CPAP.
While IM-PPV with PEEP may decrease stroke volume in hypovolemic patients with normal LV systolic function, this combination increases stroke volume in LV failure by decreasing LV afterload, and increased LV transmural pressure, provided that volume status is adequate and RV function is normal.
In RV failure, IM-PPV ideally should be avoided, although it may become necessary during invasive procedures or episodes of clinical deterioration. Volume status should be optimized, and consideration given to initiating vasopressor support prior to proceeding. Dose and choice of induction medications and pulmonary vasodilators should be guided by the objective of minimizing circulatory depression. Goals during ventilation include avoiding hypercarbia, acidosis, and hypoxia. Both de-recruitment and hyper-inflation produce a significant increase in PVR, and for the same reason, PEEP should be used with caution.
For patients in preload-dependent conditions such as cardiac tamponade, PPV may precipitate circulatory collapse, especially when sedation is administered to facilitate tracheal intubation or surgical intervention. Volume status must be optimized, with consideration given to performing tracheal intubation under awake or minimally-sedated conditions. Topicalization of the airway, maintenance of spontaneous ventilation, early initiation of vasopressor infusion, and preparation for possible urgent pericardial fluid evacuation are prudent considerations for safe management.
IM-PPV settings during extracorporeal membrane oxygenation (ECMO), where gas exchange is performed by the ECMO circuit, are tailored to minimize risk of ventilator-associated lung injury. Specific protocols and preferences differ between institutions.
Conclusions:
The CICU is an increasingly complex environment where patients require care for a spectrum of comorbid conditions including ventilatory failure. PPV is frequently required or employed for adjunctive support, and the authors have reviewed relevant concepts and discussed their relationship to specific conditions frequently encountered here. Intimate connections between aspects of ventilatory management and circulatory physiology provide novel opportunities to address clinical challenges and form hypotheses.
Perspective:
The dynamic nature of conditions require the clinician to remain engaged and constantly attentive to detail. An understanding of various aspects of ventilation management, and their interactions with cardiopulmonary physiology, is fundamental to successful management of the increasingly complex CICU population. Findings from large prospective trials to provide high-level recommendations on ventilatory management are currently sparse in this patient population.
Clinical Topics: Arrhythmias and Clinical EP, Cardiac Surgery, Diabetes and Cardiometabolic Disease, Heart Failure and Cardiomyopathies, Invasive Cardiovascular Angiography and Intervention, Pericardial Disease, Prevention, Pulmonary Hypertension and Venous Thromboembolism, Valvular Heart Disease, Vascular Medicine, Implantable Devices, SCD/Ventricular Arrhythmias, Cardiac Surgery and Arrhythmias, Cardiac Surgery and Heart Failure, Cardiac Surgery and VHD, Acute Heart Failure, Pulmonary Hypertension, Interventions and Structural Heart Disease, Interventions and Vascular Medicine, Hypertension, Mitral Regurgitation
Keywords: Acute Lung Injury, Cardiac Surgical Procedures, Cardiac Tamponade, Continuous Positive Airway Pressure, Extracorporeal Membrane Oxygenation, Heart Arrest, Heart Failure, Hypercapnia, Hypertension, Pulmonary, Hypovolemia, Intensive Care Units, Intermittent Positive-Pressure Ventilation, Mitral Valve Insufficiency, Pulmonary Edema, Respiratory Insufficiency, Stroke Volume, Systole, Vascular Resistance, Vasodilator Agents
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