This is an observational, prospective, single-centre study that will focus on patients undergoing major non-cardiac surgery requiring invasive mechanical ventilation and invasive blood pressure monitoring Hypotheses: A positive TVC (tidal volume challenge) prior to the recruitment manoeuvre (RM) predicts a decrease in CI within 5 minutes of individualised PEEP establishment of at least 10%.

1. T0: Moment prior to the start of tidal volume challenge. Baseline values
2. T1: After tidal volume challenge, moment priorate the start of the recruitment manoeuvre (RM). Mostcare and ventilator values. From this moment on, the parameters obtained from Mostcare will be analysed continuously (minute by minute) until 15 minutes after establishing the individualised PEEP.
3. T2: At minute 5 of establishing individualised PEEP. All parameters derived from basic monitoring, Mostcare, and ventilator monitoring shall be monitored and recorded. Record whether any fluid bolus has been administered.

This is an observational, prospective, single-centre study that will focus on patients undergoing major non-cardiac surgery requiring invasive mechanical ventilation and invasive blood pressure monitoring

Lung recruitment manoeuvres (RM) are performed to prevent collapsed lung parenchyma from compromising oxygenation. In order to open collapsed lung areas, intrathoracic pressure needs to be raised and this may have haemodynamic repercussions especially in patients with an overt or latent hypovolaemic state. Parameters such as stroke volume variation (SVV) or pulse pressure variation (PPV) reflect to some extent the heart-lung interaction and have been used as predictors of fluid responsiveness by exploiting this principle to detect preload-dependent patients. The tidal volume challenge (TVC) uses the same principle of heart-lung interaction with better results. TVC can be a predictor of haemodynamic tolerance to RM + individualised PEEP.

Hypotheses: A positive TVC prior to the recruitment manoeuvre (RM) predicts a decrease in CI within 5 minutes of individualised PEEP establishment of at least 10%.

Data will be collected in the surgical area. Demographic and clinical parameters will be collected from the patient's clinical history, respiratory parameters obtained from the respirator, haemodynamic parameters obtained from the Mostcare device, oxygenation parameters before and after a recruitment manoeuvre.

If the patient meets all inclusion criteria and none of the exclusion criteria, he/she will be included for data collection. If he/she benefits from a recruitment manoeuvre (air-test + clinical indication), which will be assessed by clinical indications, he/she will be entered into our study. All measurements will be taken under stable haemodynamic conditions (HR and MAP should be stable and with +-10% variation for 1 min prior to measurements), without administration of vasoactive drugs or influential surgical aggression at that time.

When the recruitment manoeuvre (RM) is performed, we will monitor all the variables by setting the following time points:

1. T0: Moment prior to the start of recruitment manoeuvre. All the variables described (Mostcare, ventilator, basic monitoring) and the administration of fluids prior to the manoeuvre shall be monitored. To avoid artefacts on the arterial waveform, a fast-flush test and assessment of dP/dtMAX should always be performed. Patients who do not have optimal arterial waveform morphology at this point will be excluded.
2. T1: At minute 1 after starting the VTC, the parameters derived from the basic monitoring and the Mostcare will be checked. From this moment on, the parameters obtained from Mostcare will be analysed continuously (minute by minute) until 15 minutes after establishing the individualised PEEP.
3. T2: At minute 5 of establishing individualised PEEP. All parameters derived from basic monitoring, Mostcare and ventilator monitoring shall be monitored and recorded. Record whether any fluid bolus has been administered.

Inclusion Criteria:

• Patients over 18 years of age; undergoing scheduled non-cardiothoracic surgery; under controlled invasive mechanical ventilation and invasive arterial monitoring; supine position; positive air test

Exclusion Criteria:

• Chronic pulmonary disease (defined as chronic obstructive pulmonary disease grade 3 or higher or any disease requiring long-term oxygen therapy); congenital cardiac malformations; severe valvular heart disease; heart failure NYHA (New York Heart Association) Grade III/IV; arrhythmias; history of reduced ventricular systolic function (FEVI <40% or TAPSE <17 cm/s); history of pulmonary hypertension; BMI >35 (due to altered lung compliance and rib cage); heart rate/respiratory rate ratio < 3.6; presence of inspiratory effort; open chest; increased intra-abdominal pressure (due to pathology or pneumoperitoneum); altered pulmonary or rib cage compliance due to surgery (trendelemburg or antitrendelemburg position); uncorrected optimal arterial waveform (resonant or damped) and presence of any contraindication to lung recruitment manoeuvres. The latter are: pulmonary emphysema, pulmonary bullae, uncontrolled haemodynamic instability, right heart failure, elevated intracranial pressure (decreased return flow through jugular veins) or lack of monitoring if necessary, bronchospasm, undrained pneumothorax.

Patients undergoing scheduled non-cardiothoracic surgery under controlled invasive mechanical ventilation and invasive arterial monitoring, without pathologies that affect heart-lung interactions and their interpretation. They would be patients who benefit from a recruitment maneuver.