12.2 Invasive Mechanical Ventilation and Heart-Lung Interactions

Modes, Set Variables, and Derived Variables

Every ventilator breath is defined by what triggers it, what limits it, and what cycles it off. The modes a CMC candidate must be able to interpret at the bedside are few.

Variables the nurse or provider sets: tidal volume by predicted body weight (PBW), respiratory rate, FiO2, PEEP, inspiratory time or flow rate and pattern, and trigger sensitivity. Variables that result and must be measured: plateau pressure (an inspiratory hold), driving pressure (plateau minus total PEEP), auto-PEEP (an expiratory hold), minute ventilation, and exhaled tidal volume.

Predicted body weight is calculated from height and sex, not actual weight: men 50 + 2.3 x (height in inches - 60); women 45.5 + 2.3 x (height in inches - 60). Lung-protective targets are tidal volume 6-8 mL/kg PBW (4-6 mL/kg in ARDS), plateau pressure 30 cmH2O or less, and driving pressure 15 cmH2O or less. A rising plateau with an unchanged tidal volume means compliance is falling - in the cardiac patient that most often means worsening pulmonary edema.

Heart-Lung Interactions: The Cardiac-Specific Core

Spontaneous inspiration lowers intrathoracic pressure, which increases venous return and increases LV transmural pressure (afterload). Positive pressure ventilation does the reverse on both counts. Understanding that single inversion explains most of the hemodynamic behavior of ventilated cardiac patients.

Peri-Intubation Arrest

Induction and the first positive pressure breaths are the most dangerous 5 minutes in the shocked cardiac patient. Three insults land simultaneously: sedatives abolish sympathetic tone and cause vasodilation and negative inotropy, positive pressure abruptly removes venous return, and any pre-existing acidemia or hypoxemia is already limiting myocardial reserve. Nursing preparation reduces this risk concretely:

• Resuscitate before you intubate: correct hypovolemia, and start or increase a vasopressor before induction rather than after the collapse.
• Have push-dose vasopressor immediately available - phenylephrine 50-200 mcg or epinephrine 5-20 mcg IV - and a norepinephrine infusion primed and connected to a functioning central or reliable large-bore access.
• Anticipate reduced induction doses in shock; ketamine 1-2 mg/kg (often 0.5-1 mg/kg in shock) or etomidate 0.2-0.3 mg/kg are preferred over propofol, which is a potent vasodilator and negative inotrope.
• Preoxygenate well, use apneic oxygenation, and avoid aggressive bag-mask ventilation that stacks breaths and drops venous return further.
• Hold or reduce vasodilator infusions such as nitroglycerin and nitroprusside immediately before induction.
• Anticipate low initial tidal volume and rate with gentle initial pressures, and check blood pressure within 1 minute of the first ventilator breaths.

Auto-PEEP and Obstructive Shock

Auto-PEEP (dynamic hyperinflation, breath stacking) occurs when expiratory time is insufficient - high respiratory rate, high minute ventilation, bronchospasm, or a small endotracheal tube. Trapped alveolar gas raises intrathoracic pressure continuously, venous return falls, and the patient develops obstructive shock with rising peak pressures, an expiratory flow waveform that never returns to zero before the next breath, and difficulty triggering the ventilator. The immediate response is to disconnect the patient from the ventilator circuit and allow full exhalation with gentle chest compression; blood pressure typically recovers within seconds, which is both the treatment and the diagnosis. Then reduce the set rate, shorten inspiratory time to lengthen expiration, treat bronchospasm, and consider raising set PEEP toward 80% of measured intrinsic PEEP to reduce triggering work in the obstructed patient.

Dynamic Preload Indices: The Conditions That Make Them Valid

Pulse pressure variation (PPV) and stroke volume variation (SVV) exploit heart-lung interaction: each positive pressure breath transiently reduces RV filling, and two or three beats later LV stroke volume falls. A steep response means the ventricle is on the ascending limb of the Starling curve. Thresholds of PPV above roughly 13% and SVV above roughly 13% predict a positive response to fluid, with a grey zone of about 9-13%.

They are valid only when all of the following hold, and CMC items are frequently written around a violated condition:

1. Fully controlled mechanical ventilation with no spontaneous respiratory effort
2. Tidal volume at least 8 mL/kg PBW (low-tidal-volume ventilation falsely lowers the values)
3. Sinus rhythm - atrial fibrillation and frequent ectopy invalidate them
4. Closed chest - an open sternum or open thorax abolishes the pressure transmission
5. No severe right ventricular failure (a variation may reflect RV afterload sensitivity rather than volume responsiveness)
6. Heart-rate-to-respiratory-rate ratio above about 3.6, normal intra-abdominal pressure, and no high-frequency ventilation

When any condition is broken, use a passive leg raise with a real-time stroke volume measurement, an end-expiratory occlusion test, or a small fluid challenge with reassessment. A static central venous pressure does not predict fluid responsiveness at any value.

Weaning, Liberation, and Weaning-Induced Cardiac Failure

Readiness and the Spontaneous Breathing Trial

Assess readiness daily: the precipitating cause is improving; PaO2/FiO2 above 150-200 with FiO2 0.5 or less and PEEP 8 cmH2O or less; pH above 7.25; hemodynamically stable on no or low-dose vasopressor; adequate cough and manageable secretions; and the ability to initiate an inspiratory effort. Pair a spontaneous awakening trial (SAT) with the spontaneous breathing trial (SBT); the paired approach shortens ventilator days. Run the SBT for 30-120 minutes on low pressure support (5-8 cmH2O), CPAP 5 cmH2O, or a T-piece. The rapid shallow breathing index (RSBI), respiratory rate divided by tidal volume in liters, is measured early in the trial; below 105 breaths/min/L favors success, and above 105 predicts failure - though it is one data point, not a veto.

Failure criteria during an SBT: respiratory rate above 35 sustained, SpO2 below 90%, heart rate rise above 20% or above 140, systolic blood pressure above 180 or below 90, new arrhythmia, diaphoresis, accessory muscle use, paradoxical abdominal motion, or agitation.

Weaning-Induced Cardiac Failure

This is the single most cardiac-specific concept in ventilator liberation and it is heavily tested. When positive pressure is withdrawn:

• Intrathoracic pressure becomes negative during inspiration, so venous return and RV preload rise - the congested LV is suddenly volume-loaded.
• LV transmural pressure rises, so afterload increases exactly when the ventricle is receiving more volume.
• The work of breathing returns to the patient, raising total oxygen consumption and triggering a catecholamine surge that raises heart rate, blood pressure, and myocardial oxygen demand.
• The result can be acute diastolic dysfunction, ischemia, and flash pulmonary edema minutes into an otherwise well-planned trial.

It accounts for a substantial share of repeatedly failed weans. At-risk patients: known HFrEF or HFpEF, significant coronary disease, aortic or mitral valve disease, chronic obstructive lung disease with high work of breathing, obesity, and chronic kidney disease with volume overload.

Recognition during the trial: hypertension and tachycardia rather than hypotension, tachypnea, new crackles, ST-segment change, a rise in pulmonary artery occlusion pressure above 18 mmHg, a fall in mixed or central venous oxygen saturation, hemoconcentration with a 5% or greater rise in hemoglobin or plasma protein as fluid shifts into the alveoli, and a rise in natriuretic peptide. Bedside echocardiography shows a rising E/e' ratio.

Management: return the patient to full support, then treat the cardiac problem rather than repeating the same trial - diuresis to a negative balance, nitrates for preload and afterload reduction, control of ischemia and rate, optimization of guideline-directed therapy, and correction of anemia. Many of these patients then wean successfully on a subsequent day.

Extubation Planning

Perform a cuff-leak test in patients at risk for laryngeal edema (prolonged intubation, traumatic or repeated intubation, large tube, prone positioning, fluid overload). A leak volume below 110 mL or less than 10-15% of the delivered tidal volume suggests risk of post-extubation stridor; systemic corticosteroids (for example methylprednisolone 20 mg IV every 4 hours for 4 doses) started at least 4 hours before extubation reduce that risk. Plan post-extubation support in advance for high-risk cardiac patients - prophylactic NIV or HFNC immediately after extubation reduces reintubation.

Alarms, Troubleshooting, and the ABCDEF Bundle

Pressure Alarms Decoded

The single most useful bedside distinction is between peak inspiratory pressure and plateau pressure.

For sudden decompensation on the ventilator, use DOPES to name the causes - Displacement of the tube, Obstruction, Pneumothorax, Equipment failure, Stacked breaths (auto-PEEP) - and DOTTS to act: Disconnect from the ventilator (this alone treats auto-PEEP), Oxygenate manually with a bag-valve device and feel the compliance, Tube position and patency (pass a suction catheter), Tweak the ventilator settings, and Sonography for pneumothorax, effusion, and cardiac function.

Sedation, Delirium, and Mobility

Use an analgesia-first approach and target a light sedation level, Richmond Agitation-Sedation Scale (RASS) -2 to 0, unless deep sedation is specifically indicated (neuromuscular blockade, refractory hypoxemia, active temperature control with shivering). Light sedation shortens ventilator days and reduces delirium. Propofol 5-50 mcg/kg/min is easily titratable but vasodilates and is negatively inotropic - a real problem at higher doses in cardiogenic shock - and requires monitoring of triglycerides and vigilance for propofol-related infusion syndrome. Dexmedetomidine 0.2-1.4 mcg/kg/h preserves respiratory drive and reduces delirium but causes bradycardia and hypotension, which matters in the patient with conduction disease or a fixed stroke volume. Benzodiazepine infusions are independently associated with delirium and should not be the default.

Implement the ABCDEF bundle: Assess and manage pain, Both SAT and SBT daily, Choice of sedation, Delirium assessment with CAM-ICU or ICDSC and management, Early mobility, and Family engagement. Early mobility is safe in selected patients on vasoactive infusions and mechanical support with appropriate staffing and line management.

Ventilator-Associated Event Prevention

Core practices: elevate the head of the bed 30-45 degrees unless contraindicated; provide regular oral care (note that routine chlorhexidine oral care is no longer recommended outside cardiac surgery populations in current SHEA/IDSA guidance); maintain cuff pressure 20-30 cmH2O; use subglottic secretion drainage tubes when intubation beyond 48-72 hours is expected; avoid unnecessary circuit changes; minimize sedation; and pursue the earliest safe extubation, since the most effective ventilator-associated pneumonia prevention is fewer ventilator days.

Across all of this the nursing frame stays constant: the ventilator is a cardiovascular device. Every change in PEEP, tidal volume, rate, or sedation changes preload, afterload, and myocardial oxygen demand, and the nurse who checks a blood pressure and a rhythm after each ventilator change is practicing at the level the CMC exam is written for.